Spike photosynthesis contributes to individual grain weight (IGW) which, along with grain number per m-2 (GN), are key components of grain yield (GY). Since IGW often plateaus at high GN, further increases in GY can be achieved by overcoming this trade-off. Although pre-anthesis growth is important for IGW establishment, assimilate supply during grain filling, provided by several photosynthetic organs, is a strong determinant of final IGW. However, the relative photosynthetic contributions of flag leaf and spike to IGW and the potential source-limitation during grain filling has not been investigated in modern UK wheat lines. Our work aims to provide physiological targets for future breeding programmes focusing on IGW improvement.

Over two field seasons (2017 and 2018), eight to sixteen wheat lines with contrasting yield components were assessed in replicated field plots. Shading was applied at GS69 to different organs (e.g. spike, leaves) to assess their contributions to IGW. In 2018, 50% of the grain on the main-stem spike were manually removed at GS69 (de-graining) to quantify genotypic differences in source-limitation by determining the IGW potential (IGWp). To assess the photosynthetic assimilation rate of the spike at saturating light (A), a bespoke cuvette was constructed and A was measured both in field and greenhouse experiments using a Li-Cor 6400 IRGA. An unexpectedly large contribution of spike photosynthesis to IGW was recorded, with significant genotypic variation (p<0.05) detected ranking from 10 to 40%. IGWp was 5% to 30% greater than IGW (p<0.05), depending on genotype, indicating that grains typically compete for a limited supply of assimilate to the grain under these growing conditions. The progressive response of IGW to de-graining was positively correlated with GN across genotypes, suggesting that genetic improvements to GN are not matched by the ability to produce adequate assimilates to fill grains. Intra-spike and intra-spikelet analysis of IGW suggested that the source-limitation was greatest in distal spikelets and grains at position 3 and 4. There was significant genotypic variation (p<0.05) for spike A, ranging from 5 to 11 µmol m-2 s-1 between lines. Spike photosynthesis significantly contributes to IGW in the UK. Evidence of source-limitation during grain filling in lines with elevated GN capacity emphasises the need to combine this trait with greater spike photosynthesis. Our work suggests that selecting lines with elevated spike photosynthesis might help achieve IGWp in lines with high GN potential, thus providing a novel avenue for yield improvement.