Higher CO₂ Assimilation in Selected Rice Recombinant Inbred Lines Is Driven by Higher CO₂ Diffusion and Light Use...

Leaf anatomy determining the light distribution within the leaf and exerting influence on CO₂ diffusion is considered to have dramatic potential for photosynthesis performance increase. In this study, we observed that two rice recombinant inbred lines, H138 and H217 (RILF₁₁ plants from Sasanishiki × IRAT10), have higher net CO₂ assimilation (An) than their parent Sasanishiki due mainly to the improvement of leaf anatomy. Our results showed that An positively correlated with anatomy traits' mesophyll cell number per cross-sectional area (NO_.mescell/A_cros) and mesophyll area (A_mes). NO._mescell/A_cros exert direct and indirect effects on An. Compared to Sasanishiki flag leaves, IRAT10, H138, and H217 have higher mesophyll cell numbers. Simultaneously, higher chlorophyll content and expression of genes encoding the light-harvesting protein of PSII and PSI (Lhcb1, 2, 3 and Lhca1, 2, 3) were recorded in IRAT10, H138, and H217, which facilitates light use efficiency. Higher electron transport rate and RuBP concentration were recorded in IRAT10, H138, and H217 flag leaves. Retinoblastoma-related gene (OsRBR1), exerting effects on mesophyll cell density, can be used to modify leaf anatomy for improving leaf photosynthesis. Additionally, higher stomatal conductance and mesophyll conductance were also recorded in H138 and H217 than in Sasanishiki. Furthermore, we modeled mesophyll conductance through anatomical traits, and the results revealed that chloroplast thickness was the dominant factor restricting CO₂ diffusion within mesophyll cells rather than cell wall thickness. Higher RuBP content accompanied by higher CO₂ concentration within the carboxylation set in H138 and H217 flag leaves contributed to higher CO₂ assimilation.