Wide variation in the suboptimal distribution of photosynthetic capacity in relation to light across genotypes of wheat

Abstract Suboptimal distribution of photosynthetic capacity in relation to light among leaves reduces potential whole-canopy photosynthesis. We quantified the degree of suboptimality in 160 genotypes of wheat by directly measuring photosynthetic capacity and daily irradiance in flag and penultimate leaves. Capacity per unit daily irradiance was systematically lower in flag than penultimate leaves in most genotypes, but the ratio (γ) of capacity per unit irradiance between flag and penultimate leaves varied widely across genotypes, from less than 0.5 to over 1.2. Variation in γ was most strongly associated with differences in photosynthetic capacity in penultimate leaves, rather than with flag leaf photosynthesis or canopy light penetration. Preliminary genome-wide association analysis identified nine strong marker-trait associations with this trait, which should be validated in future work in other environments and/or materials. Our modelling suggests canopy photosynthesis could be increased by up to 5 % under sunny conditions by harnessing this variation through selective breeding for increased γ. Plants invest nitrogen in photosynthetic enzymes in leaves. Although the optimal investment should be roughly proportional to how much light each leaf receives, plants tend to underinvest N in upper-canopy (sunlit) leaves. We examined, for the first time, whether this suboptimal pattern differs within species, by measuring light capture and photosynthetic capacity in flag and penultimate leaves across 160 wheat genotypes. We found wide variation in suboptimal N distribution, mostly driven by variation in plants’ ability to move N from penultimate to flag leaves as flag leaves develop. Selecting for less-suboptimal N distribution could increase canopy photosynthesis by several percent.