Light Intensity Alters Thermal Tolerance During Heat and Cold Stress in Canopy Plant Species

Climate change increases extreme weather events like heat waves and cold waves. Understanding plant thermal tolerance is essential for assessing their safety. Measuring the Fv/Fm after dark thermal treatments is a common way to evaluate photosynthetic thermal tolerance, but it differs from real-world conditions with light-temperature covariation. To investigate how light influences thermal tolerance measurement, we tested the photosynthetic thermal tolerance of 20 naturally growing canopy species from Xishuangbanna Tropical Botanical Garden under various light intensities. We found that weak light (300 μmol·m^-²·s^-¹) triggered species-specific responses in heat tolerance, while strong light (1000 μmol·m^-²·s^-¹) reduced heat tolerance across all tested species, except for Psidium guajava in T50_h. Weak light (200 μmol·m^-²·s^-¹) significantly reduced the cold tolerance of all 10 tested species. The species with higher gas exchange rate (E_max and A_max) were more resistant to light-induced damage under heat stress. Leaf thickness mitigated the reduction of heat tolerance caused by strong light and provided physical photoprotection under cold stress. These findings highlight the importance of considering both species-specific photoprotective capacities and ecological realism when evaluating photosynthetic thermal limits. This study helps us understand plant adaptation strategies and predict ecosystem responses to climate change.