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

Thermoregulation of leaf temperature (T ) may foster metabolic homeostasis in plants, but the degree to which T 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 on environmental drivers. Can this apparent disparity be reconciled? We continuously measured T and whole-crown net CO 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 ) across 265 d, varying in air temperature (T ) from -1 to 45°C. We compared these data to T derived from wood cellulose δ O. T exhibited substantial variation driven by T , light intensity, and vapor pressure deficit, and T was strongly linearly correlated with T with a slope of c. 1.0. T values calculated from cellulose δ O vs crown fluxes were remarkably consistent; both varied seasonally and in response to the warming treatment, tracking variation in T . The leaves studied here were nearly poikilothermic, with no evidence of thermoregulation of T towards a homeostatic value. Importantly, this work supports the use of cellulose δ O to infer T , but does not support the concept of strong homeothermic regulation of T.