Thermal sensitivity thresholds for photosystem II

A key question concerns how close tree species are to their thermal sensitivity thresholds in the warmest region of the Amazon? The data presented here show that T50 for seven focal species is at least 4 °C above the highest air temperatures ever experienced at the study site (43.9 °C). However, for most species, T5 , the breakpoint temperature, is already surpassed in the peak of the warm/dry season (Figure 6). Thus, the site temperatures are already reaching levels where incipient irreversible loss of PSII activity occurs during the dry/warm periods. Such periods are predicted to become more frequent with future warming (Yao, Luo, Huang, & Zhao, 2013). However, these inferences are based on air temperatures, while leaf temperatures are arguably a more meaningful measure of proximity to thermal sensitivity thresholds. During the dry/warm season, limited access to soil moisture restricts stomatal opening. This is likely to lead to leaf temperatures that exceed air temperatures (G. Heinrich Krause et al., 2010; Slot & Winter, 2016), at least for part of the day. Under these circumstances, plants in our study area, especially those exposed to full sunlight, may operate closer to thermal sensitivity thresholds than the simple examination of air temperatures suggests.