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.