Enhancing leaf photosynthesis from altered chlorophyll content requires optimal partitioning of nitrogen

While optimising leaf chlorophyll content ([CHL]) has been proposed as a relevant means to manipulate canopy light penetration and canopy photosynthesis, effects of modifying [CHL] on leaf photosynthesis are yet to be investigated thoroughly. A greenhouse experiment and a field experiment were conducted involving rice genotypes of different genetic backgrounds and their leaf-colour variants. Leaf photosynthesis was more influenced by alteration to yellow-leaf than to stay-green cases. Higher specific leaf area and stomatal conductance were observed in two yellow-leaf variants, while only one yellow-leaf variant showed significantly increased Rubisco carboxylation capacity (V_cmax), maximum electron transport rate (J_max), and photosynthetic nitrogen-use efficiency (PNUE). Model analysis indicated that reducing leaf [CHL] decreased the energy loss via non-photochemical quenching, but improving V_cmax, J_max, and PNUE would require an improved nitrogen distribution pattern within the leaf. Label-free quantitative proteomics confirmed that an increased investment of nitrogen in Cyt b₆/f and Rubisco was observed in the yellow-leaf variant of the genetic background with improved V_cmax, J_max, and PNUE, but not in the other background. Our results suggest that reducing [CHL] can improve leaf photosynthesis only if the saved nitrogen is optimally distributed to proteins that are more rate-limiting to photosynthesis.