Thermal safety margin (TSM) means the difference between body temperature (leaf temperature for plants) and critical temperatures that represent threshold for function or lethality. It was proposed to assess thermal safety and has been widely applied in animal studies, but its application in plants is lacking.  

In a study published in Science of the Total Environment, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences tested whether the maximum leaf temperatures vary proportionately to thermotolerance across biomes.  

The researchers investigated thermal safety margin of photosystem II (thermotolerance of PSII – the maximum leaf temperature) of dominant canopy plants in four forests across biomes (savanna, tropical rain forest, subtropical broad-leaved forest, temperate mixed forest) during a heatwave. They then compared the thermal safety margins calculated by leaf temperature and air temperature respectively. They also investigated thermal related leaf traits. 

They found that both leaf temperature and air temperature of thermal safety margin increased from hot forest to cold forest. The air temperature overestimated thermal safety margin, especially at cool biomes. During the heatwave, thermal safety margins of savanna species were the lowest, therefore, savanna species were more at risk under heat stress.  

They further found a negative relationship between thermal safety margin and thermal tolerance across biomes. The thermal safety margin decreased with the maximum leaf temperature, while the critical temperature increased with maximum leaf temperature.  

Therefore, the maximum leaf temperature instead of thermotolerance can predict thermal safety margin of plant leaves very well. 

"Our results are valuable for understanding the impact of heat stress on plant leaves, and can be applied in forest management," said LIN Hua, correspondence author of the study.

Savanna species are more at risk under heat stress. (Image by LIN Hua)