No evidence of homeostatic regulation of leaf temperature in Eucalyptus parramattensis trees: integration of CO₂ flux and oxygen isotope methodologies

Thermoregulation of leaf temperature (T_leaf) may foster metabolic homeostasis in plants, but the degree to which T_leaf is moderated, and under what environmental contexts, is a topic of debate. Isotopic studies inferred the temperature of photosynthetic carbon assimilation to be a constant value of c. 20°C; by contrast, leaf biophysical theory suggests a strong dependence of T_leaf on environmental drivers. Can this apparent disparity be reconciled? We continuously measured T_leaf and whole-crown net CO₂ uptake for Eucalyptus parramattensis trees growing in field conditions in whole-tree chambers under ambient and +3°C warming conditions, and calculated assimilation-weighted leaf temperature (T_L-AW) across 265 d, varying in air temperature (T_air) from −1 to 45°C. We compared these data to T_L-AW derived from wood cellulose δ¹⁸O. T_leaf exhibited substantial variation driven by T_air, light intensity, and vapor pressure deficit, and T_leaf was strongly linearly correlated with T_air with a slope of c. 1.0. T_L-AW values calculated from cellulose δ¹⁸O vs crown fluxes were remarkably consistent; both varied seasonally and in response to the warming treatment, tracking variation in T_air. The leaves studied here were nearly poikilothermic, with no evidence of thermoregulation of T_leaf towards a homeostatic value. Importantly, this work supports the use of cellulose δ¹⁸O to infer T_L-AW, but does not support the concept of strong homeothermic regulation of T_leaf.