Regulation of Light Absorption and Energy Dissipation in Sweet Sorghum Under Climate-Relevant CO2 and Temperature Conditions

Understanding how environmental factors regulate photosynthetic energy partitioning is crucial for enhancing crop resilience in future climates. This study investigated the light-response dynamics of sweet sorghum (Sorghum bicolor L. Moench) leaves under combinations of CO2 concentrations (250, 410, and 550 μmol mol−1) and temperatures (30 °C and 35 °C), using integrated chlorophyll fluorescence measurements and mechanistic photosynthesis modeling. Our results revealed that elevating CO2 from 250 to 550 μmol mol−1 significantly increased the maximum electron transport rate (Jmax) by up to 57%, and enhanced the effective light absorption cross-section (σ′ik) by 64% under high light and elevated temperature (35 °C), indicating improved photochemical efficiency and light-harvesting capability. Concurrently, these adjustments reduced PSII down-regulation. Increased temperature stimulated thermal dissipation, reflected in a rise in non-photochemical quenching (NPQ) by 0.13–0.26 units, accompanied by a reduction in the number of excited-state pigment molecules (Nk) by 20–33%. The strongly coordinated responses between quantum yield (ΦPSII) and σ′ik highlight a dynamic balance among photochemistry, heat dissipation, and fluorescence. These findings elucidate the synergistic photoprotective and energy-partitioning strategies that sweet sorghum employs under combined CO2 enrichment and heat stress, providing mechanistic insights for optimizing photosynthetic performance in C4 crops in a changing climate.