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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.

Radiation use efficiency can be predicted with ~70% accuracy. Canopy water content, greenness, and gas exchange spectral indices are the best predictors for RUE, biomass accumulation, and light interception., Wheat yields are stagnating or declining in many regions, requiring efforts to improve the light conversion efficiency, known as radiation use efficiency (RUE). RUE is a key trait in plant physiology because it links light capture and primary metabolism with biomass accumulation and yield, but its measurement is time consuming and this has limited its use in fundamental research and large-scale physiological breeding. In this study, high-throughput plant phenotyping (HTPP) approaches were used among a population of field-grown wheat with variation in RUE and photosynthetic traits to build predictive models of RUE, biomass, and intercepted photosynthetically active radiation (IPAR). Three approaches were used: best combination of sensors; canopy vegetation indices; and partial least squares regression. The use of remote sensing models predicted RUE with up to 70% accuracy compared with ground truth data. Water indices and canopy greenness indices [normalized difference vegetation index (NDVI), enhanced vegetation index (EVI)] are the better option to predict RUE, biomass, and IPAR, and indices related to gas exchange, non-photochemical quenching [photochemical reflectance index (PRI)] and senescence [structural-insensitive pigment index (SIPI)] are better predictors for these traits at the vegetative and grain-filling stages, respectively. These models will be instrumental to explain canopy processes, improve crop growth and yield modelling, and potentially be used to predict RUE in different crops or ecosystems.

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).

Improving biomass is an important goal for future genetic gains in yield potential in wheat, but it will also be crucial to identify physiological traits to maximize harvest index (HI, proportion of aboveground biomass in grain). Increased grain partitioning will require increased dry-matter (DM) partitioning to the spikes at anthesis as well as enhanced fruiting efficiency (FE, grains per g spike dry matter at anthesis or chaff dry matter at harvest), whilst optimizing the partitioning amongst the non-grain components to maintain post-anthesis photosynthetic capacity and soluble carbohydrate translocation. The objectives of this study were to: i) quantify genetic variation in DM partitioning among plant organs at anthesis (GS65) + 7 days and associations with spike growth and FE and ii) identify optimized partitioning traits associated with enhanced HI and grain yield, in CIMMYT elite spring wheat backgrounds. Two field experiments were conducted in 2011-12 and 2012-13 testing 26 CIMMYT spring wheat cultivars in NW Mexico in irrigated conditions in which DM partitioning was assessed in plant organs at anthesis + 7 days, and within-spike (glume, palea, lemma, rachis and awn) partitioning was assessed at harvest for a subset of 17 cultivars. Grain yield, yield components, HI and FE were assessed at harvest. Our results identified new traits for HI (decreased DM partitioning to stem internodes 2 (top down, peduncle -1) and 3, and decreased rachis DM partitioning and rachis specific weight (rachis DM per rachis unit length) and increased lemma DM partitioning), potentially allowing breeders to maximize the exploitation of enhanced carbon assimilation for grain biomass. Further work will focus on understanding the role of soluble carbohydrate re-translocation in these relationships and establishing high-throughput and cost-effective phenotyping methods for these traits for deployment in breeding.

Feature comparison of wheat seedlings obtained from high-throughput phenotyping using linear discriminant analysis shows that nitrogen uptake efficiency and nitrate availability affect root system architecture., Root architecture impacts water and nutrient uptake efficiency. Identifying exactly which root architectural properties influence these agronomic traits can prove challenging. In this paper, approximately 300 wheat (Triticum aestivum) plants were divided into four groups using two binary classifications, high versus low nitrogen uptake efficiency (NUpE), and high versus low nitrate in the growth medium. The root system architecture for each wheat plant was captured using 16 quantitative variables. The multivariate analysis tool, linear discriminant analysis, was used to construct composite variables, each a linear combination of the original variables, such that the score of the plants on the new variables showed the maximum between-group variability. The results show that the distribution of root-system architecture traits differs between low- and high-NUpE plants and, less strongly, between low-NUpE plants grown on low versus high nitrate media.

Breeding to raise yield potential through enhancing photosynthesis will have limited impact unless harvest index (HI: proportion of above-ground biomass as grain yield) is maintained or ideally increased. Boosting grain dry matter (DM) partitioning will require increased allocation of assimilates to sink organs to enhance spike growth. A high biomass spring wheat panel of 150 genotypes encompassing elite, landrace-derived and synthetic-derived lines was grown under yield potential conditions in two seasons in NW Mexico. Results showed that the incorporation of landrace-derived and synthetic-derived backgrounds into elite lines resulted in higher expression of above-ground biomass (AGDM), leaf lamina and stem DM partitioning at anthesis. However, no grain yield advantage was observed over elite lines, due to lower grain number per unit area (GN) and decreased harvest index (HI). Positive linear associations were found among spike fertility-related traits - fruiting efficiency (grains per unit of spike DM at anthesis; FE), GN and HI - which were, in turn, related positively with grain yield (GY). Stem-internode 3 length and internode 3 DM partioning were negatively associated with spike partitioning index (SPI: ratio of spike DM to total above-ground DM at anthesis) and GN, suggesting an enhanced competition for assimilates between the spike and stem internode 3 during stem elongation. Within-spike DM partitioning analysis (glume, lemma, palea, rachis, awn) showed decreased partitioning to awns was associated with increased FE and thousand grain weight (TGW). While the use of exotic material can enhance biomass, special attention needs to be paid in the selection for novel DM partitioning traits that raise HI and GN coming from the elite genepool. The selection for grain partitioning traits in wheat breeding combined with sources expressing high biomass can potentially allow breeders to increase grain carbon assimilation that will deliver higher yields.

Wheat represents a major crop, yet the current rate of yield improvement is insufficient to meet its projected global food demand. Breeding root systems more efficient for water and nitrogen capture represents a promising avenue for accelerating yield gains. Root crown phenotyping, or shovelomics, relies on excavation of the upper portions of root systems in the field and measuring root properties such as numbers, angles, densities and lengths. We report a new shovelomics method that images the whole wheat root crown, then partitions it into the main shoot and tillers for more intensive phenotyping. Root crowns were phenotyped using the new method from the Rialto × Savannah population consisting of both parents and 94 doubled-haploid lines. For the whole root crown, the main shoot, and tillers, root phenes including nodal root number, growth angle, length, and diameter were measured. Substantial variation and heritability were observed for all phenes. Principal component analysis revealed latent constructs that imply pleiotropic genetic control of several related root phenes. Correlational analysis revealed that nodal root number and growth angle correlate among the whole crown, main shoot, and tillers, indicating shared genetic control among those organs. We conclude that this phenomics approach will be useful for breeding ideotype root systems in tillering species.

Our objective was to investigate the relationships between spike traits, grain number and yield potential and their physiological basis in a doubled-haploid (DH) population derived from a cross between a CIMMYT spring wheat (Triticum aestivum L.) advanced line of large-spike phenotype (LSP2; +Tin1 tiller inhibition gene) and the UK winter wheat cultivar Rialto (R; −Tin1) of conventional spike phenotype. Field experiments were carried out in high radiation, irrigated conditions in NW Mexico in two seasons. Comparing the two groups of +Tin1 and −Tin1 DH lines, results showed the presence of the +Tin1 gene for tiller inhibition increased spike partitioning index (spike DM/above-ground DM at GS61 + 5 d; SPI) from 0.32 to 0.34 (+6.3%) (P < 0.01) and grains spike−1 by 5.1 (+13.9%) (P < 0.001), but reduced spikes m−2 by 20.7 (−5.7%) (P < 0.01). Overall a significant increase in grains m−2 of 865 (+6.6%) was observed in +Tin1 DH lines compared to −Tin1 DH lines (P < 0.05), but the effect on grain yield was not statistically significant. Above-grouund biomass at anthesis was not significantly affected by the presence/absence of the Tin1 gene; although results indicated the presence of the Tin1 gene increased photosynthetically active radiation interception from onset of stem elongation to anthesis, but decreased radiation-use efficiency during this phase. Our results indicated that introgressing the +Tin1 gene into modern wheat germplasm may offer scope to increase grains spike−1 and grains m−2 in irrigated, high radiation environments.

In Mediterranean regions drought is the major factor limiting spring barley and durum wheat grain yields. This study aimed to compare spring barley and durum wheat root and shoot responses to drought and quantify relationships between root traits and water uptake under terminal drought. One spring barley (Hordeum vulgare L. cv. Rum) and two durum wheat Mediterranean cultivars (Triticum turgidum L. var durum cvs Hourani and Karim) were examined in soil-column experiments under well watered and drought conditions. Root system architecture traits, water uptake, and plant growth were measured. Barley aerial biomass and grain yields were higher than for durum wheat cultivars in well watered conditions. Drought decreased grain yield more for barley (47%) than durum wheat (30%, Hourani). Root-to-shoot dry matter ratio increased for durum wheat under drought but not for barley, and root weight increased for wheat in response to drought but decreased for barley. The critical root length density (RLD) and root volume density (RVD) for 90% available water capture for wheat were similar to (cv. Hourani) or lower than (cv. Karim) for barley depending on wheat cultivar. For both species, RVD accounted for a slightly higher proportion of phenotypic variation in water uptake under drought than RLD.

Future genetic progress in wheat grain yield will depend on increasing above-ground biomass and this must be achieved without commensurate increases in N fertilizer inputs to minimise environmental impacts. Our objective was to quantify variation in grain yield, above-ground biomass and N-use efficiency (NUE) and associated traits in a panel of diverse hexaploid wheat germplasm comprising: (i) landraces from the AE Watkins collection, (ii) synthetic-derived hexaploid lines in a cv. Paragon spring wheat background and (iii) UK modern cultivars including cv. Paragon under low N and high N conditions. A field experiment was carried out in two seasons examining 15 genotypes (five landraces, five synthetic-derived (SD) hexaploid lines and five UK modern cultivars) under low N and high N conditions at Nottingham University farm, UK. Machine-harvested grain yield, above-ground biomass and NUE were measured. Physiological traits were assessed including flag-leaf light-saturated photosynthetic rate (Amax) and relative chlorophyll content (SPAD) under HN conditions; and flag-leaf senescence duration and rate and Normalized Difference Vegetative Index (NDVI) under LN and HN conditions. Under HN conditions, the modern cultivars overall produced higher grain yield than the SD lines (+9.7%) and the landraces (+60.4%); and the modern cultivars and SD lines also produced higher biomass than the landraces (30.3% and 28.4%, respectively). Under LN conditions, reduction in grain yield and biomass compared to HN conditions was least for the landraces (?1% and ?8.6%, respectively), intermediate for the SD lines (?7.4 and ?10.2%, respectively) and highest for the modern cultivars (?9.3 and ?24.6%, respectively). As a result, the SD lines had higher biomass (+17%) than the modern cultivars under LN conditions. Under HN conditions the synthetic derivatives (23.8 ?mol m?2 s?1) and modern cultivars (241.1 ?mol m?2 s?1) had higher pre-anthesis Amax than the landraces (19.7 ?mol m?2 s?1) (P < 0.001). Pre-anthesis Amax was strongly positively linearly associated with above-ground biomass (R2 = 0.63, P < 0.001) and grain yield (R2 = 0.75, P < 0.001) amongst the 15 genotypes. Flag-leaf Amax was also positively linearly associated with flag-leaf relative chlorophyll content at anthesis (R2 = 0.74; P < 0.001). Comparing the SD lines to the recurrent parent Paragon, under HN conditions one line (SD 22) had higher pre-anthesis flag-leaf Amax than Paragon (P < 0.05). Under LN conditions one line (SD 24, +27%) had higher yield than Paragon (P < 0.05) and two lines (SD 24 and SD 38, +32% and +31%, respectively) had more biomass than Paragon (P < 0.05). Our results indicated that introgressing traits from synthetic-derived wheat and landraces into UK modern wheat germplasm offers scope to raise above-ground biomass and grain yield in moderate-to-low N availability environments.

As the world population grows, there is a pressing need to improve productivity from water use in irrigated and rain-fed agriculture. Foliar diseases have been reported to decrease crop water-use efficiency (WUE) substantially, yet the effects of plant pathogens are seldom considered when methods to improve WUE are debated. We review the effects of foliar pathogens on plant water relations and the consequences for WUE. The effects reported vary between host and pathogen species and between host genotypes. Some general patterns emerge however. Higher fungi and oomycetes cause physical disruption to the cuticle and stomata, and also cause impairment of stomatal closing in the dark. Higher fungi and viruses are associated with impairment of stomatal opening in the light. A number of toxins produced by bacteria and higher fungi have been identified that impair stomatal function. Deleterious effects are not limited to compatible plant-pathogen interactions. Resistant and non-host interactions have been shown to result in stomatal impairment in light and dark conditions. Mitigation of these effects through selection of favourable resistance responses could be an important breeding target in the future. The challenges for researchers are to understand how the effects reported from work under controlled conditions translate to crops in the field, and to elucidate underlying mechanisms.

Embryo and caryopsis dormancy, abscisic acid (ABA) responsiveness, after-ripening (AR), and the disorder pre-harvest sprouting (PHS) were investigated in six genetically related wheat varieties previously characterized as resistant, intermediate, or susceptible to PHS. Timing of caryopsis AR differed between varieties; AR occurred before harvest ripeness in the most PHS-susceptible, whereas AR was slowest in the most PHS-resistant. Whole caryopses of all varieties showed little ABA-responsiveness during AR; PHS-susceptible varieties were responsive at the beginning of the AR period whereas PHS-resistant showed some responsiveness throughout. Isolated embryos showed relatively little dormancy during grain-filling and most varieties exhibited a window of decreased ABA-responsiveness around the period of maximum dry matter accumulation (physiological maturity). Susceptibility to PHS was assessed by overhead misting of either isolated ears or whole plants during AR; varieties were clearly distinguished using both methods. These analyses allowed an investigation of the interactions between the different components of seed development, compartments, and environment for the six varieties. There was no direct relationship between speed of caryopsis AR and embryo dormancy or ABA-responsiveness during seed maturation. However, the velocity of AR of a variety was closely associated with the degree of susceptibility to PHS during AR suggesting that these characters are developmentally linked. Investigation of genetic components of AR may therefore aid breeding approaches to reduce susceptibility to PHS.

Grain texture is an important component of end-use quality in wheat. The effects of water availability on the components of texture; vitreosity, determined using a Light Transflectance meter (LTm), grain hardness measured using the single-kernel characterisation system (SKCS), and protein content, were studied in field experiments of winter wheat in the UK in 2001/2002 and 2002/2003. Experiments were grown on a drought prone soil and employed a mapping population of 46 doubled haploid (DH) lines and their parents, Beaver (+1BL/1RS, soft wheat) and Soissons (1B, hard wheat). The results showed that drought increased hardness in both seasons, but the effect was never sufficient to move a line from the soft class into the hard class. Puroindoline (PIN)-a:b peak height ratio explained ca. 78% of the variation in hardness, and drought also appeared to increase the amounts of PINs in the grain. Minor quantitative trait loci (QTLs) were found for hardness on chromosomes 2A, 2D, 3A and 6D, also associated with QTLs for PINs. Vitreosity also increased in response to drought in both seasons. Variation in vitreosity explained 7–11% of the overall variation in texture within a hardness class, with hardness increasing on average by 2.2 SKCS units for each 10% increase in the proportion of vitreous grains. The relationship between vitreosity and protein content was poor, despite the fact that protein content also increased in response to drought. Minor QTLs associated with both protein content and vitreosity were found on chromosomes 1B, 4D and 5D. A minor QTL for vitreosity was also found on chromosome 2D. However, there appeared to be no direct relationship between alleles at the Ha locus, the gene which controls the difference between hard and soft wheats, and vitreosity. A positive relationship between the presence of the 1BL/1RS translocation and the proportion of vitreous grains was identified, suggesting that vitreosity was strongly linked to changes in protein quality.

Source or sink limitation of grain filling in cereals is often inferred from experiments in which the source:sink ratio is manipulated by shading, defoliation or grain removal. However, interpretation of this type of experiment is usually qualitative rather than quantitative in nature and the extent of any imbalance between the source and sink is not known. The objectives of the current work were: (1) to provide a detailed analysis of radiation interception, radiation-use efficiency (RUE) and carbohydrate storage reserves in winter barley in order to quantify the potential supply of photosynthates for grain filling; (2) to estimate the variation in source–sink balance between environments. Field experiments were conducted on cv Pearl at six sites in the UK and over 3 years. Crops were grown under a comparable husbandry regime at each site and received a full fertilizer and crop protection programme. When the cumulative interception of post-anthesis photosynthetically active radiation (PAR) was plotted against the increase in biomass to determine RUE, the pattern of response differed between sites and years; for some site/years the response was linear, for others it was non-linear where RUE decreased during the latter stages of grain filling. The extent and statistical significance of non-linearity was determined from the quadratic term of fitted 2nd order polynomials. There was no significant association between climatic variables, such as temperature, radiation or rainfall, and the value of the quadratic term of RUE. Neither could non-linearity of RUE be explained in terms of the shedding of leaf tissue during canopy senescence. There were weak associations ( r 2 r 2 = 0.63) was found between the source:sink ratio (green area per grain) at GS 59 and non-linearity of RUE. These results suggest that a major factor leading to the reduction in RUE during the second half of grain filling at some sites was feedback inhibition from a limited sink capacity. This conclusion is supported by a fairly strong positive association between RUE non-linearity and the apparent contribution of stem carbohydrate reserves to grain yield ( r 2 = 0.47). The potential assimilate supply for grain filling was estimated as (maximum post-anthesis RUE × PAR intercepted) + stem soluble carbohydrate reserves at GS 59. The potential supply exceeded the measured yield at all sites except one implying that crops were predominantly sink limited. The size of the excess, which is a measure of the relative source–sink balance during grain filling, differed widely between site/years.

Highlight A phenotyping pipeline was used to quantify seedling root architectural traits in a wheat double haploid mapping population. QTL analyses revealed a potential major effect gene regulating seedling root vigour/growth., Seedling root traits of wheat (Triticum aestivum L.) have been shown to be important for efficient establishment and linked to mature plant traits such as height and yield. A root phenotyping pipeline, consisting of a germination paper-based screen combined with image segmentation and analysis software, was developed and used to characterize seedling traits in 94 doubled haploid progeny derived from a cross between the winter wheat cultivars Rialto and Savannah. Field experiments were conducted to measure mature plant height, grain yield, and nitrogen (N) uptake in three sites over 2 years. In total, 29 quantitative trait loci (QTLs) for seedling root traits were identified. Two QTLs for grain yield and N uptake co-localize with root QTLs on chromosomes 2B and 7D, respectively. Of the 29 root QTLs identified, 11 were found to co-localize on 6D, with four of these achieving highly significant logarithm of odds scores (>20). These results suggest the presence of a major-effect gene regulating seedling root vigour/growth on chromosome 6D.

Crops frequently display genotype x environment interaction for yield and end-use quality in response to different environments, particularly when stresses such as water limitation and temperature are components of the interaction. Plant breeders usually exploit this variation via phenotypic selection to develop varieties with both general and specific adaptation. However the individual genes and physiological processes underlying the basis of general and specific adaptation have rarely been elucidated. We are combining large-scale QTL analysis of several doubled haploid populations of wheat, grown over different environments and seasons, with detailed physiological analysis, to dissect the genes and mechanisms responsible for yield and yield x environment variation in adapted European winter germplasm. Analysis of populations grown under irrigated and non-irrigated conditions on drought-prone soils has revealed individual genes showing stable and differential expression over environments, and the analysis has also identified physiological traits that contribute to differential yield performance. Genes on the homoeologous group 2 chromosomes were associated with flag leaf senescence (stay-green) variation and were the most significant in drought interactions. Variation for stem soluble carbohydrate reserves was associated with the 1RS arm of the 1BL/1RS translocated chromosome, and was positively correlated with yield under both irrigated and non-irrigated conditions, and thus general adaptability. Separate analyses of populations grown over three seasons in England, Scotland, France and Germany revealed QTL for yield performance showing both general and specific effects. A stable QTL on chromosome 6A, consistent in different populations, showed significant effects over seasons and environments, whilst other QTL were specific to season and/or environments.

International audience; Our objective was to investigate the determinants of genetic variation in N accumulation, N partitioning and N remobilization to the grain post-flowering and associations with flag-leaf senescence, grain yield and grain N% in 16 wheat cultivars grown under high N (HN) and low N (LN) conditions in the UK and France. Overall, cultivars ranged in leaf lamina N accumulation at anthesis from 5.32 to 8.03 gN m(-2) at HN and from 2.69 to 3.62 gN m(-2) at LN, and for the stem-and leaf-sheath from 5.45 to 7.25 gN m(-2) at HN and from 2.55 to 3.41 gN m(-2) at LN (P < 0.001). Cultivars ranged in N partitioning index (proportion of above-ground N in the crop component) at anthesis for the leaf lamina from 037 to 0.42 at HN and 0.34 to 0.40 at LN; and for the stem-and leaf-sheath from 0.39 to 0.43 at HN and from 0.35 to 0.41 at LN (P < 0.001). The amount of leaf lamina N remobilized post-anthesis was negatively associated with the duration of post-anthesis flag-leaf senescence amongst cultivars in all experiments under HN. In general, it was difficult to separate genetic differences in lamina N remobilization from those in lamina N accumulation at anthesis. Genetic variation in grain yield and grain N% (through N dilution effects) appeared to be mainly influenced by pre-anthesis N accumulation rather than post-anthesis N remobilization under high N conditions and under milder N stress (Sutton Bonington LN). Where N stress was increased (Clermont Ferrand LN), there was some evidence that lamina N remobilization was a determinant of genetic variation in grain N% although not of grain yield. Our results suggested that selection for lamina N accumulation at anthesis and lamina N remobilization post-anthesis may have value in breeding programmes aimed at optimizing senescence duration and improving grain yield, N-use efficiency and grain N% of wheat. (C) 2013 The Authors. Published by Elsevier B.V. All rights reserved.

In Mediterranean regions drought is the major factor limiting spring barley and durum wheat grain yields. This study aimed to compare spring barley and durum wheat root and shoot responses to drought and quantify relationships between root traits and water uptake under terminal drought. One spring barley (Hordeum vulgare L. cv. Rum) and two durum wheat Mediterranean cultivars (Triticum turgidum L. var durum cvs Hourani and Karim) were examined in soil-column experiments under well watered and drought conditions. Root system architecture traits, water uptake, and plant growth were measured. Barley aerial biomass and grain yields were higher than for durum wheat cultivars in well watered conditions. Drought decreased grain yield more for barley (47%) than durum wheat (30%, Hourani). Root-to-shoot dry matter ratio increased for durum wheat under drought but not for barley, and root weight increased for wheat in response to drought but decreased for barley. The critical root length density (RLD) and root volume density (RVD) for 90% available water capture for wheat were similar to (cv. Hourani) or lower than (cv. Karim) for barley depending on wheat cultivar. For both species, RVD accounted for a slightly higher proportion of phenotypic variation in water uptake under drought than RLD.

Increasing photosynthetic capacity by extending canopy longevity during grain filling using slow senescing stay-green genotypes is a possible means to improve yield in wheat. Ethyl methanesulfonate (EMS) mutated wheat lines (Triticum aestivum L. cv. Paragon) were screened for fast and slow canopy senescence to investigate the impact on yield and nitrogen partitioning. Stay-green and fast-senescing lines with similar anthesis dates were characterised in detail. Delayed senescence was only apparent at higher nitrogen supply with low nitrogen supply enhancing the rate of senescence in all lines. In the stay-green line 3 (SG3), on a whole plant basis, tiller and seed number increased whilst thousand grain weight (TGW) decreased; although a greater N uptake was observed in the main tiller, yield was not affected. In fast-senescing line 2 (FS2), yield decreased, principally as a result of decreased TGW. Analysis of N-partitioning in the main stem indicated that although the slow-senescing line had lower biomass and consequently less nitrogen in all plant parts, the proportion of biomass and nitrogen in the flag leaf was greater at anthesis compared to the other lines; this contributed to the grain N and yield of the slow-senescing line at maturity in both the main tiller and in the whole plant. A field trial confirmed senescence patterns of the two lines, and the negative impact on yield for FS2 and a positive impact for SG3 at low N only. The lack of increased yield in the slow-senescing line was likely due to decreased biomass and additionally a possible sink limitation.

Vertical leaf nitrogen (N) gradient within a canopy is classically considered as a key adaptation to the local light environment that would tend to maximize canopy photosynthesis. We studied the vertical leaf N gradient with respect to the light gradient for wheat (Triticum aestivum) canopies with the aims of quantifying its modulation by crop N status and genetic variability and analyzing its ecophysiological determinants. The vertical distribution of leaf N and light was analyzed at anthesis for 16 cultivars grown in the field in two consecutive seasons under two levels of N. The N extinction coefficient with respect to light (b) varied with N supply and cultivar. Interestingly, a scaling relationship was observed between b and the size of the canopy for all the cultivars in the different environmental conditions. The scaling coefficient of the b-green area index relationship differed among cultivars, suggesting that cultivars could be more or less adapted to low-productivity environments. We conclude that the acclimation of the leaf N gradient to the light gradient is a whole-plant process that depends on canopy size. This study demonstrates that modeling leaf N distribution and canopy expansion based on the assumption that leaf N distribution parallels that of the light is inappropriate. We provide a robust relationship accounting for vertical leaf N gradient with respect to vertical light gradient as a function of canopy size.

The rate of organ emergence and the duration of developmental phases are key components of plant fitness to its environment. Here, we describe a hybrid genetic algorithm for the estimation of the parameters of complex non-linear simulation models that we used to estimate the varietal parameters of a well-evaluated ecophysiological model (Sirius) of wheat phenology. The aim of this study was to analyze the correlations between the varietal parameters of Sirius and to determine the minimum number of parameters that need to be estimated in order to accurately simulate the effect of the genotype and environment on wheat anthesis date. A panel of 16 bread wheat cultivars was grown under controlled conditions with different vernalization and daylength treatments and in a field nursery with different sowing dates in order to characterize their vernalization and photoperiodic requirements and earliness per se. These cultivars were also grown in the field in two consecutive growing seasons in France and in the UK along a 800 km latitude transect, where the dynamics of leaf appearance and anthesis date were determined. Variation in both final leaf number and anthesis date in response to the environment and the genotype was predicted with a mean error of 0.55 leaves and 3.94 d, respectively, after estimation of only three of the seven varietal parameters of Sirius: the phyllochron, the response of vernalization rate to temperature, and the daylength response of leaf production. Among them, the phyllochron was reasonably well estimated and showed a positive association with earliness per se (Spearman's coefficient of rank correlation = 0.59). This study showed that the number of varietal parameters in Sirius was overestimated and that considering only three varietal parameters reduced the correlations between the parameters and the root mean square error of prediction for final leaf number and anthesis date. We conclude that a phenomenogical model of wheat development can be used to estimate key phenological parameters that are difficult to determine in the field; offering the possibility to conduct large-scale quantitative genetic studies to understand better the genetic control of flowering time in cereals.

A substantial increase in grain yield potential is required, along with better use of water and fertilizer, to ensure food security and environmental protection in future decades. For improvements in photosynthetic capacity to result in additional wheat yield, extra assimilates must be partitioned to developing spikes and grains and/or potential grain weight increased to accommodate the extra assimilates. At the same time, improvement in dry matter partitioning to spikes should ensure that it does not increase stem or root lodging. It is therefore crucial that improvements in structural and reproductive aspects of growth accompany increases in photosynthesis to enhance the net agronomic benefits of genetic modifications. In this article, six complementary approaches are proposed, namely: (i) optimizing developmental pattern to maximize spike fertility and grain number, (ii) optimizing spike growth to maximize grain number and dry matter harvest index, (iii) improving spike fertility through desensitizing floret abortion to environmental cues, (iv) improving potential grain size and grain filling, and (v) improving lodging resistance. Since many of the traits tackled in these approaches interact strongly, an integrative modelling approach is also proposed, to (vi) identify any trade-offs between key traits, hence to define target ideotypes in quantitative terms. The potential for genetic dissection of key traits via quantitative trait loci analysis is discussed for the efficient deployment of existing variation in breeding programmes. These proposals should maximize returns in food production from investments in increased crop biomass by increasing spike fertility, grain number per unit area and harvest index whilst optimizing the trade-offs with potential grain weight and lodging resistance.

Publisher Summary This chapter considers the genetic basis of yield potential and the implications for breeding, identifies barriers to future gains in yield potential, and assesses the scope for the use of physiological tools to select for high yield potential. Yield potential is the yield of cultivars when grown in environments to which they are adapted with nutrients and water nonlimiting and with pests, diseases, weeds, and other stresses effectively controlled. It is somewhat theoretical since “optimal” radiation and temperatures, for example, most likely interact with genotype and growth stage. This differs from the attainable yield, which corresponds to the best yield achieved through skillful use of the best available technology. On-farm yields normally realize from 60 to 80% of attainable yield; the gap relates to the physical environment being suboptimal on most farms and to moderate use of fertilizer and crop protection measures in environmentally sensitive farming. The yield gap may therefore be quite large, especially in the more marginal environments.

Recent advances in crop research have the potential to accelerate genetic gains in wheat, especially if co-ordinated with a breeding perspective. For example, improving photosynthesis by exploiting natural variation in Rubisco's catalytic rate or adopting C(4) metabolism could raise the baseline for yield potential by 50% or more. However, spike fertility must also be improved to permit full utilization of photosynthetic capacity throughout the crop life cycle and this has several components. While larger radiation use efficiency will increase the total assimilates available for spike growth, thereby increasing the potential for grain number, an optimized phenological pattern will permit the maximum partitioning of the available assimilates to the spikes. Evidence for underutilized photosynthetic capacity during grain filling in elite material suggests unnecessary floret abortion. Therefore, a better understanding of its physiological and genetic basis, including possible signalling in response to photoperiod or growth-limiting resources, may permit floret abortion to be minimized for a more optimal source:sink balance. However, trade-offs in terms of the partitioning of assimilates to competing sinks during spike growth, to improve root anchorage and stem strength, may be necessary to prevent yield losses as a result of lodging. Breeding technologies that can be used to complement conventional approaches include wide crossing with members of the Triticeae tribe to broaden the wheat genepool, and physiological and molecular breeding strategically to combine complementary traits and to identify elite progeny more efficiently.

Root system architecture (RSA) is important in optimizing the use of nitrogen. High-throughput phenotyping techniques may be used to study root system architecture traits under controlled environments. A root phenotyping platform, consisting of germination paper-based pouch and wick coupled with image analysis, was used to characterize root seedling traits in 30 landrace-derived bread wheat genotypes and the bread wheat parent Paragon under hydroponic high N (HN) and low N (LN) conditions. In addition, two glasshouse experiments under HN and LN conditions were carried out to measure whole plant performance including flag-leaf photosynthetic rate, N uptake and biomass per plant for 13 wheat genotypes of which eight were common with those in the root phenotyping hydroponic experiment. There were significant differences in RSA traits between genotypes for seminal root number per plant, lateral root number per plant, seminal root length per plant and seminal root angle, with transgressive segregation for landrace-derived lines above the elite parental cultivar Paragon under HN and LN conditions. In the glasshouse experiments, genetic variation in flag-leaf photosynthesis rate was found in landrace-derived genotypes in the range 25.9–33.3 µmol m−2 s−1 under HN and in N uptake in the ranges 0.37–0.48 g N plant−1and 0.21–0.30 g plant−1 under HN and LN conditions, respectively (P < 0.05), with transgressive segregation above Paragon. Plant Nitrogen Nutrition Index also showed transgressive segregation in the landrace-derived lines above Paragon under HN and LN conditions. Greater maximum root depth and more lateral roots per plant in the hydroponic screen were each correlated with increased biomass per plant under LN conditions. Results from this study demonstrated genetic variation for seedling RSA traits in landrace-derived lines above the elite parental cultivar Paragon, which potentially could be utilized to improve N-use efficiency in breeding programmes.