Search

The description of long photoperiod sensitivity in wheat and barley is a cause of confusion for researchers working with these crops, usually accustomed to free exchange of physiological and genetic knowledge of such similar crops. Indeed, wheat and barley scientists customarily quote studies of either crop species when researching one of them. Among their numerous similarities, the main gene controlling the long photoperiod sensitivity is the same in both crops (PPD1; PPD-H1 in barley and PPD-D1 in hexaploid wheat). However, the photoperiod responses are different: (i) the main dominant allele inducing shorter time to anthesis is the insensitive allele in wheat (Ppd-D1a) but the sensitive allele in barley (Ppd-H1) (i.e. sensitivity to photoperiod produces opposite effects on time to heading in wheat and barley); (ii) the main ‘insensitive’ allele in wheat, Ppd-D1a, does confer insensitivity, whilst that of barley reduces the sensitivity but still responds to photoperiod. The different behaviour of PPD1 genes in wheat and barley is put in a common framework based on the similarities and differences of the molecular bases of their mutations, which include polymorphism at gene expression levels, copy number variation, and sequence of coding regions. This common perspective sheds light on a source of confusion for cereal researchers, and prompts us to recommend accounting for the photoperiod sensitivity status of the plant materials when conducting research on genetic control of phenology. Finally, we provide advice to facilitate the management of natural PPD1 diversity in breeding programmes and suggest targets for further modification through gene editing, based on mutual knowledge on the two crops.

As crop yields are pushed closer to biophysical limits, achieving yield gains becomes increasingly challenging and will require more insight into deterministic pathways to yields. Here, we propose a wiring diagram as a platform to illustrate the interrelationships of the physiological traits that impact wheat yield potential and to serve as a decision support tool for crop scientists. The wiring diagram is based on the premise that crop yield is a function of photosynthesis (source), the investment of assimilates into reproductive organs (sinks) and the underlying processes that enable expression of both. By illustrating these linkages as coded wires, the wiring diagram can show connections among traits that may not have been apparent, and can inform new research hypotheses and guide crosses designed to accumulate beneficial traits and alleles in breeding. The wiring diagram can also serve to create an ever-richer common point of reference for refining crop models in the future.

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.

Identifying traits for improving sink-strength is a bottleneck to increasing wheat yield. The interacting processes determining sink-strength and yield potential are reviewed and visualized in a set of 'Wiring Diagrams', covering critical phases of development (and summarizing known underlying genetics). Using this framework, we reviewed and assembled the main traits determining sink-strength and identified research gaps and potential hypotheses to be tested for achieving gains in sink-strength. In pre-anthesis grain number could be increased through: (i) enhanced spike growth associated with optimized floret development and/or a reduction in specific stem-internode lengths and (ii) improved fruiting efficiency through accelerated rate of floret development, improved partitioning between spike or optimized spike cytokinin levels. In post-anthesis grain sink-strength could be augmented through manipulation of grain size potential via ovary size and/or endosperm cell division and expansion. Prospects for improving spike vascular architecture to support all rapidly growing florets, enabling the improved flow of assimilate, are also discussed. Finally, we considered the prospects for enhancing grain weight realization in relation to genetic variation in stay-green traits as well as stem carbohydrate remobilization. The Wiring Diagrams provide a potential workspace for breeders and crop scientists to achieve yield gains in wheat and other field crops. Research of the authors on physiology and genetics of wheat yield potential has been funded by many different sources over the years. Most recent grants include the International Wheat Yield Partnership (IWYP) projects funded by the Biotechnology and Biological Research Council of the UK (e.g., IWYP48 BB/N021061/1, IWYP64 BB/ N020871/2, IWYP163 BB/S005072/1, and IWYP25FP), as well as projects funded by other donors (State Research Agency of Spain: AGL2015-69595-R and RTI2018-096213-B-100).

Special Issue: Feeding the World: The Future of Plant Breeding Asymmetry of investment in crop research leads to knowledge gaps and lostopportunities to accelerate genetic gain through identifying new sources andcombinations of traits and alleles. On the basis of consultation with scientistsfrom most major seed companies, we identified several research areas withthree common features: (i) relatively underrepresented in the literature; (ii) highprobability of boosting productivity in a wide range of crops and environments;and (iii) could be researched in‘precompetitive’space, leveraging previousknowledge, and thereby improving models that guide crop breeding and man-agement decisions. Areas identified included research into hormones, recombi-nation, respiration, roots, and source–sink, which, along with new opportunitiesin phenomics, genomics, and bioinformatics, make it more feasible to explorecrop genetic resources and improve breeding strategies. The authors acknowledge the following scientists for helping to achieve a consensus on research bottlenecks with significant implications for crop improvement: Yann Manes (Syngenta), Jeremy Derory (Limagrain), Bruno Marty (BASF), and Hans Braun (recent director of the Global Wheat Program, CIMMYT). We acknowledge Renee Lafitte (Bill and Melinda Gates Foundation) for helpful feedback on the manuscript and Fatima Escalante for valuable assistance in coordinating edits to the manuscript and its formatting. M.R. acknowledges the International Wheat Yield Partnership (https://iwyp.org/) for establishing a precedent of integrating different research strands in wheat for targeted prebreeding and the Foundation for Food and Agricultural Research (https://foundationfar.org/) for supporting a translational research and prebreeding pipeline at CIMMYT to identify and stack climate resilience traits in wheat.

Awnless and awned wheat is found across the globe. Archeological and historical records show that the wheat spike was predominantly awned across the many millennia following domestication. Thus, ancient farmers did not select against awns at least until the last millennium. Here, we describe the evolution and domestication of wheat awns, quantifying their role in spike photosynthesis and yield under contrasting environments. Awns increase grain weight directly (increasing the size of all grains) or indirectly (increasing the failure of distal grains), but not as a consequence of additional spike photosynthesis. However, a trade-off is produced through decreasing grain number. Thus, favorable effects of awns on yield are not consistently found across environments. The authors’ research on wheat yield physiology has been funded by many different sources and we are grateful to all of them. The latest projects on wheat yield physiology were RTI2018-096213-B-100 and PID2019-106650RB-C21 (both funded by the State Agency for Research of Spain). J.L.A. also acknowledges the support of ICREA Acadèmia Generalitat de Catalunya, Spain.

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) .

This chapter provides an analysis of the processes determining the yield potential of wheat crops. The structure and function of the wheat crop will be presented and the influence of the environment and genetics on crop growth and development will be examined. Plant breeding strategies for raising yield potential will be described, with particular emphasis on factors controlling photosynthetic capacity and grain sink strength.

Source traits are currently of great interest for the enhancement of yield potential; for example, much effort is being expended to find ways of modifying photosynthesis. However, photosynthesis is but one component of crop regulation, so sink activities and the coordination of diverse processes throughout the crop must be considered in an integrated, systems approach. A set of ‘wiring diagrams’ has been devised as a visual tool to integrate the interactions of component processes at different stages of wheat development. They enable the roles of chloroplast, leaf, and whole-canopy processes to be seen in the context of sink development and crop growth as a whole. In this review, we dissect source traits both anatomically (foliar and non-foliar) and temporally (pre- and post-anthesis), and consider the evidence for their regulation at local and whole-plant/crop levels. We consider how the formation of a canopy creates challenges (self-occlusion) and opportunities (dynamic photosynthesis) for components of photosynthesis. Lastly, we discuss the regulation of source activity by feedback regulation. The review is written in the framework of the wiring diagrams which, as integrated descriptors of traits underpinning grain yield, are designed to provide a potential workspace for breeders and other crop scientists that, along with high-throughput and precision phenotyping data, genetics, and bioinformatics, will help build future dynamic models of trait and gene interactions to achieve yield gains in wheat and other field crops. Research of the authors on physiology and genetics of wheat yield potential has been funded by many different sources over the years. Recent grants include the International Wheat Yield Partnership (IWYP) projects funded by the Biotechnology and Biological Research Council of the UK [BB/N021061/1, BB/ N020871/2, BB/S005072/1] (IWYP48, IWYP64, IWYP163 and IWYP25FP, respectively), as well as projects funded by other donors (State Research Agency of Spain: AGL2015-69595-R and RTI2018-096213-B-100).

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.

In this chapter, the authors analyze the changes produced by breeders in wheat yield during just the last century, when scientific genetic improvement took place after the rediscovery of Mendel's laws. The main objectives in writing the chapter were to identify the contributions of breeding to the improvement in wheat grain yield and the major physiological attributes that were modified with grain yield and grain protein as a result of the genetic improvement of wheat grain yield potential. In this chapter, the authors have outlined most of these changes with the objective of summarizing those traits that could be useful in future breeding. Austin et al. and Slafer et al. showed that genetic improvement in wheat grain yield has led to decreases in grain protein percentage. Future breeding should exploit the genetic variability in physiological attributes to increase total biomass of the crop, maintaining present levels of partitioning between vegetative and reproductive tissues.

Increases in grain yield in bread wheat from breeding and associated physiological changes, Gustavo A. Slafer et al productivity of oats - genetic gains and associated physiological changes, Pirjo Peltonen-Sainio progress in barley breeding, Luigi Cattivelli, et al physiological basis of the genetic improvement of corn, Matthijs Tollenaar et al genetic improvement in soybean - physiological attributes, James R. Frederick and John D. Hesketh physiological characteristics related to yield improvement in sunflower (Helianthus annuus L.), Victor O. Sadras and Francisco J. Villolobos genetic contributions to yield in cotton, Thomas W. Culp genetic improvement and associated physiological changes in the potato, Daniel O. Caldiz genetic improvement in physiological traits of rice yield, Mohammad Ashraf et al.

This chapter analyzes past breeding efforts, their affects on yield-related physiological traits, and the challenge of further raising the yield potential of the rice varieties. The principal emphasis in the chapter is on the physiological traits that are related to yield. Understanding the relationship of physiological traits and yield will assist intelligent use of physiological components in breeding for high yield. As regards physiological traits, the promotion of photosynthetic activity by fertilizer N should be as great as possible, and that of respiration as little as possible. In Korea, high-yielding varieties derived from indicajaponica hybridization had large sink capacity and more inner vascular bundles than japonica varieties. Therefore, rice yield can be increased by increasing total dry matter and/or harvest index. Studies on sink-source relationships in rice have led rice physiologists to believe that physical environments need not limit rice yield beyond the current levels.

The potato (Solarium tuberosum L.) originated in the Peruvian Andes. During a period of 180 years, the first and probably unknown contribution of breeding to the physiological response of the potato allowed its adaptation to European conditions. When discussing the genetic improvements in new potato cultivars, it is necessary to consider those achieved in tuber dormancy and sensitivity to physiological aging, because both traits are capable of modifying potential crop growth. When the potato tuber is planted at an adequate physiological age, according to the corresponding length of the growing season and final purpose of the crop (seed, ware, industry), sprout growth will proceed at a rate mainly determined by temperature if no pathogens, water, or nutrients are limiting factors. In areas where only one crop per year is grown, varieties with a long absolute dormant period are needed to reduce storage losses due to sprouting, evaporation, and respiration.

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).

Earliness per se (Eps) genes may play a critical role in further improving wheat adaptation and fine-tuning wheat development to cope with climate change. There are only few studies on the detailed effect of Eps on wheat development and fewer on the interaction of Eps with the environment and other genes determining time to anthesis. Furthermore, it seems relevant to study every newly discovered Eps gene and its probable interactions as the mechanisms and detailed effects of each Eps may be quite different. In the present study, we evaluated NILs differing in the recently identified Eps-7D as well as in Ppd-D1 at three temperature regimes (9, 15 and 18 °C) under short day. The effect of Eps-7D on time to anthesis as well as on its component phases varied both qualitatively and quantitatively depending on the allelic status of Ppd-D1 and temperature, being larger in a photoperiod-sensitive background. A more noticeable effect of Eps-7D (when combined with Ppd-D1b) was realised during the late reproductive phase. Consequently, the final leaf number was not clearly altered by Eps-7D, while floret development of the labile florets (florets 2 and 3 in this case, depending on the particular spikelet) was favoured by the action of the Eps-7D-late allele, increasing the likelihood of particular florets to become fertile, and consequently, improving spike fertility when combined with Ppd-D1b. This research was funded by the Spanish Research Agency (AEI), project AGL2015-69595-R, and the International Wheat Yield Partnership (IWYP), project IWYP25FP.

Earliness per se (Eps) genes are reported to be important in fine-tuning flowering time in wheat independently of photoperiod (Ppd) and vernalisation (Vrn). UnlikePpdandVrngenes,Epshave relatively small effects and their physiological effect along with chromosomal position are not well defined. We evaluated eight lines derived from crossing Paragon and Baj (late and early flowering respectively), vernalisation insensitive, to study the detailed effects of two newly identified QTLs,Eps-7DandEps-2Band their interactions under field conditions. The effect of both QTLs were minor but their effect was modulated by the allelic status of the other. While the magnitude of effect of these QTLs on anthesis was similar, they are associated with very different profiles of pre-anthesis development which also depends on their interaction.Eps-7Daffected both duration before and after terminal spikelet while not affecting final leaf number (FLN) soEps-7D-earlyhad a faster rate of leaf appearance.Eps-2Bacted more specifically in the early reproductive phase and slightly altered FLN without affecting the leaf appearance rate. BothEps-7Dand2Baffected the spike fertility by altering the rate of floret development and mortality. The effect of the latter was very small but consistent in that the -lateallele tended to produced more fertile florets.

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.

Earlinessper se(Eps) genes are important to fine tune adaptation, and studying their probable pleiotropic effect on wheat yield traits is worthwhile. In addition, it has been shown that some Eps genes interact with temperature. We studied two NILs differing in the newly identifiedEps-7Dbut carrying insensitivePpd-D1 in the background under three temperature regimes (9, 15 and 18 °C) and two photoperiods (12 h and 24 h).Eps-7Daffected time to anthesis as expected and theEps-7D-lateallele extended both the period before and after terminal spikelet. The interaction effect ofEps-7D× temperature was significant but not cross-over: the magnitude and level of significance of the difference between NILs with thelateorearlyallele was affected by the growing temperature (i.e. difference was least at 18 °C and largest at 9 °C), and differences in temperature sensitivity was influenced by photoperiod. Rate of leaf initiation was faster in NIL withEps-7D-earlythan with thelateallele which compensated for the shorter duration of leaf initiation resulting in similar final leaf number between two NILs.Eps-7D-late consistently increased spike fertility through improving floret primordia survival as a consequence of extending the late reproductive phase.

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.

Physiological causes for grain weight determination in maize (Zea mays L.) are not clear. Source–sink relationships during grain filling modulate grain weight, and there are controversies regarding the degree of source limitation that may exist during grain filling. We aimed to analyze likely causes of the esponsiveness of maize grain weight to defoliation and degraining treatments imposed 15 d after silking, quantifying the responsiveness of grain weight to these source–sink manipulations in a large number of field conditions (52 background conditions in which source–sink manipulations were imposed). Grain weight was largely unresponsive to increases in source availability but was diminished by defoliations in six out of seven experiments. Interestingly, grain weight reductions due to defoliation were not hierarchical (grains from different positions along the ear responded similarly) and were not worsened by imposing a simultaneous heat stress. Heat affected the grain growth capacity directly, and indirect effects (through reducing source strength due to accelerated senescence) were not evident. The penalty imposed by heat was neither increased by defoliation nor diminished by degraining, and the reduction in grain weight was similar for grains with different potential size. Our study reinforced the concept that maize yield is limited by the sink strength during grain filling, even when grain weight may respond to reductions in the grain filling source–sink ratio. We gratefully acknowledge technicians of the Crop Physiology Laboratory for help in the management of the experiments, particularly with the tedious degraining required at relatively large scale in field plots. We thank Lucas Borrás (University of Rosario, Argentina) and Sotirios Archontoulis (Iowa State University, IA) for their comments and suggestions in earlier versions of this paper, as well as María Otegui (Associate Editor of Crop Science) for valuable suggestions made directly on the PDF file of the original submission. Funding was provided by Project 8031 of the Regional Fund for Agricultural Technology (FONTAGRO). R.A. Ordóñez held a University of Lleida predoctoral scholarship.

Ppd-1a reduced the number of fertile florets due largely to lowering floret survival, and the effect was not proportional to the duration of the late reproductive phase., As wheat yield is linearly related to grain number, understanding the physiological determinants of the number of fertile florets based on floret development dynamics due to the role of the particular genes is relevant. The effects of photoperiod genes on dynamics of floret development are largely ignored. Field experiments were carried out to (i) characterize the dynamics of floret primordia initiation and degeneration and (ii) to determine which are the most critical traits of such dynamics in establishing genotypic differences in the number of fertile florets at anthesis in near isogenic lines (NILs) carrying photoperiod-insensitive alleles. Results varied in magnitude between the two growing seasons, but in general introgression of Ppd-1a alleles reduced the number of fertile florets. The actual effect was affected not only by the genome and the doses but also by the source of the alleles. Differences in the number of fertile florets were mainly explained by differences in the floret generation/degeneration dynamics, and in most cases associated with floret survival. Manipulating photoperiod insensitivity, unquestionably useful for changing flowering time, may reduce spike fertility but much less than proportionally to the change in duration of development, as the insensitivity alleles did increase the rate of floret development.

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 addition to its role in vernalization, temperature is an important environmental stimulus in determining plant growth and development. We used factorial combinations of two photoperiods (16H, 12H) and three temperature levels (11, 18 and 25 °C) to study the temperature responses of 19 wheat cultivars with established genetic relationships. Temperature produced more significant effects on plant development than photoperiod, with strong genotypic components. Wheat genotypes with PPD-D1 photoperiod sensitive allele were sensitive to temperature; their development was delayed by higher temperature, which intensified under non-inductive conditions. The effect of temperature on plant development was not proportional; it influenced the stem elongation to the largest extent, and warmer temperature lengthened the lag phase between the detection of first node and the beginning of intensive stem elongation. The gene expression patterns of VRN1, VRN2 and PPD1 were also significantly modified by temperature, while VRN3 was more chronologically regulated. The associations between VRN1 and VRN3 gene expression with early apex development were significant in all treatments but were only significant for later plant developmental phases under optimal conditions (16H and 18 °C). Under 16H, the magnitude of the transient peak expression of VRN2 observed at 18 and 25 °C associated with the later developmental phases.

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.

Grain growth and leaf senescence are the main processes during post-silking in maize. An unanswered question is how changes in sink strength (the number of grains to be filled) affect leaf senescence. We studied the effect of changes in the source-sink ratio during grain filling on the dynamics of green leaf area and photosynthetic rate. Maize plants were exposed to ear removal, pollination prevention, degraining or defoliation treatments during post-silking in two sowing dates. Green leaf area, leaf photosynthetic rate and leaf greenness were periodically measured. At maturity, yield, grain weight, and stem dry weight were determined. Green leaf area loss showed a first phase of slow senescence followed by a second phase of rapid senescence in all conditions. The response of green leaf area duration (GLAD) to the source-sink treatments depended on environmental conditions: ear removal increased GLAD while defoliations reduced it in the early sowing date, though GLAD was unaffected by these treatments in the late sowing date. But any change in senescence was less than proportional to the changes in the source-sink ratio. Leaf photosynthetic rate or leaf greenness (SPAD) did not change with the source-sink treatments. Defoliation caused a remarkable remobilisation of reserves from the stem, which did not fully mitigate the reduction in grain weight. It is concluded that (i) manipulations of source-sink balances modified slightly leaf longevity but not the functionality of the remaining leaf area (leaf photosynthetic rate), the response of GLAD being dependant on environmental conditions, and (ii) grain weight was more susceptible than GLAD to reductions in source-sink ratio. LGA held a ‘Juan de la Cierva’ post-doctoral contract funded by the Spanish Ministry of Science during the experimental phase of this research. Funding was provided by Project 8031 of the Regional Fund for Agricultural Technology (FONTAGRO).

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.