Source-sink balance in wheat under abiotic stress: Dissecting the spatio-temporal dynamics of grain filling

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Wheat is one of the most important staple food crops and high grain yields are essential for global food security. In extensive previous work we confirmed that limitations to wheat yield potential are primarily determined by traits implicated in source-sink relations: Modern cultivars show increases in biomass, nutrient use efficiency, harvest index, light interception and radiation utilisation efficiency along with an elevated grain number. However, all still carry many detrimental alleles for source-sink traits.We performed extensive phenotyping of source- and sink-related traits under diverse environmental and management scenarios in a large panel of historical winter wheat cultivars. This enabled us to identify modern cultivars with enhanced source strength, facilitated by high water and nitrogen uptake efficiency, high biomass potential and/or high radiation interception/use efficiency, or with remarkable sink strength imparted mainly by a high number of grains per unit area. However, these putative physiological advantages did not always result in increased grain yield: Instead, such genotypes often showed sink deficits in terms of irregular grain formation, spatio-temporal variability for grain development, or other source-sink tradeoffs.The aim of this subproject in the Package Proposal “Wheat source-sink relationships and limitations (WheatSouSi)” is to generate detailed data describing the spatial distribution of grain number and grain size in relation to rachis row number on main stems and secondary tillers of genetically diverse winter wheat cultivars, grown under different environmental constraints, and compare this information with other comprehensive source-sink datasets collected in other subprojects. We will also analyze the relationships between stem soluble carbohydrates, pollen viability, grain set and grain weight, in a spatial context along the length of the spike, under different environmental constraints. This data will be used to evaluate benefits and trade-offs of different allelic combinations of key genes involved in fertility and fecundity in relation to the availability of stem soluble carbohydrates under optimal vs. stress conditions. Finally, we will perform joint analyses with cooperating subprojects to investigate abiotic stress responses at different developmental timeponts, to implement stem carbohydrate data into source-sink models, and to evaluate allelic shifts related to traits under co-selection with grain yield as a consequence of breeding. Collectively, these comprehensive joint analyses will give novel insight into the impact of genetic gain for grain yield on source-sink relationships in modern wheat.