Ecophysiological relevance of PSII QY temperature sensitivity

During the dry and hot period of the seasonal cycle, leaf to air temperature differentials (TleafTair ), in evergreen trees in some tropical sites have been measured to be as high as 18°C (Fauset et al., 2018; Pau, Detto, Kim, & Still, 2018; Tribuzy, 2005). Furthermore, during the parts of the day when transpirational cooling capacity is limited by stomatal closure (Slot et al., 2018), leaf temperatures reach as high as 48°C in some tropical trees (Slot & Winter, 2016). Comparing these leaf temperatures with the range of T5 measured, consistent with the ideas presented in Mau, Reed, Wood, and Cavaleri (2018) and Doughty and Goulden (2008), it is likely that some sensitive species are already experiencing temperatures that are affecting PSII functioning. Given the species level variability and largely unexplored mechanisms of thermal protection strategies of trees, it is probable that there will be varied range of response to high temperature conditions across species. Early leaf senescence is triggered by exposure to high temperatures (De la Haba, De la Mata, Molina, & Agüera, 2014; Way, 2013). Strategies such as increased leaf turnover rates could therefore be triggered by high temperatures with implications for carbon acquisition by the trees. Transpirational cooling mechanisms under high temperatures could be limited and vary between species, making species with less water available for cooling more susceptible to thermal stress. However, some species continue to transpire even at extremely high temperatures (Drake et al., 2018).
The Fv /Fm orF0 based fluorescence data only indicate the responses of PSII (See Supporting Information Figure S3 for Fv /Fm andF0 relationship). However, the limits of other processes such as thermal protection, stomatal and metabolic controls of CO2 assimilation are largely unknown except in a few model plant species. Hence, such integrated definitions of thermal thresholds could be more meaningful and provide a better understanding of the thermal sensitivity of trees or plants in general. Thus, concepts such as thermal safety margins (O’Sullivan et al., 2017; Sunday et al., 2014) could be expanded further to derive ecophysiologically meaningful conclusions (See Supporting Information 7 and Table S4).
Overall, the data presented here show that there is significant diversity in the temperature sensitivity of photosynthesis across tropical evergreen trees (Cseh et al., 2005; Holm, Várkonyi, Kovács, Posselt, & Garab, 2005), much of which remains uncharacterised. It is probable that with increasing temperatures, especially during the summer, the leaf temperature differences between species, together with differences in transpiration cooling capacity, will reveal large interspecific variations in tree sensitivity to extreme heat conditions.