Trade-off between grain yield and grain protein concentration is modulated by canopy photosynthesis in Japanese wheat cultivars

A negative correlation between grain yield and protein concentration hinders an efficient nutrient supply. To analyze the causation of this inverse relationship, we compared the leaf and canopy photosynthesis of two Japanese wheat cultivars. Gas exchange measurements of leaves revealed that Kitahonami (a high-yielding and low-protein cultivar) maintained high stomatal conductance from pre-anthesis to late grain-filling stage while stomata of Yumechikara (a low-yielding and high-protein cultivar) were closed during the daytime, leading to decreased photosynthesis. We simulated canopy photosynthesis using a model and quantified the contribution of distinct stomatal behavior to canopy carbon gain. Although daily canopy photosynthetic gain was comparable or slightly low in Yumechikara when model parameters obtained in the morning were used, the gain was substantially lower in Yumechikara than in Kitahonami when midday parameters were used. Canopy respiratory loss in Yumechikara was greater than that in Kitahonami during the middle of the grain-filling period because of its high canopy nitrogen content, leading to a considerable difference in net canopy carbon gain between the cultivars. Our study suggests one of the pathways for a lower carbon gain of a high protein cultivar and the greater nitrogen content does not necessarily result in a greater carbon gain.