Shade tolerance is associated with foliar adaptations, improved radiation use efficiency, and photosynthetic rate in rice

Low-light (LL) stress imposes a major constraint on rice yield in densely planted and monsoonal environments, yet the mechanistic basis of shade tolerance remains insufficiently resolved. We investigated four rice genotypes under simulated LL conditions, including two LL-tolerant varieties (Purnendu and Swarnaprabha) and two LL-susceptible varieties (IR64 and IR8). Responses were systematically analysed from the flag leaf to the fourth leaf. Comprehensive evaluation included measurements of light interception, chlorophyll fluorescence, gas exchange, carbohydrate content, chloroplast ultrastructure, and the expression of fourteen photosynthesis-related genes. Our findings demonstrate that LL tolerance in rice cannot be explained by adaptation of a single leaf; rather, it results from a coordinated strategy involving integrated changes at morphological, physiological, biochemical, and gene expression levels throughout the entire canopy. Tolerant genotypes exhibited only 30-35% loss in photons from the flag leaf to the fourth leaf, whereas susceptible genotypes lost up to 75%. Architectural traits such as plant height, leaf area plasticity, and leaf angle accounted for 84% of the variation in radiation use efficiency, while tolerant genotypes sustained higher photosynthetic efficiency, carbohydrate reserves, and robust gene expression across all layers. These insights identify concrete trait targets for breeding LL-resilient rice varieties, supporting stable yield in dense and light-limited environments.