As climate change fuels more frequent heatwaves and cold spells, a plant's ability to tolerate temperature extremes is vital for its survival. This tolerance is often assessed by measuring the chlorophyll fluorescence parameter Fv/Fm in darkness. But in reality, high temperatures often accompany strong light, while low temperatures coexist with weak light.

In a study published in Plant, Cell & Environment, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences and their collaborators revealed that light intensity significantly influences how well plants can withstand extreme temperatures, challenging current assessment methods conducted in darkness.

Researchers tested the photosynthetic thermal tolerance of 20 naturally grown tree species in XTBG, and measured the thermal tolerance of Photosystem II (PSII) under heat stress at different light intensities and cold tolerance under low light during cold stress. 

They found that the effects of weak light on thermal tolerance were species-specific, and weak light significantly reduced the cold tolerance of all 10 tested species. They also found that strong light significantly reduced thermal tolerance in all tested species except for a non-significant change in Psidium guajava. Under strong light, the critical temperature threshold for heat damage decreased by 1.96°C to 13.96°C, and the temperature causing 50% damage decreased by 0.92°C to 5.26°C.

Moreover, species with higher maximum photosynthetic (A_max) and transpiration (E_max) rates showed greater resistance to light-induced damage during heat stress. Thicker leaves helped mitigate the reduction in thermal tolerance caused by strong light and provided physical protection against light during cold stress.

This study shows that while measurements taken in darkness are used to compare differences between different species, they overestimate a plant's true temperature tolerance capacity in natural environments where light is always present. "This means that assessments of plant thermal safety margins based solely on dark-adapted measurements may be inaccurate," said LIN Hua from XTBG.

This study provides insights into how plant vulnerability to climate extremes should be evaluated, and has crucial implications for understanding plant resilience under climate change.