Tropical forest trees with higher thermal optima of photosynthesis exhibit lower PSII heat stability

Tropical tree species vary in photosynthetic temperature sensitivity, with species from warmer habitats or those acclimated to higher temperatures typically displaying higher thermal optima for net CO2 assimilation (Topt,Anet). Sustaining photosynthesis at elevated temperatures probably requires increased allocation of resources (ATP, NADPH, nitrogen, and carbon) towards heat stress management, particularly PSII repair. However, under extreme heat, repair demands may exceed available resources, potentially limiting acclimation. It is unclear whether higher Topt,Anet reflects inherently greater PSII heat stability. We studied 11 tropical tree species across a topographic (hilltop, slope, and valley) and thermal gradient (summer peaks: 46.1, 40.1, and 31.8 °C, respectively) in India's Central Western Ghats forest, measuring photosynthetic temperature responses and PSII thermal tolerance (T5, the temperature causing 5% PSII efficiency decline) at peak summer. We found an inverse correlation between T5 and Topt,Anet (P=0.005): lower Topt,Anet was associated with higher PSII heat stability (higher T5), and vice versa. This could suggest a trade-off between investing resources to achieve higher Topt,Anet and maintaining PSII heat stability. Species may struggle to simultaneously acclimate to elevated temperatures and remain resilient to extreme heat events. These findings have implications for understanding tropical forest tree responses to climate warming.