Variations in the responses of PSII to heat

The negative relationship between T5 and decline width (Figure 4) indicates a range of response strategies in trees. The two extremes of PSII thermal sensitivity could be described as ‘sensitive’ and ‘tolerant’ (See Figure 7). Tolerators were found to sustain their PSII QY values up to a remarkably high temperature point (high T5 ). They were characterised by a rapid decline to near zero levels at higher temperatures beyondT5 . Conversely, heat ‘sensitive’ responses are characterised by a sensitivity of PSII to much lower (lowT5 ) temperatures. The decline in PSII QY to near zero levels in these trees, however, occurs gradually over a very wide temperature range. Hence, in heat sensitive trees, when the leaf temperatures are at this ‘decline width’ range, PSII is already negatively affected by temperature stress. The slow decline could potentially indicate PSII protection mechanisms that ensue mediated by heat shock proteins and/or xanthophyll mediated thermal protection mechanisms. Given the change in PSII activity, it is reasonable to hypothesise that electron flow could switch from the non-cyclic pathway to cyclic electron transport around PSI, a mechanism that is known to protect PSII against thermal inactivation (Essemine, Xiao, Qu, Mi, & Zhu, 2017; Sun, Geng, Du, Yang, & Zhai, 2017).
High temperature tolerant trees, on the other hand have better protected PSII which can continue to function up to a very high temperature point. By sustaining PSII activity under extremely high leaf temperature conditions, tolerators could maintain better photosynthetic electron flow rates than thermosensitive species. The effective thermal sensitivity of heat tolerant trees could be linked to stomatal controls and transpiration cooling. However, evaporative cooling can reduce the need for investment in thermal tolerance mechanisms. If transpiration cooling is strong, the mechanisms that facilitate high PSII thermal tolerance might not be required. Hence, thermal tolerance of leaf metabolism must consider the molecular physiology of the leaf.