Ecophysiological relevance of PSII QY temperature
sensitivity
During the dry and hot period of the seasonal cycle, leaf to air
temperature differentials (Tleaf –Tair ), 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.