Effect of different heat stress durations on the thermotolerance of tropical tree species and its relation with temperature response of carbon uptake

Abstract

As the frequency of heatwaves and their severity are increasing, understanding the upper thermal limits and thermal safety margins of different biomes has become important. This has been previously studied by various groups at the global level. Use of various assays to measure thermotolerance across different groups creates discrepancy to compare globally. Understanding the heat tolerance relationships with leaf functional traits will help in predicting the fate of the trees in the future warming periods. High thermotolerance is shown to promote carbon uptake by plants at higher temperatures. This study tries to understand if heat stress periods of 15 and 30 minutes has any effect on thermotolerance which is measured as the temperature at which Fv/Fm declines by 50% (T50) and temperature sensitivity in response curve (measured as the difference of T95 and T5). It was seen that variation temperature sensitivity in Fv/Fm response curve was species identity dependent and T50 decreased by an order of 1°C from 15 to 30 minutes of heat exposure period across 17 species. This suggests that studies of 15 and 30 minutes heat treatment durations are now comparable. In another experiment for two species, six heat stress periods (10, 15, 20, 30, 40 and 50 minutes) were given. T50 decreased while temperature sensitivity did not show significant change. In the study of leaf traits relationship with T50 for 17 species of Pune, India, LMA (leaf dry mass per unit area), LDMC (leaf dry weight by fresh weight) and Leaf area were estimated. LDMC and leaf Area showed positive and negative correlation with T50 respectively, suggesting that leaves with low LDMC and high area will be more susceptible to rising temperatures in future climate change scenarios. However, LMA did not show any correlation. This may be because of low sample size. To understand how thermotolerance parameters (T5 and T50) correlate with temperature response of carbon uptake parameters (Tmax, Topt, Popt and thermalbreadth) a separate study was done on 11 species of Sirsi, India. It was seen that T5 showed weak negative correlation with Tmax but T50 did not show any significant correlation with carbon uptake parameters suggesting that these events may be independent. Studying with larger sample size will be needed to understand these correlations.