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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).

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) .

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.

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.

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.

The understanding of crop phenology is critical for both improvement of yield potential and adaptation to stress. Matching ‘critical’ phases (when the most important yield components are determined) and best environmental conditions is crucial to maximize yield. The processes regulating crop development are complex and are strongly influenced by genetic and environmental factors. In this chapter, we describe the main developmental stages, delimiting major phenological phases of wheat and soybean, and the relationships between crop phenology, adaptation and yield determination. Analysis of environmental control of development is restricted to the main drivers: temperature, including temperature per se and vernalization (for wheat), and photoperiod. The effects of major genes are outlined: Vrn, Ppd and Eps in wheat and Dt (growth habit) and genes of the series E and J in soybean. Finally, we integrate the environmental and genetic effects on phenology into the determination of crop adaptation and yield potential.

In Mediterranean environments with scarce and variable rainfall and unfertile soils, rain-fed cereals are exposed to different severity of water stress, which often combines with heat stress and nitrogen deficit. In this chapter, we revise three elements that are commonly accepted for cereal production in Mediterranean type-environments. First, the notion that annual crops in these environments are exposed to ‘terminal drought’. Using physiological and environmental analysis, we showed that this label overemphasizes grain filling, and neglects the most critical pre-flowering period; this has implications for both crop management and breeding. Second, the notion that barley performs better than wheat at the lowest end of the rainfall range. We emphasize the lack of solid scientific evidence supporting this view, and briefly consider the implications for patterns of land allocation. Third, the notion that nitrogen fertilization may reduce yield in low rainfall sites and seasons. We argue that, again in this case, the scientific evidence does not fully support this viewpoint, and discuss the opportunities to improve yield and water-use efficiency by better management of nitrogen fertilization.

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

A better understanding of the effects of different source?sink ratio during the grain filling period on grain growth may be relevant in order to further increase cereal grain yield. The main objective of the present work was to determine the effect of different manipulations of the source?sink ratios in wheat and barley grown at four different environmental conditions on responsiveness of sinks (grain growth and yield) and sources (spike photosynthesis and water soluble carbohydrates in the stems). Four treatments were imposed 7 days after anthesis in two contrasting locations with low- and high-inputs conditions in wheat (cv. Soissons) and barley (cv. Sunrise): they were a control, a treatment removing all the spikelets from the upper half of the spikes (TS), and shadings decreasing incident radiation by 75% on the whole canopy (SW) or only on the leaves (having the top area of the meshes individual holes for each spike to be exposed to solar radiation, SL). As expected grain yield was closely related to grain number per m2. Average grain weight was reduced by shading treatments far more markedly in Sw than in SL. Interestingly, significant amounts of water soluble carbohydrates in the stems remained at maturity in SL and Sw treatments and spike photosynthesis in SL was consistently higher than in the unshaded controls in both species. These results may be an indication that wheat and barley are not source-limited during grain filling and that only when subjected to an extremely severe stress, grain size would be reduced due to lack of enough assimilates available to fill them. Fil: Serrago, Roman Augusto. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura; Argentina Fil: Alzueta, Ignacio. Universidad de Buenos Aires. Facultad de Agronomía. Departamento de Producción Vegetal. Cátedra de Cerealicultura; Argentina Fil: Savin, Roxana. Universitat de Lleida. Departament de Producció Vegetal i Ciència Forestal; España. Universitat de Lleida. Agrotecnio; España Fil: Slafer, Gustavo Ariel. Universitat de Lleida. Departament de Producció Vegetal i Ciència Forestal; España. 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

Publisher Summary This chapter discusses the particularities of development of wheat and soybean to highlight the importance of identifying the genetic and environmental controls of the phenological pattern. This knowledge is a prerequisite to understand, predict, and manipulate the association between crop cycles, the resources, and the environmental constraints to favor the coincidence of the critical period with the most favorable conditions. Although the cycle to match crops and environmental factors has been determined in most production systems, further improvement is feasible by manipulation of critical periods. The critical period may occur before (e.g., in wheat) or after (e.g., in soybean) flowering, but it is clear that in both species, in spite of their large morphological and physiological differences, the growth during this period defines crop yield in most environments. Improving the knowledge of genetic and environmental drivers of the expression of the genes that control flowering time should improve the precision in positioning the critical period, when the highest level of resources is expected and stresses are less likely.

In Southern Mediterranean regions, farmers usually grow barley monocultures in drought-prone areas while wheat is grown in better-watered regions. This is based on an assumption of better performance of barley than wheat in terms of productivity and resource capture. Information about the comparative performance of these species under scarce resource availability is rudimentary. The aim of the present work was to compare the performance of bread and durum wheat and barley in terms of water use, nitrogen uptake, water use efficiency and nitrogen use efficiency under Mediterranean environments. Four experiments were conducted on farmers’ fields at Agramunt, in the province of Lleida (Catalonia, north-eastern Spain) during 2004/05, 2005/06 and 2006/07. Treatments consisted of barley, bread wheat and durum wheat grown under different combinations of water (rainfed or irrigated) and nitrogen availabilities. Water and nitrogen availabilities resulted in a wide range of water use efficiency (from 6.3 to 23 kggrain ha−1 mm−1) and nitrogen use efficiency (from 2.9 to 31.8 kggrain kgavailable N−1 in soil). Nitrogen uptake was closely and positively related to grain yield and total biomass at maturity for the three species. Nitrogen fertilization modified yield of the three species, mainly through changes in water use efficiency (R2 = 0.75). Differences between the species were low but existed in relative terms; they were not always in favor of barley. In fact, barley did not consistently outperform bread or durum wheat in terms of resource use or resource use efficiency under poor-yielding conditions. The relative differences between wheat and barley in terms of grain yield (analyzed as wheat yield/barley yield) were explained by the relative differences between them in terms of nitrogen use efficiency and water use efficiency (analyzed as wheat resource use efficiency/barley resource use efficiency).

Physiological bases of Mediterranean wheat yield improvements have been less explored than in other regions, particularly so during the period following the Green Revolution. Due to the common terminal stress of Mediterranean regions, it could be hypothesized that contemporary cultivars would exhibit improved average grain weight and yield stability compared with a traditional cultivar. Despite the lack of clear evidence in the literature, farmers in Mediterranean regions may prefer growing traditional cultivars over modern ones under the poorest environments and allocate the best lands to more contemporary cultivars. For instance, in our region this has been the case with Anza (a traditional, but semi-dwarf, cultivar) and Soissons (more modern and with a putatively higher potential yield than Anza). We analysed in detail for these two cultivars yield determination and its responsiveness to a very wide range of growing conditions (across six experiments, yield ranging from less than 1 to more than 8 Mg ha−1). Both cultivars responded to the improved yielding conditions noticeably but Soissons did so more strongly, revealing a clearly higher yield potential. The higher yield stability of the traditional cultivar did not imply consistently better performance under the lowest yielding conditions. The main component explaining the yield advantage of the contemporary cultivar was the number of grains per m2 though the traditional cultivar exhibited a sort of partial compensation due to possessing a consistently higher average grain weight. Across all conditions, the overall fruiting efficiency of the contemporary cultivar (111.4 ± 7.5 grains gspike−1) was significantly higher than that of the traditional one (88.3 ± 3.9 grains gspike−1), which was the basis of a higher spike fertility in the contemporary than in the traditional cultivar, observed in most of the spikelets consistently across environmental conditions with a clear trend to increase the number of spikelets exhibiting a significant difference in fertility between the two cultivars and the overall difference in spike fertility between them. Analysing in detail the weight of each individual grain in the last two experiments, we showed that the improved spike fertility due to a higher fruiting efficiency did not bring about reductions in grain weight potential and the dominant cause of the partial compensation was the increase in the proportion of grains in distal positions (constitutively smaller than proximal grains) in the contemporary cultivar.

It has been broadly accepted that barley performs better than wheat, particularly under severe stress conditions of the Mediterranean basin. However, this assumption is not based on rigorous evidence. This study compared grain yield, grain yield components and biomass accumulation performances of barley, bread wheat and durum wheat under a wide range of water and nitrogen (N) availabilities on a location in the Mediterranean basin during four consecutive growing seasons (2003/04, 2004/05, 2005/06 and 2006/07). The different experimental treatments enabled the comparison of the three species under wide grain yield ranges. Yields under these treatments ranged from 0.9 to 10.2 Mg ha −1 . Grain number per unit area better explained grain yield differences in the three species across experimental years and treatments. Photothermal quotient (PTQ) during the critical growth period (from 20 days pre- to 10 days post-flowering) explained the differences in grain number for all three species ( r 2 = 0.64; r 2 = 0.84 and r 2 = 0.56 for barley, bread wheat and durum wheat, respectively). Biomass differences between wheat and barley were generated mainly by their differences in post-flowering growth. Post-flowering growth was positively and directly related to the number of grains set per m 2 . It was confirmed that it is not universally true that barley yields better than wheat under Mediterranean environments.