High-resolution temperature responses of leaf respiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function

We tested whether snow gum (Eucalyptus pauciflora) trees growing in thermally contrasting environments exhibit generalizable temperature (T) response functions of leaf respiration (R) and fluorescence (F_o). Measurements were made on pot-grown saplings and field-grown trees (growing between 1380 and 2110m a.s.l.). Using a continuous, high-resolution protocol, we quantified T response curves of R and F_o - these data were used to identify an algorithm for modelling R-T curves at subcritical T's and establish variations in heat tolerance. For the latter, we quantified T_max [T where R is maximal] and T_crit [T where F_o rises rapidly]. T_max ranged from 51 to 57°C, varying with season (e.g. winter>summer). T_crit ranged from 41 to 49°C in summer and from 58 to 63°C in winter. Thus, surprisingly, leaf energy metabolism was more heat-tolerant in trees experiencing ice-encasement in winter than warmer conditions in summer. A polynomial model fitted to log-transformed R data provided the best description of the T-sensitivity of R (between 10 and 45°C); using these model fits, we found that the negative slope of the Q₁₀-T relationship was greater in winter than in summer. Collectively, our results (1) highlight high-T limits of energy metabolism in E.pauciflora and (2) provide a framework for improving representation of T-responses of leaf R in predictive models.