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There is a generalised belief that barley is more resilient to stress than wheat. Consequently, land allocation to wheat and barley is based on a presumed differential sensitivity to stresses between these species. Previous field experiments showed that yield of both crops tended to be similar under both low- and high-productivity conditions under Mediterranean environments, but in some particular studies barley advantages are reported. Therefore, the aim of this study was to determine the comparative performance of wheat and barley across a wide range of conditions through the literature published over the first 20 years of this century considering exclusively studies where both cereals had been grown side-by-side in field experiments. We compiled a rigorous database from papers of high-standard journals reporting actual yield data (we excluded simulated yields) measured in experiments under field conditions where both crops were grown side-by-side in the same experiment (excluding studies analysing wheat and barley grown separately). Yield distribution for both species across studies was symmetrical with an average of c. 440 g m−2 and a range from c. 50-900 g m−2. The yields of the lowest quartile ranged from 58 to 293 g m−2, being the comparison of performance in this quartile suitable for detecting differences in low-yielding conditions. Considering individual papers, it is possible to identify any possible outcome of the differential performance of these cereals. However, considering all papers together it was clear that there was not a consistent advantage of barley over wheat at most stressful conditions. Indeed, at very low yielding conditions wheat tended to outyield barley. The trends described above were not influenced by the data-points based on comparisons of barley with the tetraploid wheats, nor by our own experimental data-points. Thus, we confirm in this review that barley does not consistently outyield wheat under low yielding conditions, despite the generalised belief that it does. We are grateful to Prof Ignacio Romagosa (University of Lleida and AGROTECNIO) for generous advice on statistical procedures. Research of the authors on wheat and barley yield physiology has been funded along years by many different grants from different funding bodies; most recently mainly from the Spanish State Agency for Research (latest projects were RTI2018–096213-B-100 and PID2021–127415OB-I00 on wheat and PCI2019–103536 on barley).

Wheat (Triticumspp.) grain yield (GY) is highly associated with grain number per unit area (GN m-2). Biomass accumulation and partitioning are essential to understand pre-anthesis spike growth dynamics which determines spike dry matter at anthesis (SDMa) - a GN determinant. Spike growth takes place during the stem elongation period (SE), from terminal spikelet to anthesis, following leaf and spikelet initiation (LS) from sowing to terminal spikelet. In this study, bread wheat cultivars were examined under Mediterranean semi-arid conditions to determine (i) the varietal differences in pre-anthesis phase duration, (ii) whether this variability influences biomass partitioning and spike-related traits, and (iii) to what extent, the genotypic variations in pre-anthesis phase duration and spike growth are associated with yield components. A panel of Israeli commercial bread wheat cultivars were grown in the field during 2016-17 (three environments) and 2017-18 (two environments) and characterized for pre-anthesis phases, floral conditions and spike fertilityviahistological measurements, spike traits and dry matter accumulation and partitioning at anthesis and maturity and for yield components. Significant variability in the timing of pre-anthesis phases was detected within the tested panel. LS duration, and occasionally SE, favourably related with a better dry matter of fertile florets spike-1(at anthesis) and SDM (at both anthesis and maturity). Two cultivar pairs ‘Zahir-Yuval’ and ‘Negev-Gedera’, which flowered concurrently, revealed significant differences in the durations of LS and SE phases across the environments. Longer LS (e.g., in cultivars Zahir and Negev) exhibited increased spikelets number spike-1, whereas longer SE (e.g., in Yuval and Gedera) enhanced spike fertility through improving the survival rate of floret primordia (FSR%) of central spikelets. However, there was a trade-off for FSR at the proximal and distal spike portions, resulting reduction of final GN (or GY) in cultivars with longer SE. It is concluded that, in this panel, the duration of both LS and SE contribute to spike fertility. However, under short wheat growing cycle, LS duration seemed a stronger driver than SE for GN and yield enhancement. These highlights the novel importance of pre-anthesis phases, especially the role of LS in wheat yield increment during the short growing cycle. The varietal combination with variable LS and SE duration could be implemented in the breeding pipeline and used as pre-breeding materials for GN improvement. Furthermore, the findings will improve pre-anthesis traits adoption in Mediterranean bread wheat future breeding programs.

Most of the required increases in food production over the next decades are expected to be achieved through increases in crop yield. As wheat is essential for food security it is worrying that its yield gains over the last two decades were small. To achieve further yield increases it is critical to continue increasing number of grains per unit area (GN m(-2)), the trait best related to yield. In this context, it is relevant to identify the main determinants of GN m(-2) in response to genetic and environmental factors as well as the trade-offs between them. In the present study we compiled a large database across the literature to analyse the relative importance of components when affected by genetic or environmental factors, producing small or large changes in GN m(-2) and its components, either numerical (the number of spikes per m(2), SN m(-2); and the number of grains per spike, GN spike(-1)) or physiological (spike dry weight at anthesis, SDWa; and fruiting efficiency, FE) determinants. The database included 367 papers published in: (i) Field Crop Research (FCR), (ii) European Journal of Agronomy (EJA), (iii) Crop Science (CS) and (iv) Crop and Pasture Science (CPS, formerly Australian Journal of Agricultural Research) between 1990 and 2020. The complete dataset was split into classes, depending on the source of experimental variation, environment or genotype and was normalised to remove the differences between experiments and determine the environmental and genotypic effects within each experiment. Normalised data showed that the responsiveness of GN m(-2)was similarly explained by changes in both SN m(-2) and GN spike(-1), but in terms of physiological components SDWa was more relevant than FE for explaining the variations in GN m(-2). Considering the numerical components of GN m(-2) genotypic and environmental factors modified more GN spike(-1) than in SN m(-2). On the other hand, physiological components were differently modified by genotype and environment: for genotypic effects FE was more critical than SDWa and the other way around for environmental factors. A tradeoff between numerical and physiological components was observed although was greater between physiological than between numerical components. Research of the authors on wheat yield physiology has been funded along years by many different grants from different funding bodies; most recently mainly from (i) the International Wheat Yield Partnership (IWYP25FP) , (ii) , the Spanish State Agency for Research (latest project was RTI2018-096213-B-100) , and (iii) Argentine Fund for Scientific and Technological Research (latest project PICT RAICES 2019-04333) .

Under dryland systems of the Mediterranean basin, water management is widely recognised critical for improving grain yield. It has been proposed that nitrogen (N) fertilisation may be a tool to increase grain yield by increasing water use efficiency. We tested in a multilocation-multiyear study in four different areas of the Mediterranean Basin if durum wheat and barley may respond positively to N fertilisation under rainfed cultivation. The study involved 16 experiments including different genotypes of durum wheat and barley sown at Morocco, Jordan, North-eastern Spain, and Southern Italy, with different nitrogen doses applied at sowing or early in crop development from 2003/04-2007/08 growing seasons. Grain yield increased 26% in average for years, locations and species compared with their respective unfertilised controls. The increased yield in response to N fertilisation was noticeable under low (< 2 Mg ha(-1)) and under high yield potential cases (> 5 Mg ha(-1)). There were only exceptional cases with slightly negative or no responses to N fertilisation. Grain number per unit area, and not grain weight, was the main component related to grain yield. Also, total biomass at maturity was closely related to grain yield. Barley and durum wheat responded similarly to N fertilisation. This study was supported by WatNitMED (Management improvements of WUE and NUE of Mediterranean strategic crops) , EC-FP6-2002-INCO project funded by the European Union.

Average grain weight (AGW) is a major component of wheat yield. When attempting to elucidate mechanisms behind treatments effects on AGW, the distribution of the weight of individual grains may be critical. Determining the individual weight of thousands of grains in each sample would be unmanageable. Then, when individual sizes must be considered, researchers either weigh individually a very minor proportion of the grains or determine for the complete sample individual linear dimensions (length, width, area) through an image processing equipment. We aimed to generate a single model equation to trustworthily convert grain linear dimensions to grain weights. Firstly, we used a set of data to build and calibrate a model for the relationship between weight and linear dimensions of individual grains. Then, we validated the model calibrated with independent data. Grain area was a better predictor of grain weight than length and width of grains. Initially, we generated a single linear model but (i) the intercept was incongruently negative and therefore (ii) we forced the linear regression through the origin, but that consistently overestimated the weight of small grains and underestimated large grains. Finally, we fitted the data again with a power curve model and forced the intercept to zero (with the log-transformed data) obtaining the model (ŷ = x1.32) to estimate individual grain weight from grain area. The model was validated with (i) independent data from the same studies used to build the model, (ii) data from other completely independent experiments, and (iii) data from the literature. Considering the diversity of genotypes and environments in the model generation and validation, the proposed power curve model could be trustworthily used to estimate grain weights from measured areas. Funding was provided by projects AGL2015-69595R and RTI2018-096213-B-100 funded by the Agencia Estatal de Investigación (AEI) of Spain. Jinwook Kim held a pre-doctoral research contract from AGAUR (the Agency for Management of University and Research Grants of Catalonia).

38 Pags.- 13 Figs.- 2 Tabls. The definitive version is available at: https://www.sciencedirect.com/science/journal/11610301, Genetic gains in wheat yield are required to meet future demand. In order to design strategic crosses, it is essential to quantify the genetic variation among elite cultivars on traits responsible for the adaptation and the generation of yield components. We explored the degree of variation within well-adapted commercial cultivars on developmental phases (vegetative, early reproductive and late reproductive), final leaf number and phyllochron, leaf and spikelet plastochron, number of spikelets, florets, and dynamics of floret primordia development. Variation in flowering time among cultivars was minor but they differed in the partitioning of that time into particular phases. Main differences were found for the length of the late reproductive phase, partly linked to those in final leaf number. Variations on number of fertile florets at anthesis were mostly explained by floret survival, due to differential ability of labile florets to maintain normal development between cultivars. These differences were related to the duration of the floret development, but not to the synchrony of development among florets. These findings provide useful information for breeders when selecting genotypes for strategic crosses, as considering floret survival through duration of floret development in one of the parents (for which these processes shall be characterised in the elite germplasm) might increase the likelihood of transgressive segregation towards high-yielding offspring., Funding was provided by ADAPTAWHEAT (a project funded by the European Commission under the 7th Framework Programme for Research and Technological Development) and by project AGL2012- 35300 funded by the Ministry of Economy and Competitiveness of Spain. HO held a FPI scholarship from the Spanish Ministry of Science and Innovation, and PP held a PhD scholarship from the University of Lleida.

Future grain yield (GY) improvements require the identification of beneficial traits within the context of high yield potential and not just based on the pleiotropic effect of traits such as crop height and heading date. We evaluated 1937 lines from Nested Association Mapping (NAM) population derived from 13 bi-parental varietal crosses under field conditions. We selected 493 lines with similar time to anthesis to that of the two checks used in the study (across and within each family) which reduced the range of plant height in the selected lines. Yield components were measured in these 493 lines from which 231 lines were selected by excluding lines with lowest number of grains so excluded low yielding lines. Later the subset of 231 lines were evaluated in two field experiments (2016−17, CS1 and 2017−18, CS2). Numerical and physiological components of grain yield were measured. The two-step selection maximised GY within an acceptable range of variation for height and anthesis. GY in 231 lines showed very high G × E interaction. Taking both seasons together, we selected lines from upper and lower quartile GY groups to identify stable beneficial trait combinations for improved GY. Differences in GY were explained by grain number driven by increased spike dry weight at anthesis (SDWa) and fruiting efficiency (FE). Increased GY was accompanied by sink limitation. The data points towards increases in grain number as the route towards future GY increases in wheat breeding. Funding for the experimental work was partly provided by projects AGL2015-69595-R, from the Spanish Research Agency (AEI), and IWYP25FP, from the International Wheat Yield Partnership (IWYP). PB held a pre-doctoral research contract from the agency for Management of University and Research (AGAUR) from the Generalitat de Catalunya.

Variations in wheat yield are largely explained by changes in grain number per m2 which is linked to the number of fertile florets at anthesis. This, in turn is the outcome of developmental processes which control floret initiation and mortality. Earliness ‘per se’ (Eps) genes are involved in fine-tuning time to anthesis in wheat (and other cereals) but their effect on development prior to anthesis is less well studied. We aimed to determine effects of Eps genes on spike fertility, quantifying aspects of floret developmental which influence this trait. Field experiments were carried out to record floret primordia generation/degeneration dynamics in near isogenic lines (NILs) with contrasting Eps alleles (late flowering vs early flowering alleles; Eps-late and Eps-early, respectively) derived from the Avalon x Cadenza (AxC) cross with the Eps gene on either chromosome 1D or 3 A and from the Spark x Rialto (SxR) cross with the Eps gene on chromosome 1D. Eps NILs varied in spike fertility: Eps-late alleles increased fertility. Although the effect was in general slight, the magnitude was affected by the particular alleles and the cross used to produce the NILs. Differences in the number of fertile florets were explained by differences in the dynamics of floret development. NILs with Eps-late alleles improved the development of a small number of labile florets allowing them to complete their development to become fertile florets instead of dying, as in lines carrying early alleles. Thus, these alleles improved floret fertility mainly through reducing the rate of floret mortality with no influence on the dynamics of floret primordia initiation or in maximum number of floret primordia. Therefore, Eps genes could be exploited in wheat breeding not only to fine-tune time to anthesis but also to improve spike fertility.

Earliness per se (Eps) genes are critical for fine-tuning flowering time and likely to modify other developmental traits as well. Effects of Eps alleles on components comprising flowering time of hexaploid wheat have not been studied. In this work, we evaluated under field conditions the effects of Eps alleles on flowering time and on a number of developmental traits using four groups of near isogenic lines (NILs) with contrasting Eps alleles (late vs early alleles). The NIL groups were derived from the Avalon x Cadenza (AxC) cross with the Eps effects on either chromosome 1D or 3A and from the Spark x Rialto (SxR) cross with the Eps effect on chromosome 1D. Eps-D1-late alleles delayed flowering (and the effect was stronger in AxC than in SxR); while Eps 3A did not have any effect. Differences in flowering time between NILs with Eps-D1-early and –late alleles were mainly associated with the effects of this gene on the duration of the late reproductive phase consistently for both crosses. However, the effects of Eps-D1 on other developmental traits were different depending on the specific cross used to produce the NILs. The Eps-D1-early allele from AxC did not affect FLN, reduced spikelet initiation phase and, in turn, decreased the spikelet number (though less than proportionally as it accelerated the rate of spikelet initiation as well), whilst Eps-D1-early allele from SxR affected marginally FLN, but had no effects on either spikelet initiation phase or spikelet number. The results reported from this study in Mediterranean conditions conflict quantitatively (Eps-D1) or qualitatively (Eps 3A) with results observed previously in the UK, likely evidencing the existence of Eps x temperature interactions.

Variation in photoperiod sensitivity in wheat plays a major role in the crop adaptation to wide agronomic environments. Photoperiod insensitivity is provided by Ppd-Aa, Ppd-B1a and Ppd-D1a alleles. Effects of the genome, doses and source of the particular Ppd-1a alleles on time to anthesis has not been analysed simultaneously in the same experiments, even less under field conditions; and the effects on particular phases rather than on considering only the total time to anthesis and on phyllochron have not been considered for this range of allele combinations. We carried out field experiments during two consecutive growing seasons to assess differences in time to anthesis, in its component phases, in final leaf number, phyllochron and tillering across wheat isogenic lines differing in specific Ppd alleles and homoeoalleles. In addition to confirming that the introgression of Ppd-1a alleles advanced anthesis time (on average by 307, 251 and 191 °C d if there were 3, 2 or 1 insensitivity alleles introgressed, respectively), we found that the variation between photoperiod insensitive genotypes was largely dependent on the varietal source of Ppd-B1a which could be stronger or weaker than Ppd-D1a depending on the donor considered. All components of time to flowering: the particular sub-phases (P < 0.001) as well final leaf number (P < 0.001) and phyllochron of late-appearing leaves (P < 0.05) were sensitive to Ppd-1a alleles, but the strength of particular alleles on particular components was different, so that similar adjustments in time to anthesis could be achieved with different partitioning of developmental time between the considered phases. We also found that although they did not affect phyllochron of the first 7 leaves, that of the leaves appearing later was consistently reduced in lines carrying Ppd-1a. Tillering was sensitive too, but not final number of spikes due to compensations between tillering and tiller mortality. Funding was provided by ADAPTAWHEAT (a project funded by the European Commission under the 7th Framework Programme for Research and Technological Development) and by project AGL2012-35300 funded by the Ministry of Economy and Competitiveness of Spain. HO held a FPI scholarship from the Spanish Ministry of Science and Innovation and PP held from University of Lleida scholarship. We wish to thank Ignacio Romagosa for valuable advice in the statistical procedures and Andreu Casellas for excellent technical assistance.

Environmental variability has increased in recent decades and there are high expectations that heat waves will affect wheat productivity. Therefore, methods to quantify the effects of high temperature on wheat growth, yield, and yield-related traits under field conditions are greatly needed. In this opinion paper, we first describe the different approaches to quantify these effects, briefly highlighting their pros and cons. Portable polyethylene tents over the plots are cost-effective and convenient when relatively large plots are preferred and/or when there is no access to electricity in the field. This method shares a drawback with other approaches due to the tent reducing light intensity and altering other environmental factors like wind, humidity and evapotranspiration. We discuss to what degree there is an actual confounding effect (incorrectly assigning changes to heat effects that might be in part due to lower radiation). In that context, we not only discuss the issue theoretically, but also present results from a field experiment comparing a control (open uncovered plot), and a "roof-only" treatment (covering the plot with a polyethylene roof alone; thus impacting incoming radiation but not temperature). These treatments were imposed at either booting or the onset of grain filling, and testing two wheat cultivars having contrasting grain numbers per m2 and average grain weight. Supporting a theoretical framework that this reduced radiation would be compensated by increased radiation use efficiency, we show empirically that the roof-only treatment (which reduced incoming photosynthetically active radiation by 10.7 ± 0.2 % at noon on sunny days, but did not alter the temperature at any time of the day) did not result in any yield penalty with respect to the uncovered control plots. Thus, portable polyethylene tents are a reliable method to impose heat treatments in field experiments. Although other methods are more sophisticated and may be perceived as having fewer confounding factors, portable polyethylene tents are ideally suited to irrigated field experiments where electricity is not available, where relatively large plots are needed, and for research carried out on a limited budget. Funding was provided by project RTI2018-096213-B-100 funded by the Spanish State Research Agency (AEI). Jinwook Kim held a pre-doctoral research contract from AGAUR (the Agency for Management of University and Research Grants of Catalonia).

In a large comparison of experimental and published data across a wide range of environmental conditions, a cross-over interaction of durum vs . bread wheat yield was found to be the reverse of expectations, where the former would outyield the latter under low-yielding conditions and vice-versa . In this study, we aimed to establish the physiological basis of this species cross-over interaction, analysing whether it was established before or after anthesis and to determine if it was related to differences in sink-strength. Our study indicates that the consistent species-by-environment interaction for yield is the consequence of a few traits related to sink-strength that are determined around anthesis. Thus, differences in the spike weight and N accumulated at anthesis, which produced important variations in the number of grains and presumably generated differences in sink-strength, seemed to be involved. This conclusion is supported by the strong relationship between grain yield and the biomass produced during grain filling, in addition to the fact that the variation in individual grain weight, as well as its rate or the duration of grain filling, was poorly related to yield and the post-anthesis biomass.

In a previous paper (Field Crops Res. 203, 114-127), we showed that the difference in yield potential between a contemporary and a traditional cultivar was due to differences in fruiting efficiency, likely derived from differences in spike fertility (fertile florets per spike) while having similar spike dry weights at anthesis. In this study, we determined the mechanistic bases of these genotypic differences in spike fertility analysing the initiation of all floret primordia per spike (up to 8), the maximum number of florets initiated per spikelet, and the associated floret developmental rates and their fate to become fertile florets under contrasting photoperiod (natural vs extended) and nitrogen availability (50 or 200 kg N ha−1) during the stem elongation phase. Under potential growing conditions (natural photoperiod, high nitrogen availability), the contemporary cultivar owed its higher spike fertility to the improved rate of floret development, which mainly determined an improved level of floret primordia survival to produce fertile florets. The sensitivity of the floret developmental patterns to faster development due to exposure to an extended photoperiod and, to a larger extent, a reduction in N availability was similar for both cultivars, providing a basis for the consistent differences in spike fertility across a range of environments. The response again determined a main effect through increasing floret mortality reducing therefore the level of fertile florets per spikelet in these conditions. Funding was provided by project AGL2009-11964 granted by the Spanish Ministry of Science and Innovation. AF held a FPU scholarship from the Spanish Government.

Efforts to enhance the yield of wheat require increases in the number of grains per m2 (GN) which depends on spike fertility, i.e. the number of fertile florets or grains per spike. Fruiting efficiency (FE), the number of grains set per unit spike dry weight at anthesis, has been proposed as an alternative trait to improve spike fertility and thus GN. The use of FE in realistic breeding programs requires genetic variation within well-adapted, high-yielding modern cultivars and a lack of trade-off with other yield components. We quantified FE variation within Spanish elite material and determined whether the variation implied trade-offs with spike dry weight or grain weight (GW). Field experiments under contrasting nitrogen conditions were carried out with nine elite genotypes (eight modern cultivars and one advanced line) in which the number of fertile florets and grains were determined for different spikelets and the whole spike. Furthermore, the partitioning of dry matter was investigated at anthesis and maturity. Overall, the yield was strongly related to GN because environmental and genetic factors contributed to this relationship, although as expected for elite material the environmental effect was much greater than the genotype. Environmental effects on GN mainly influenced spike dry weight at anthesis (SDWa) whereas most genotypic differences were due to FE variation. Averaging across environments, the FE was c. 110–170 grains gspike−1. Although some G × E interactions were observed, there was no reversal in ranking between genotypes with extreme FEs across environments, and genotypic differences across environmental conditions were consistent (h2 = 0.46). The relationship between FE and the individual weight of fertile florets tended to be negative, although rather than a constitutive reduction in floret size driving the improvements in FE, this relationship appeared to reflect the increased proportion of smaller grains (distal florets in each spikelet) due to changes in spike fertility. FE was not related to grain weight (either the average of all grains or that of the most proximal grains). Therefore, differences in FE among the elite genotypes did not imply constitutively-linked changes in grain weight. When chaff dry weight was used as a proxy for SDWa to estimate genotypic variation in FE, we observed the consistent underestimation of FE and also changes in the ranking of genotypes. Funding was provided partially by ADAPTAWHEAT (a project funded by the European Commission under the 7th Framework Programme for Research and Technological Development) and partially by project AGL2012-35300 funded by the Ministry of Economy and Competitiveness of Spain.

The objective of this study was to compare the performance of two different remotely sensed techniques in detecting the effects of terminal heat stress and N fertilization on final maize aerial biomass (AB) and grain yield (GY). The study was conducted under field conditions for two consecutive growing seasons. Six N treatments combining three doses [0, 100, 200 Kg N ha −1 ] and two timings [at V4 and at 15 days before silking] were applied. Within each N treatment three heat treatments were applied (pre-flowering, post-flowering and the control treatment at ambient air temperature). Remote sensing measurements were taken with a multispectral band camera to measure the normalized difference vegetation index (NDVI) and a digital Red/Green/Blue (RGB) camera to measure the normalized green red difference index (NGRDI). Both indices failed to predict the GY of pre-flowering heat-treated plants due to grain set establishment problems that could not be detected by vegetation indices which are designed to capture differences in green canopy area. In contrast, both the NGRDI and the NDVI correlated positively with GY and AB in the control heat treatment and to a lesser extent in the post-flowering heat treatment. Under the control heat treatment, the NGRDI exhibited higher correlations with AB and GY than the NDVI across the N fertilization treatments. Since the NGRDI is formulated based only on the reflectance in the visible regions (VIS) of the spectrum (Green and Red) without dependence on the near infrared regions (NIR), it performs better than the NDVI. This is because it overcame the reported saturation patterns at high leaf area index and was more efficient at capturing even small differences in leaf colour (chlorophyll content) due to the different applied N treatments. Also, the NGRDI seemed to be a more seasonally independent parameter than the NDVI, which is more affected by temporal variability within the field, and thus the NGRDI predicted AB and GY better than the NDVI when combining the data of the two growing seasons.

Maize yield has to be increased in the next decades in order to satisfy world demand. This increase has to be achieved in a scenario of climate change particularly characterised by heat stress. Several agronomic and genetic strategies for increased tolerance to high temperatures will be necessary. Some results in other crops showed interactions between high temperature stress and nitrogen (N) availability. To the best of our knowledge, this interaction has not been tested in maize. Therefore, the aim of this study was to explore, under field conditions, whether the magnitude of yield penalty imposed by elevated canopy air temperature around flowering or during early grain filling is affected by N availability. In particular we aimed to (i) quantify the magnitude of yield losses by heat stress around flowering and during early grain filling, (ii) determine whether N fertilisation affects these magnitudes, and (iii) identify whether the effects are indirect (through affecting growth) or directly on the grain set and/or grain growth capacity. Four field experiments were carried out during four consecutive years in NE Spain. The treatments consisted in a factorial combination of one or two hybrids, two or three levels of N fertilisation and three temperature conditions. The temperature treatments consisted of a control (plots grown under natural temperature throughout the growing season) and two treatments in which the temperatures of the canopy were increased in the field for relatively short periods. All experiments were well watered to avoid water stress. The elevated canopy air temperature treatment only increased the maximum temperature in a relatively small magnitude (mean daily temperature was increased at the ears height by c. 1 °C each day of treatment). However, yield penalty imposed by heat stress was in general very noticeable and dramatic when the treatment included the critical period for grain number determination, around silking. The damage was much stronger in the long- than in the short-cycle hybrid. Exposing the plants to elevated canopy air temperature during the critical period reduced harvest index, from values of around 46% in unheated conditions to 20% under elevated canopy air temperature, being the reduction higher under N200 than under N0 fertilisation treatments. As far as we are aware, we showed for the first time in maize grown in field conditions, that the losses in yield in response to elevated canopy air temperature treatment were magnified by the N availability. The effect of N on emphasising the penalties seemed not to be a direct effect of this nutrient but and indirect effect through affecting growth. The effect was through affecting the capacity of the plants to set grains and to a lesser extent to allow grain weight to be maximised; and it was independent of any (potentially additional) effects on either uncoupling anthesis and silking or on pollen amount and viability.

The introgression of awns as a source of photoassimilates during grain filling has received has long been of particular attention in wheat. Although the benefits for average grain weight (AGW) seem consistent, those for yield still remain unclear, and the causes for the improved AGW have not been examined. In this study we carried out three field experiments. In two of them, one in CIMMYT (Mexico) and the other in Lleida (Spain), we compared awned-awnless near isogenic lines (NILs) subjected to defoliation and degraining during the effective period of grain filling, whilst in another experiment we analysed the responses of modern cultivars to awn detachment 10 days after anthesis under two nitrogen levels. There were not consistent differences in yield between the awned-awnless NILs neither across the locations nor across genetic backgrounds. This lack of effect on yield was associated with a compensatory effect of the presence of awns on decreasing grain number while increasing AGW. The manipulation treatments did not consistently affect AGW neither in the NILs nor in the modern cultivars. Focusing on the responses of the NILs, firstly there were some statistically significant responses in AGW but they were both mostly very small and inconsistent between defoliation and degraining treatments as well as between genetic backgrounds. Secondly these responses failed to be consistently more noticeable in the awnless than in the awned lines (as expected if the presence of awns increases AGW through increasing photosynthetic capacity during grain filling). The increase on AGW due to the presence of awns seemed ascribed to both an indirect and a direct effect. The former would simply reflect the consequence of awns reducing grain number: this would naturally be the consequence of an increased failure of distal florets to become fertile which would have reduced the proportion of grains constitutively smaller as well. A direct true effect on the capacity of the grains to grow also emerged with the analysis of individual size of each particular grain for the awned-awnless NILs revealing a constitutively higher potential size in the awned NILs, even for the largest grains. Funding for the experimental work at Lleida was provided by project AGL2015-69595-R from the Spanish Research Agency (AEI). Part of the funding for the experiment was supported by BASF Agricultural Solutions. RSB was supported by Juan de la Cierva program JDC-Formación (FJCI-2016-28164). JWK held a pre-doctoral research contract from the Government of Catalonia. CRA and GM were supported by the Sustainable Modernization of Traditional Agriculture (MasAgro) an initiative from the Secretariat of Agriculture and Rural Development (SADER) and CIMMYT and the International Wheat Yield Partnership (IWYP). JLA acknowledges the support of ICREA Academia, Generalitat de Catalunya, Spain.

Understanding factors underpinning the variability in nitrogen (N) utilization efficiency (NUtE) [i.e. grain yield per unit of N uptake at maturity (NupMAT)] is critical to direct future improvements in breeding and agronomic management. To our knowledge, no study has summarized changes in wheat NUtE across a wide range of experimental conditions. We performed a synthesis-analysis using published data to provide a global perspective of NUtE trends in wheat by (i) benchmarking against yield limited by NupMAT, and (ii) assessing factors contributing to the variation in NUtE. The final database encompassed 55 studies (n = 529). A nonlinear model explained yield as a function of NupMAT. The gap between actual yield and NupMAT-limited yield was negligible at the lowest range of NupMAT and increased to ∼2000 kg ha−1 as NupMAT levels increased. Hence, opportunities to enhance yield through improving NUtE would be more likely at greater-than-average yield and NupMAT levels. The negative correlation between grain protein concentration and the residuals between NUtE and NupMAT indicated a challenge to increase yield without penalizing grain protein. Further, there are greater opportunities to increase NUtE in fall- than winter-sown wheat. Identifying the determinants of NUtE will enable to narrow the gap between actual and NupMAT-limited yields.

High-temperatures reduce yield of wheat and with global warming episodes of heat waves (only few days of high maximum temperatures) during grain filling will become more frequent. It has been recently reported that the magnitude of the yield penalties imposed by high temperatures under field conditions may interact with nitrogen (N) availability both in barley and maize. We determined, under field conditions, the penalties imposed by post-anthesis high-temperatures waves (increased maximum –but not minimum– temperatures during part of the grain filling period) on wheat yield under contrasting soil N supply during two consecutive years. The high temperature treatment was imposed for 10 d starting 10 d after anthesis by placing over the crops transparent polyethylene film (125 μm) mounted on wood structures of 1.5 m height above the ground. This high-temperature and the unheated controls were imposed on 5 modern and well adapted cultivars under contrasting N availabilities (376, 268 and 68 KgN ha−1). Averaged across N conditions, high-temperature treatments reduced yield by c. 1.5 Mg ha−1 (a loss of c. 17%) even though the treatment was rather mild in terms of different average temperature during grain filling. The magnitude of the loss was consistently shaped by the N condition in which the treatment was imposed: yield penalty produced by high-temperature increased from less than 1 to 2.6 Mg ha−1 (which represents losses from 10 to 25%) in parallel with the increased N supply. The penalties were related to both yield components (grain number and average grain weight) which also were more severely penalised under high than under low N supply. As episodes of high-temperature waves will become more frequent in the future the tools used to establish the needs of N fertilisation should be revised as the rates maximising yield (or gross margin) might induce higher sensitivities to these episodes. Also simulation models used to upscale physiological responses to regional or even global domains might need to be revised to include the effect of heat waves (which would be larger per °C increase than what is estimated from experiments increasing temperature during the whole day and over longer periods) as well as the interaction with N supply. Authors thank Semillas Batlle S.A. for their support in the setting up and maintenance of the field plots. Funding was provided by project AGL2012-35300 of the Ministry of Economy and Competitiveness of Spain.

Future increases in grain number may be achieved through improving fruiting efficiency (FE; grains set per unit spike dry weight at anthesis). In recent experiments it was found a negative relationship between the average weight of the grains and FE. The objective of this paper was to determine whether this negative relationship was constitutive (i.e. the increased FE caused all grains to be smaller) or resulted from the increased proportion of grains of smaller potential size. Four experiments, involving 8 different environmental conditions, were carried out during two consecutive growing seasons (2008–2009 and 2009–2010) in which two durum wheat cultivars contrasting in FE and average grain weight were compared. In these conditions we determined FE and carried out a comprehensive study analysing in detail the weights of each individual grain along each of the spikelets. Averaging all experimental conditions, there was a negative relationship between average grain weight (GW) and fruiting efficiency (FE). Donduro with low FE produced in average heavier grains than Vitron. However, when considering individual grains there were, in general, no differences in GW. Therefore, the negative relationship between average grain weight and FE would not be constitutive, and would mostly represent the increase in the proportion of distal grains within the spikelets or of grains from apical spikelets as a consequence of an increased FE, reducing the average size of the grain but mostly unaffecting the size of particular grains. This provides further and stronger support to the idea of using FE as a criterion to further raise yield potential in wheat breeding programmes.

A major aim in breeding programs is to improve nitrogen use efficiency (NUE). The critical level of specific leaf nitrogen (SLNc) maximising growth and yield is a relevant trait influencing NUE. The relationship between growth or yield and SLN is hyperbolic indicating that levels of SLN higher than the critical would tend to decrease NUE. It has been recently proposed that maize breeding may profit of this relationship by phenotyping SLN and selecting for genotypes with SLN = SLNc (SLN SLNc would bring about high yields but at the expense of low NUE); being the SLNc = 1.5 gN m −2 . For this proposal to be realistic, genotypic variation in SLNc should be negligible. We aimed to determine the degree of genetic variation in SLNc in a range of modern maize hybrids. For this purpose, we carried out four field experiments with 11 contrasting maize hybrids grown under fertilized (200 kgN ha −1 ) or unfertilized conditions in each of the four experiments. At silking, leaf area and N concentration of leaves were determined and at maturity, yield and biomass were measured. Analysing the whole dataset together ( n = 88) we found a very large range of all variables measured and calculated: yield varied between c. 6 and c. 18 Mg ha −1 and SLN from c. 0.7 to c. slightly more than 2.5 gN m −2 . Within that context we found a significant bi-linear regression between yield and SLN ( p −2 . The variation was also substantial calculating SLNc with an alternative methodology and if leaf N content instead of SLN were used as independent variable. Genotypic differences in SLNc were positively related to N uptake but unrelated to other traits for which hybrids varied, including yield potential. Additionally, genotypes with low SLNc show a tendency for a better NUtE. We concluded that it does not seem appropriate to phenotype for a particular value of SLNc as any particular threshold might bring about selected lines with either a yield potential lower than possible and/or with a relatively lower than possible NUE.

It is frequently assumed that durum wheat is more tolerant to stress than bread wheat. Unfortunately few research papers compare the performance of both species side-by-side under a wide range of environments in field conditions. We aimed to compare durum and bread wheat performance in a field study under contrasting treatments of water and nitrogen during three experimental seasons. In addition we compiled a comprehensive database with data from field experiments in which both species were grown in the same field conditions. A cross-over interaction of yield from bread vs. durum wheat was found, but oppositely to the hypothesis, bread outyielded durum wheat in the low-yielding conditions while durum wheat tended to have higher potential yield. We then divided the database in decades in which the cultivars were released and found that in the 1960s bread wheat outyielded durum wheat in almost any comparison whilst in the 2000s durum wheat outyielded bread wheat in most comparisons. Grain weight was constitutively higher in durum than in bread wheat, likely associated with a lower fruiting efficiency in the former; but in both types of wheat grain number per m2 was the component responsible for yield sensitivity to environmental changes. Differences in yield were also related to differences in water and nitrogen use efficiencies: under low-yielding conditions bread wheat was consistently more efficient than durum wheat and under high-yielding conditions durum wheat was more efficient.

Wheat yield is frequently analysed in terms of yield components (spikes per m2, grains per spike, grain size), and correlations between components are commonly reported, but partially understood. Compensations between components are one of the main barriers to improve yield using this approach. Understanding better the nature of these relationships is therefore important. We compiled a large database (n = 846) of wheat yield (ranging from 1.3 to 14.8 Mg ha−1) under field conditions to further investigate the relationships between yield components from the viewpoint of phenotypic plasticity. We found two main hierarchies between plasticities. Hierarchy 1, grains per m2 ≫ grain size, has been widely described before and is supported empirically and theoretically; here this hierarchy lends confidence in our analytical method. Hierarchy 2, spikes per m2 > grains per spike, is demonstrated for the first time. These hierarchies are interpreted in terms of coarse and fine regulation of grain yield (Hierarchy 1), and grains per m2 (Hierarchy 2). This means small changes in yield or grains per m2 can be accounted for by any of their components, but large changes in yield can only be accommodated by grains per m2, and large changes in grains per m2 are primarily associated with heads per m2. Further, we showed that the rankings of responses of yield components depend on the nature of the driver, i.e. genotype or environment. Both hierarchies are much stronger when the driver is environmental factors (e.g. water supply). When the driver is genotype, and this means we are not strictly dealing with plasticity but trait responsiveness to genotype, the rankings are weaker. Strong, negative relationships between components were found for small changes in yield or in grains per m2 for both environmental and genetic factors, implying strong trade-offs. For large changes, trade-offs were generally less relevant but much stronger for components of yield than for components of grains per m2. For large changes in grains per m2 associated with environmental factors, the negative relationship between grains per spike and of spikes per m2 was weak but when they were associated with genetic factors, the negative relationship was strong. In conclusion, our study reinforces the hierarchy of plasticities between components of grain yield, reveals a hierarchy between the components of grains per m2, and demonstrates that the strength of the trade-off between components of either yield or grains per m2 depends upon the magnitude of yield changes and, for large changes, the source of variation. A practical corollary is that breeding and agronomy could potentially achieve modest to intermediate yield increase involving any components, which would exhibit some trade-offs. Agronomic interventions are more likely to lead to large yield improvement, mainly through increase in grains per m2 with negligible trade-offs in grain size.

The frequency and intensity of extreme high temperature events are expected to increase with climate change. Higher temperatures near anthesis have a large negative effect on maize (Zea mays, L.) grain yield. While crop growth models are commonly used to assess climate change impacts on maize and other crops, it is only recently that they have accounted for such heat stress effects, despite limited field data availability for model evaluation. There is also increasing awareness but limited testing of the importance of canopy temperature as compared to air temperature for heat stress impact simulations. In this study, four independent irrigated field trials with controlled heating imposed using polyethylene shelters were used to develop and evaluate a heat stress response function in the crop modeling framework SIMPLACE, in which the Lintul5 crop model was combined with a canopy temperature model. A dataset from Argentina with the temperate hybrid Nidera AX 842 MG (RM 119) was used to develop a yield reduction function based on accumulated hourly stress thermal time above a critical temperature of 34 °C. A second dataset from Spain with a FAO 700 cultivar was used to evaluate the model with daily weather inputs in two sets of simulations. The first was used to calibrate SIMPLACE for conditions with no heat stress, and the second was used to evaluate SIMPLACE under conditions of heat stress using the reduction factor obtained with the Argentine dataset. Both sets of simulations were conducted twice; with the heat stress function alternatively driven with air and simulated canopy temperature. Grain yield simulated under heat stress conditions improved when canopy temperature was used instead of air temperature (RMSE equal to 175 and 309 g m−2, respectively). For the irrigated and high radiative conditions, raising the critical threshold temperature for heat stress to 39 °C improved yield simulation using air temperature (RMSE: 221 g m−2) without the need to simulate canopy temperature (RMSE: 175 g m−2). However, this approach of adjusting thresholds is only likely to work in environments where climatic variables and the level of soil water deficit are constant, such as irrigated conditions and are not appropriate for rainfed production conditions. Fil: Gabaldón Leal, C.. Centro Alameda del Obispo; España Fil: Webber, H.. Universitat Bonn; Alemania Fil: Otegui, Maria Elena. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Producción Vegetal; Argentina Fil: Slafer, Gustavo Ariel. Universidad de Lleida; España. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Institució Catalana de Recerca i Estudis Avancats; España Fil: Ordóñez, R. A.. Universidad de Lleida; España Fil: Gaiser, T.. Universitat Bonn; Alemania Fil: Lorite, I. J.. Centro Alameda del Obispo; España Fil: Ruiz Ramos, M.. Universidad Politécnica de Madrid; España Fil: Ewert, F.. Universitat Bonn; Alemania

The number of seeds per unit land area, the major yield component in soybean (Glycine max (L.) Merrill) is largely determined after the beginning of flowering, particularly from R3 to R6. Environmental factors increasing crop growth rate (e.g. radiation) or extending the duration of the reproductive phases (e.g. photoperiod) increase the number of seeds. We aimed to compare the mechanisms by which photoperiod and radiation affect the definition of final seed number during the critical period of R3–R6. Two field experiments were conducted with indeterminate soybeans at intermediate maturity group. All plots in each experiment were grown under natural conditions until the beginning-pod stage (R3); and from then onwards different treatments were imposed. Treatments consisted of the factorial combination of two levels of radiation (natural or shading) and two photoperiod regimes (natural or extended). Extended photoperiod increased the duration of reproductive phases, the number of nodes and the number of pods produced on the nodes that flowered during or after the applications of the treatments. Shading had negligible effects on development and node number, but reduced crop growth rate and also reduced the number of pods produced on most nodes of the plants. The number of seeds was positively related to the crop growth rate during R3–R6, but photoperiod increased the number of seeds produced per unit of crop growth rate, due to the lengthening of the phase. The number of seeds was therefore even better related to accumulated growth during R3–R6, irrespective of the factor that increased the accumulated biomass (higher daily radiation or longer duration of the phase) suggesting that long photoperiods increased the number of pods and seeds established per unit land area, mainly through increasing the total resource availability during a phase that is critical for the determination of seed number in soybean. However, photoperiod regulation involved additional changes in the development, evidenced by changes in the pod distribution pattern within the canopy.

Highlights • Time to terminal spikelet and from then to anthesis were largely independent. • The length of the stem elongation phase was slightly but positively related to grains per m2. • Fruiting efficiency was critical for determining grain number, but it was also negatively related to grain weight. • The length of the stem elongation phase seems to have imposed an upper threshold for fruiting efficiency., Developmental patterns strongly influence spike fertility and grain number, which are primarily determined during the stem elongation period (i.e. time between terminal spikelet phase and anthesis). It has been proposed that the length of the stem elongation phase may, to an extent, affect grain number; thus it would be beneficial to identify genetic variation for the duration of this phase in elite germplasm. Variation in these developmental patterns was studied using 27 elite wheat lines in four experiments across three growing seasons. The results showed that the length of the stem elongation phase was (i) only slightly related to the period from seedling emergence to terminal spikelet, and (ii) more relevant than it for determining time to anthesis. Thus, phenological phases were largely independent and any particular time to anthesis may be reached with different combinations of component phases. Yield components were largely explained by fruiting efficiency of the elite lines used: the relationships were strongly positive and strongly negative with grain number and with grain weight, respectively. Although fruiting efficiency showed a positive trend with the duration of stem elongation that was not significant, a boundary function (which was highly significant) suggests that the length of this phase may impose an upper threshold for fruiting efficiency and grain number, and that maximum values of fruiting efficiency may require a relatively long stem elongation phase.

Highlights • The Rht8 introgression was associated with robust height reduction across different nitrogen and water treatments. • The Rht8 NIL had increased root-lodging resistance at agronomically-relevant nitrogen levels. • Decreased spike length associated with Rht8 resulted in a semi-compact spike. • Yield penalty associated with Rht8 was driven by reduced grain number and spike number. • The yield penalty was overcome at low nitrogen and in irrigated conditions., Reduced height 8 (Rht8) is the main alternative to the GA-insensitive Rht alleles in hot and dry environments where it reduces plant height without yield penalty. The potential of Rht8 in northern-European wheat breeding remains unclear, since the close linkage with the photoperiod-insensitive allele Ppd-D1a is unfavourable in the relatively cool summers. In the present study, two near-isogenic lines (NILs) contrasting for the Rht8/tall allele from Mara in a UK-adapted and photoperiod-sensitive wheat variety were evaluated in trials with varying nitrogen fertiliser (N) treatments and water regimes across sites in the UK and Spain. The Rht8 introgression was associated with a robust height reduction of 11% regardless of N treatment and water regime and the Rht8 NIL was more resistant to root-lodging at agronomically-relevant N levels than the tall NIL. In the UK with reduced solar radiation over the growing season than the site in Spain, the Rht8 NIL showed a 10% yield penalty at standard agronomic N levels due to concomitant reduction in grain number and spike number whereas grain weight and harvest index were not significantly different to the tall NIL. The yield penalty associated with the Rht8 introgression was overcome at low N and in irrigated conditions in the UK, and in the high-temperature site in Spain. Decreased spike length and constant spikelet number in the Rht8 NIL resulted in spike compaction of 15%, independent of N and water regime. The genetic interval of Rht8 overlaps with the compactum gene on 2DS, raising the possibility of the same causative gene. Further genetic dissection of these loci is required.

Yield differences among wheat cultivars and its responsiveness to resource availability are usually related to grain number per m 2 , and further increases in grain number are required, but now beyond differences in time to anthesis and plant height (that are mostly optimised in traditional growing regions). The objective of this paper was to determine and quantify differences in yield physiology between different modern well adapted durum wheats grown in contrasting conditions (in addition we also tested if a higher amount of resources due to removal of competition could affect yield through the same determinants to nitrogen (N) availability). Four experiments were carried out during two growing seasons (2008–09 and 2009–2010); all in all under eight different growing conditions (Gc1–Gc8). In the first season four different modern cultivars were used and in the second season we selected the two highest-yielding cultivars, which differed in their responses to N in terms of spike fertility. Yield was closely related to biomass and N uptake due to both growing conditions and genotypic effects in all experiments. The existence of variation among modern cultivars in these yield determinants is relevant as further increases in yield must be achieved beyond further increases in partitioning. Yield differences among cultivars were based on their differences in grains per m 2 or average grain weight depending on which cultivars were compared. Overall grain number was related to spike dry weight at anthesis, and no evidences were found for a direct involvement of N in grain number determination, beyond the general effect of N availability on spike dry matter. However, this overall effect was mainly driven by growing conditions whilst differences between genotypes seemed associated with either spike dry weight at anthesis or fruiting efficiency, which varied significantly between cultivars. Interestingly, grain weight was consistently higher in the cultivars with lower fruiting efficiency than in cultivars maximising the number of grains set per unit of spike dry weight at anthesis suggesting a constitutive component for the commonly reported negative relationship between grain weight and grain number. In conclusion, the main differences in yield between modern, well adapted wheats were related to their biomass and N uptake and to the sink-strength during post-anthesis, being this strength increased either by having more grains or grains of potentially greater size, depending on the specific cultivar and as a consequence of a trade-off between fruiting efficiency and grain size.

In order to identify genotypes of durum wheat expressing improved yield and determining whether selection for individual plant efficiency in producing grains would improve plant and crop yield, we firstly compared the performance of segregating mutants and control cultivars and then selected divergent lines for number of grains per unit stem length, representing extreme source-sink balances further characterising the offspring lines. Research included three field experiments with individual segregating plants within the semi-arid region of the Ebro Valley during the 2006/07, 07/08 and 08/09 growing seasons; a fourth experiment with segregating plants conducted under controlled conditions at Rothamsted in 2008/09 and finally a conventional field plot experiment with the most promising lines derived from the selection. In the initial characterization of the selected mutants, there was a clear response to the selection made in M2, and differences in plant yield were due to differences in grains per plant and biomass. There was a clear response to selection for sink-strength in improving the number of grains exhibited by the offspring lines through the generations (M2–M5) and the overall differences between the two divergent groups were consistently clear throughout the whole process. Response to selection was similarly expressed in field and in controlled conditions, implying high broad sense heritability of grains per plant (when there were no consistent differences in height and then it reflects grains per unit stem length). The best selections outyielded plants of the controls, without reducing the protein concentration of the grains. It seems clear that increasing the efficiency of conversion of growth during stem elongation into grains may be a relevant trait to further raise yield and that the trait used in the present study as a proxy of such efficiency (the number of grains per unit stem length) seemed appropriate as a selection criteria in early generations as it expressed similarly in isolated plants and in crop stands.

Yield components are relatively easy to measure and their interpretation is intuitive. However, strong environmental influences, genetic and physiological controls, and evolutionary constraints collectively lead to lack of independence among yield components that restrict their value in breeding and agronomic applications. Here, we first sketch a framework of plant responses to environmental factors to highlight the modulation of yield components by resources and their interplay with non-resource factors including developmental cues (e.g. maternal effects), extreme events (e.g. frost), predicting factors (e.g. photoperiod) and synchronising and integrating information (e.g. spectral composition of light). We suggest that, to the extent that non-resource cues allow plants to predict future availability of resources, simple resource-based models may be sufficient to capture the macroscopic responses of yield components to the environment. Next, we expand the original concept of hierarchy of plasticities between grain size (a relatively stable trait) and grain number (a plastic trait) to test the hypothesis of a broader hierarchy in the plasticities of yield components. Using published data for wheat, rice, barley and triticale, we verified that heritabilities capture the established hierarchy between plasticity of grain size and number. Median heritabilities of 0.31 for tiller number, 0.58 for inflorescence number, 0.59 for grains per inflorescence, and 0.79 for grain size supported the hierarchy of plasticities: tiller number > inflorescence number ≈ grains per inflorescence > seed size. The heritability of grain yield was consistently higher than the heritability for tillering, consistently lower than the heritability for grain size, and suggestively close to the heritability of inflorescence number and grains per inflorescence, the components of grain number per unit ground area. We conclude that understanding the environmental regulation of yield components in cereals would benefit from a dual focus on yield-related traits per se and their plasticity.

Stem lodging, a permanent displacement of cereal stems which may reduce wheat yields significantly, affects wheat crops in both developed and developing countries, mainly under high-yielding conditions. Although breeding has decreased these losses by reducing the likelihood of the plant lodging through reduced height (mainly by the introgression of dwarfing genes), lodging continues to affect wheat yields. We aimed to quantify how wheat breeding has modified the sensitivity of yield to lodging, identifying physiological causes for yield reductions. Two field experiments were carried out in Mediterranean Spain during the 2006/07 and 2007/08 growing seasons. The first season two genotypes (Aragon 03 and ID-2151) and two lodging treatments (control and lodged near anthesis) were analysed. The second season four genotypes (Aragon 03, Estrella, Anza and ID-2151) and three lodging treatments (control, lodged from either jointing to maturity or anthesis to maturity) were imposed. In addition, in the 2007/08 experiment the source–sink ratios of five representative spikes of each plot were modified through hand trimming all spikelets from one side of each spike soon after anthesis. Yield losses due to early lodging were reduced by wheat breeding: yield penalties in the oldest and newest genotypes were 61% and 43% (mean values of both lodging treatments), respectively while it was 57% in intermediate cultivars (mean value of both lodging treatments and cultivars). Lodging decreased both the number of grains per m 2 and the average grain weight (AGW). The decrease of the number of grains per m 2 was associated with reductions in the crop growth, while the AGW loss seemed related to both a reduction of the availability of assimilates to fill grains and a direct effect on grain size potential. Lodged plants exhibited stronger source-limitation during grain filling than those un-lodged, while the response of the grain weight to sink-strength reductions in lodged plants was very similar to that in the un-trimmed controls, indicating that grain filling in un-lodged and in strongly lodged canopies was similarly balanced in terms of source–sink ratios.

Under Mediterranean environments, farmers usually prefer to sow barley rather than wheat as it is generally believed that barley yields more under stressful conditions. As terminal stresses such as high temperature and water are common constraints in Mediterranean regions, higher grain weight stability may confer a clear advantage in order to maintain higher yields. The objective of the present study was to compare the stability in terms of grain weight and its components for barley, bread wheat, and durum wheat, exploring a wide range of nitrogen and water availabilities in experiments conducted in a Mediterranean region. Grain weight ranged from 23.8 to 47.7 mg grain−1, being higher for durum wheat than barley and bread wheat. Durum wheat presented higher variability both in maximum grain filling rate and duration of grain filling period than bread wheat or barley. The three species responded similarly in terms of grain nitrogen content to changes in the environmental conditions explored. It is concluded that in terms of grain weight barley is as stable as bread wheat. However, durum wheat presented a lower stability than barley and bread wheat.

36 Pag., 3 Tabl., 2 Fig. The definitive version is available at: http://www.sciencedirect.com/science/journal/03784290, Extending the phase of stem elongation (SE) has been proposed as a tool to further improve yield potential in small-grain cereals. The genetic control of pre-heading phases may also contribute to a better understanding of phenological traits conferring adaptability. Given that an optimized total time to heading is one of the most important traits in a breeding program, a prerequisite for lengthening SE would be that this and the previous phase (leaf and spikelet initiation, LS) should be under different genetic control. We studied the genetic control of these two pre-heading sub-phases (from sowing to the onset of stem elongation, LS, and from then to heading, SE) in terms of Quantitative Trait Loci (QTL) in a barley double-haploid population derived from the cross Henni × Meltan, both two-rowed spring North European barley cultivars. DH lines (118) and their parents were studied in four field trials in North-Eastern Spain. Genetic control of a number of traits related to leaf appearance and tillering dynamics, which could be important for an early crop canopy structure, were also studied. LS and SE are, at least partially, under a different genetic control in the Henni × Meltan population, mainly due to a QTL on chromosome 2HS. The QTLs responsible for a different control of LS and SE did not seem to correspond with any major gene reported in the literature. Moreover shortening LS, so as to lengthen SE without modifying heading date, would not necessarily imply a negative drawback on traits that could be important for early vigour, such as phyllochron and the onset of tillering., GB held a pre-doctoral FPU scholarship from the Spanish Ministry of Science and Innovation. This study was partially funded by the European Union – INCO - MED program (ICA3-CT2002-10026) Mapping Adaptation of Barley to Drought Environments (MABDE) and by the Spanish Ministry of Science and Innovation competitive grants AGL2006-07814, AGL2007-63625 and AGL2008-05541. The Centre UdL-IRTA forms part of the Centre CONSOLIDER on Agrigenomics.

The stem elongation phase seems critical in yield potential determination in barley ( Hordeum vulgare L.). Extending its duration, without modifying total time to anthesis, has been proposed as a promising breeding tool. A prerequisite for its use is that the duration of phases before and after jointing (that including leaf and spikelet initiation, LS, and that of stem elongation, SE) should be under different genetic control. In addition, little is known about the implications of changes in the duration of LS and SE upon other developmental traits which could affect other aspects of yield generation, such as phyllochron and tillering. Thus, the objectives of the present work were to study the genetic variability in LS and SE, in traits related to leaf appearance and tillering, as well as their relationships, in a double-haploid (DH) population derived from the cross Henni × Meltan. DH lines and both parents were studied in four field trials. Despite the similarity in development between parents, there was significant genetic variability in duration of both LS and SE ( i.e . considerable transgressive segregation was observed), with no major genetic correlations found between them. Although some significant genetic correlations were found between duration of phases and leaf appearance and tillering traits, it has been shown that modifying the duration of LS does not necessarily imply concomitant changes in traits that could be important for an early expansion of the crop canopy (i.e. phyllochron, onset and rate of tillering).

Past breeding achievements in grain yield were mainly related to increases in harvest index (HI) without major changes in biomass production. As modern cultivars have already high HI, future breeding to improve grain yield will necessarily focus on increased biomass. Improved biomass would depend on our capacity to improve the amount of photosynthetically active radiation intercepted by the crop (IPAR%) or the efficiency with which the canopy converts that radiation into new biomass (radiation use efficiency, RUE). Four field experiments with a set of wheat cultivars selected, bred and introduced in the Mediterranean area of Spain and that represent important steps in wheat breeding in Spain were conducted in order to identify whether and how wheat breeding in this area affected the amount of IPAR% and RUE both before and after anthesis. Although there was genotypic variability, cultivars did not show any consistent trend with the year of release of the cultivars for their biomass, pre and post-anthesis IPAR%, Crop growth rate (CGR) or RUE but, the post-anthesis CGR and RUE of the two oldest genotypes were lower than that of the other cultivars. As the oldest genotypes have lower number of grains per m2 than their modern counterparts, it is suggested that post-anthesis RUE in these cultivars was reduced by lack of sinks and therefore further increases in grains per unit area in modern cultivars could permit to improve biomass via increases in post-anthesis RUE.

Bread wheat has been frequently characterised as sink-limited during grain filling but the degree of sink-limitation could vary with the contribution of breeding in increasing the number of grains per unit land area. This hypothetical change in the level of sink-limitation due to breeding has been insufficiently documented. Two source–sink manipulation experiments under field conditions with three released cultivars and an advanced breeding line representing important steps in wheat breeding in the Mediterranean area of Spain were conducted in order to quantify whether genetic improvement of grain yield in Mediterranean wheat modified the source–sink balance during grain filling, and how it affected grain weight and post-anthesis photosynthetically active radiation intercepted by the crop (IPAR) and radiation use efficiency (RUE). Average grain weight of control and trimmed spikes during grain filling was not significantly affected by halving the number of growing grains in the two oldest cultivars, but it was significantly increased in the most modern line, and had an intermediate response in the intermediate cultivar. In those cases in which a certain degree of source-limitation during grain growth occurred the magnitude of the response reflected a co-rather than a source-limitation. Considering grains from different positions within the spikes the smaller (distal) grains responded more markedly than the larger (proximal) grains. No differences in post-anthesis IPAR were found between the trimmed and control sub-plots for any of the genotypes. However, trimming the spikes reduced post-anthesis RUE, a fact corroborated by a simultaneous reduction in leaf net photosynthetic rate at noon. It seems that bread wheat breeding has tended to reduce the strength of the sink-limitation during post-anthesis even under Mediterranean conditions, and consequently the most modern line presented a sort of co-limitation.

To publish papers that have impact is indispensable for good scientists. It is frequently accepted that the impact factor (IF) of a journal reflects the likelihood of a paper to have impact. Many scientists may then attempt publishing their papers in journals with the highest possible IF. This is relevant for crop scientists as in many cases our hypotheses may be relevant to agronomy/agriculture but the product of our work may be published in the more basic plant science journals, which in general have higher IF than agronomic journals. To test whether agronomic journals may be better vehicles than plant science journals for crop physiological work, I did an analysis of my own publications (only “standard” papers, disregarding reviews) relating their annual citation rates against the IF of the journal in which they were published. The relationship was virtually inexistent, and my most frequently quoted papers were published in agronomic rather than in plant sciences journals. Conversely, restricting the analysis to the papers published within agronomic journals there was a relationship between the record of citations of the papers and the IF of the journals, particularly if the analysis is restricted to the most quoted papers in each journal. I then analyzed the cases of three other well-recognized crop scientists, to check whether the findings with my results were exceptional, and in the three cases results were remarkably similar. Therefore, it seems that the journal IF might be a valuable criterion for choosing the best journal in which to publish our best contributions only when comparing within agronomic journals, but not within combinations of agronomy and plant science journals. This should be also taken into account in evaluation processes when comparing scientists working in related but not identical fields.

The critical period for grain yield determination has not been determined for triticale. We aimed to identify it, determining the relative importance of both the major yield components and the dry matter acquisition by the spikes at anthesis. A field experiment was carried out with two triticales, differing in tillering capacity, subjected to shading treatments at five different timings from early tillering to maturity. Results showed that reductions in grain yield were more significant when shading was imposed during 3 weeks before and 1 week after anthesis. Reductions in grain yield by shading treatments were associated with lower number of grains per m2 more than with changes in the average grain weight. Reductions in grains per m2 were due to reductions in the number of fertile florets per spike, affecting grains per spike. The assimilate acquisition by the spikes during the critical period was a key determinant of floret survival. Grain number per m2 was related with photothermal quotient during 30 days before anthesis and spike dry weight at anthesis, though the goodness of the prediction compared with wheat, was lowered by poorer grain setting percentage.

Three field experiments comparing bread wheats representing different eras of genetic improvement in the Mediterranean area of Spain (one landrace, seven cultivars released since 1950 and two advanced breeding lines) were carried out at the province of Lleida (Catalonia, north-eastern Spain). Experiments were conducted during the 2004/05, 2005/06 and 2006/07 growing seasons at Gimenells and during 2005/06 at Foradada in order to quantify gains in yield, and in its physiological determinants, during much of the last century. At Gimenells, the first two seasons were conducted under relatively high-yielding backgrounds (Gimenells, irrigated) and in the last season under moderately stressed conditions (Gimenells, rainfed), while at Foradada the experiment was conducted under a severely stressed low-yielding environment. For almost all the traits analyzed in this paper, there were only significant differences when cultivars were grown under relatively high-yielding environments. In this conditions, grain yield increased with the year of release of the cultivars from 1940 to the 1970s, with no clear further improvements thereafter. On the other hand, total biomass did not show any association with the year of release of the cultivars. Results showed that, even when breeding was performed under Mediterranean conditions, harvest index was the main attribute responsible for yield improvements. Stem height was linearly and negatively related with harvest index, implying a marked height reduction until the 1970s, with no clear further reductions thereafter. In general yield was lineally and positively associated with number of grains per m2, while average grain weight did not exhibit any clear trend with the year of release of the cultivars. The increase in number of grains was more associated with that in grains per spike than with differences in spikes per m2. Finally, the increase in number of grains per m2 was associated with both number of grains per unit of spike dry weight at anthesis, or “fruiting efficiency”, and spike dry weight at anthesis.

A better understanding of responsiveness of time to anthesis to water stress in triticale and wheat might help to explain their differences in performance under stress and to identify which attributes or components of time to anthesis are most affected. Three experiments were carried out during the 2004 and 2005 growing seasons to (i) evaluate genotypic variability in phenology responsiveness to drought in triticale and to (ii) explore the variation in the duration of pre-anthesis phases occurring before and after the onset of stem elongation as well as their responsiveness to water stress. Important variation was found among triticale cultivars on time to anthesis, allowing classification of cultivars into long, intermediate and short cycle. Differences in the duration of the stem elongation phase (i.e. from terminal spikelet initiation (TS) to anthesis (ANT)) were as high as 200 °Cd for cultivars with similar time to flowering. Although water stress reduced the time to anthesis (in average by ca. 125 °Cd), cultivars showed a differential responsiveness to drought in phases occurring before or after TS. Some cultivars that were highly sensitive to drought in the emergence (Eme)–TS phase were insensitive in the phase TS–ANT and vice-versa. Water shortage tended to reduce the grain filling period and was most evident when a Mediterranean environment treatment was simulated.

Spike growth during the stem elongation phase is critical for the establishment of the number of fertile florets (FF). A longer period of spike growth would lead to an increased number of FF and grains in wheat. The aim of this study was: (i) to analyze in three different commercial cultivars the impact of a photoperiod extension (NP + 10) during the stem elongation phase on the number of FF and (ii) to determine whether the photoperiod extension effect might be reversed by increasing the availability of assimilates by a direct modification of the source/sink ratio at booting. The number of FF per spike was reduced by extended photoperiod in the experiment sown in an optimum date, though no differences were observed when sown much later, probably due to the poor environmental conditions of a spring sowing. The detrimental effect of lengthening the photoperiod on the number of FF per spike was reversed by trimming the spike at booting in two of the three cultivars analyzed. This reversion, together with the effect of photoperiod extension on FF associated with similar effects on spike growth, may be considered as a confirmation that, at least in part, the detrimental effects of photoperiod extension on FF would be mediated by changes in assimilate supply to the developing florets. No differences were observed between photoperiod treatments on the onset of rapid spike growth; however, extended photoperiod reduced the duration of spike growth phase without differences in the maximum rate of spike growth in two of the three cultivars. Although a positive association was found between the number of FF and spike dry weight at anthesis, differences among cultivars were observed in this relationship, suggesting different carbohydrates partitioning (i.e., FF per unit of spike weight) among them.

Nitrogen (N) uptake at heading and at maturity was evaluated under four N treatments (20, 50, 110 and 160 kgN ha−1 at sowing) in four malting barley cultivars released to the market from 1944 to 1998 in Argentina. Grain N concentration was slightly lower in the newest cultivar than in the oldest one. Grain N yield showed cultivar × N interaction: modern cultivars tended to increase their grain N yield in response to N fertilization more than old cultivars. Grain N yield was correlated to grain yield as well as to total above-ground biomass. Nitrogen fertilization affected total N in above-ground biomass at maturity, but no differences among cultivars were found. Nitrogen harvest index of the most modern cultivar (0.76) was higher than that of the oldest cultivar (0.67). Modern cultivars tended to have a higher N content in ears at heading than old cultivars, and the magnitude of the differences increased with N availability. Physiological nitrogen efficiency for grain yield was significantly higher in the newest than in the oldest cultivar (57 and 43 g grain g N − 1 , respectively).

Field experiments with soybean demonstrated that the extension of photoperiod after flowering increases both the duration of the post-flowering phase and the production of seeds. These results suggest that cultivars with increased duration of the post-flowering phase could be selected to improve soybean yields. The aims of this paper were to: (a) evaluate the ability of the CROPGRO-soybean model to reproduce the experimental relationships between seed number and duration of the critical phase between first and last pod, and (b) assess the putative benefits and trade-offs of variable durations of the critical phase in a south–north transect in the Pampas of Argentina. We varied the genetic coefficients accounting for photothermal requirements and photoperiod sensitivity of soybean A5409-RG (maturity group V), reducing pre-flowering phase while maintaining time to maturity. The model produced sound estimates of crop phenology and reproduced the positive relationship between seed production and the duration of the critical period between first and last pod found in field studies. Linking the model with long-term climate records for six locations between 31 and 37°S, indicated that substantial yield gains could result from increasing the relative duration of post-flowering phases. A hypothetical cultivar with flowering time typical of cultivars of maturity group III and total cycle typical of cultivars of group V, outyielded the traditional cultivar of group V in most cases. For well-watered crops, extension of the critical period had a positive effect in yield in all seasons and locations, whereas for rainfed crops, the extension of the critical period increased yield in 95% of the simulated scenarios. The magnitude of the effect was strongly asymmetric: yield gains were up to 1 Mg ha −1 whereas losses were below 0.2 Mg ha −1 . The modelled results in this paper indicate that shortening pre-flowering period without changing the duration of the whole cycle should increase yields in a broad range of latitudes and environmental conditions.

In temperate cereals are commonly accepted that determination of grain number (GN) and grain weight (GW) scarcely overlap during the crop cycle. However, the assumption that GW is determined exclusively after anthesis needs to be critically reviewed in the light of reports published over the few years where temperature treatments imposed before anthesis decreased GW of bread wheat. Although these evidences suggest that both GW and GN could be affected by environmental conditions before anthesis little is known about the effect of pre-anthesis temperature on these two main yield components in wheat, barley and triticale at field conditions. In addition, the effect of temperature on GW and GN at different stages prior to anthesis has been scarcely evaluated. The objectives of the current study were: (i) to evaluate the overall response, and specific differences, of GN and GW to pre-anthesis temperature, and (ii) to study the effect of different timings of high temperature at pre-anthesis on GN and GW in wheat, barley and triticale. Three fully irrigated field experiments were carried out in three successive seasons. At each season, a wheat, barley and triticale high yielding cultivar was evaluated at three temperature regimes: control, and two timings of heating before anthesis. During the first and second seasons, the timings of heating were booting-anthesis and heading-anthesis. In the thirst season, the timings were beginning of stem elongation-booting and booting-anthesis. Plots were arranged in a split-plot design with three replicates, where the main plot was assigned to thermal regime and the sub-plots to crop species. To apply heat, transparent chambers equipped with thermostatically controlled electric heaters were used. The thermal regime was controlled by sensors connected to a temperature regulator and recorded using data loggers. Temperature within the chambers was stable across developmental stages, crops, and seasons; it averaged 5.5 °C higher than air temperature. Thermal treatments consistently reduced grain yield (p < 0.05), the magnitude of the effect ranged between 5 and 52%. The highest effect was found when temperature increased during stem elongation (yield decrease: 46%), lowest when treatments were imposed during heading-anthesis (15%) and intermediate for treatments imposed during booting-anthesis (27%). Most effects of thermal treatments on yield were due to parallel effects on GN. However, thermal treatments significantly (p < 0.05) decreased GW during the three seasons. The most effecting treatment on GW was when the crops were heated during the B-A period, i.e. GW decreased up to 23%.

Average grain weight is a major yield component contributing to its variation, especially in Mediterranean regions where grain weight is frequently exposed to terminal stresses affecting grain growth. Most of the literature agrees that wheat grain growth is hardly limited by the source. However, no source–sink ratios studies seem to have been conducted in the Mediterranean region to determine to what degree wheat grain growth is actually limited by the source in these particular regions. We conducted two field experiments in Catalonia (north-eastern Spain), where an old cultivar (Anza) and a more recently released one (Soissons) were sown in a range of different nitrogen and water availabilities and sowing dates. This was to analyse the degree of source limitation for grain growth. Sink size was modified by removing half of the spikelets c . 10 days after anthesis, virtually doubling the availability of assimilates per grain effectively growing. Trimming the spikes did not produce significant changes in grain growth rate or duration of grain filling. Consequently, grain weight did not respond noticeably to the reduction in sink demand and any eventual response has been far from representing a strong competition among grains during grain filling.

This study explored whether the average grain weight of wheat tends to be reduced when grain number is increased due to either competition or, to a consistent increase in the relative proportion of grains of smaller weight potential. Three field experiments considering environmental, genetic and environmental × genetic effects on yield and its main components were analysed during the 2003/2004 growing season in two different locations within the Mediterranean area of Catalonia, Spain. The relationship between grain weight and number of grains per unit land area was analysed for both the average of all grains (AGW) and for grains in specific positions of the main-shoot spikes: proximal (CPg) and distal (CDg) grains of central spikelets, and proximal grains of the near apical (APg) and near basal (BPg) spikelets. The proportional contribution of grains per spike for the different grain positions and the relative contribution of spikes per m 2 made by the main shoot or tillers were also examined. In the three experiments, AGW was reduced when grain number was increased due to genetic and/or environmental factors. However, the slopes of the straight-lines that represented the negative relationship between grain number and grain weight were lower (less negative) and less significant for CPg ( b = −0.20, P > 0.20), CDg ( b = 0.06, P > 0.20) and BPg ( b = −0.21, P > 0.20) than for AGW ( b = −0.40, P

Lesquerella fendleri and Lesquerella mendocina are two species with potential for domestication as crops for semiarid regions. Understanding the environmental influences on development is a critical step for the introduction of a wild species into cultivation. Under controlled conditions these species responded differently to temperature: L. fendleri phenological approach toward flowering responded linearly to temperature, whereas initiation of flowering in L. mendocina was relatively insensitive to temperature. L. fendleri exhibited a quantitative response to supra-optimal temperatures (with rate of development reduced with further increases in temperature) whereas L. mendocina showed a qualitative response, no flower development at supra-optimal temperatures. In this work undertaken in the field we studied phenological development in L. fendleri and L. mendocina as a function of planting date, quantified the time required to reach particular phenological stages under the various thermal environments, and compared these results with those previously obtained with controlled conditions. We also studied the influence of photoperiod on plant phenology in field situations and through experiments done under controlled conditions. Development rate for both species varied with sowing date with plant cycles shorter in spring sown plants, even if measured in thermal time. L. mendocina plants sown in late spring displayed a biennial cycle. These results are consistent with those obtained under controlled conditions. However, cycle shortening in thermal time with delays in sowing date suggested that factors other than temperature also influenced phenology of these two species. Further studies under controlled conditions showed that phenological development of L. fendleri plants was also altered by photoperiod, with plants displaying a typical long-day response. At the highest temperatures used in these studies L. mendocina plants did not respond to photoperiod. The possibility that incident radiation is involved in L. mendocina response to sowing date is discussed.

A detailed and accurate phenotyping of mapping populations is an important “bottleneck” for the understanding of the phenotype–genotype relationships. Grain number per unit area (GN), the main wheat yield component, can be analyzed through physiological components as spike dry weight at flowering, determined by crop growth rate and biomass partitioning to spike during stem elongation phase, and fruiting efficiency. The phenotypic variability of these physiological traits responsible for variation in GN and the relationships among them was analyzed in an elite wheat doubled-haploid (DH) population grown in two different environments. Positive transgressive segregation (i.e. DH lines that exceed parental phenotypic values) was observed for all GN determining traits, suggesting that increases in GN could be achieved through their improvement. However, focusing on top DH lines, which represent a feasible genetic improvement, fruiting efficiency was the most relevant physiological trait for consistently improving GN, and thereby grain yield, in both environments. Fil: García, Guillermo Ariel. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Serrago, Roman Augusto. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: González, Fernanda Gabriela. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Buenos Aires Norte. Estación Experimental Agropecuaria Pergamino; Argentina Fil: Slafer, Gustavo Ariel. Institució Catalana de Recerca i Estudis Avancats; España. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina Fil: Reynolds, Matthew P.. Centro Internacional de Mejoramiento de Maíz y Trigo; México Fil: Miralles, Daniel Julio. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina

Pod and seed number are the most important yield components in soybean ( Glycine max (L.) Merrill) crops. Crop growth rate during post-flowering and the duration of the period when pod and seeds are formed explain much of the variation of soybean yields across genotypes and environments. Exposing post-flowering stages to long photoperiod has been found to extend the period R3–R6 and to increase seed number in soybeans grown under field conditions. In this paper, post-flowering development and yield components responses to photoperiod were quantitatively analysed and the degree of coupling between both responses was investigated. Indeterminate soybean cultivars, A-5409 (maturity group V) and Dekalb CX-458 (maturity group IV), were grown under field conditions and exposed to natural photoperiod from sowing to the beginning pod stage (R3). From then onwards, they were either kept under natural daylength or exposed to four photoperiod regimes that were artificially extended in relation to natural daylength by 1.5, 3.0, 4.5 or 6.0 h. All the extended photoperiod regimes increased the duration of R3–R6 period. Both cultivars showed a quantitative type of response through the whole range of explored photoperiods, though A-5409 exhibited a stronger sensitivity. Development responses during the R6–R8 phase were less noticeable and more variable. Exposing plants to extended photoperiod increased the number of nodes per plant and improved node fertility, thus increasing the number of pods and seeds produced per unit area. Average seed weight tended to be reduced in plants exposed to extended photoperiod and the magnitude of these effects depended both on cultivars and treatments. However, seed size was reduced in ca. 20% while seed number was increased by more than 75% due to the treatments. These results strengthen the hypothesis that manipulating photoperiod sensitivity during post-flowering in indeterminate soybean may actually be an avenue to increase seed number and yield.