Temperature-Dependent and Temperature-Independent Changes in Photosynthetic Traits Across the Day in the Tropical Tree Calophyllum inophyllum

Abstract

Tropical forests play a crucial role in global carbon cycling but face increasing stress from rising temperatures and atmospheric drought (vapor pressure deficit, VPD). While most studies assess photosynthetic performance at fixed time points, short-term diurnal variation in photosynthetic parameters remains underexplored. We examined how the optimum temperature of photosynthesis, photosynthetic capacity, electron transport rate, and thermal tolerance vary throughout the day in tropical tree species Calophyllum inophyllum. Morning and afternoon measurements were conducted using gas exchange (FAsTeR and Dynamic A-Ci methods) and chlorophyll fluorescence. Photosynthetic capacity declined significantly in the afternoon, with lower light-saturated photosynthesis rates, A_max (−29%), maximum carboxylation rate, Vc_max (−32%), and triose-phosphate utilization rate, TPU (−21%) than in the morning, despite consistent measurement conditions. These declines were not driven by stomatal or mesophyll conductance, suggesting biochemical downregulation. In contrast, thermal tolerance metrics (T₅₀ and T_crit) remained stable between morning and afternoon, indicating decoupling between heat tolerance and metabolic performance. Our findings support a physiological strategy in C. inophyllum that prioritizes morning carbon gain and modulates photosynthetic efficiency throughout the day without compromising heat tolerance. This dynamic regulation underscores the need to account for diurnal variation and temperature-sensitive constraints into models of tropical forest function. Relying on photosynthetic capacity estimated under favorable morning conditions risks overestimating carbon uptake under future climate scenarios.