Abstract:

Objective: This study aims to investigate the effect of different heat wave frequencies and contrasting water additions on the photosynthetic characteristics and growth ofSchima superba seedlings, so as to provide theoretical basis for revealing its adaptation mechanism to extreme high temperature events. Method: In this study, S. superba seedlings, a common broad-leaved tree species, were chosen as the experimental materials. An artificial heat wave simulation device was used to simulate the occurrence of natural heat waves. The seedlings were subjected to water control and replenishment treatments with different heat wave frequencies (no heat wave, single heat wave, two heat waves). The differences in the photosynthetic characteristics and growth rate of S. superba seedlings were analyzed for their resistance and resilience ability to different frequency of heat wave stress. Result: 1) With water addition, the single heat wave treatment had insignificant effects on the photosynthetic characteristics. S. superba was able to regulate leaf temperature by significantly increasing its transpiration rate (Tr), thus reducing water use efficiency (WUE) significantly, but there was no significant change in net photosynthetic rate (P_n). 2) Under controlled water supply condition, a single heat wave significantly decreased S. superba P_n by 49%, while there were no significant changes in Tr and WUE. However, the treatments significantly slowed down the recovery process of photosynthetic characteristics of S. superba. 3) Under the influence of two heat waves, regardless of water control and replenishment conditions, S. superba P_n significantly decreased (about 46% and 32%) during heat wave stage, respectively. Although it was able to still recover after the heat wave, the recovery process was hindered. The effects of multiple heat waves were additive, manifested as a weakening of the resistance and recovery ability of S. superba to heat waves. Conclusion: Our results indicate that the photosynthetic characteristics and growth rates of S. superba demonstrate divergent resistance and resilience to heat waves with the different water supply. The seedlings can maintain high temperature resistance through physiological regulation mechanism, but this mechanism would be disturbed when water supply is not adequate. Moreover, the interaction of high temperature and drought significantly affect the photosynthetic characteristics and growth rate of S. superba, and the effects of compound stress are stronger than single factor effects. These results suggest that the occurrence of combined seasonal high temperature and drought stress in subtropical regions, or the occurrence of high-frequency successive heat waves may significantly change the structure and function ofS. superba natural forests and plantations, as well as its stability and sustainability.

Key words: climate extremes, water supply, drought, net photosynthetic rate, resistance, recovery, heat wave