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

Thermoregulation of leaf temperature (Tleaf) may foster metabolic homeostasis in plants, but the degree to which Tleaf 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 Tleaf on environmental drivers. Can this apparent disparity be reconciled? We continuously measured Tleaf 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 (TL-AW) across 265 d, varying in air temperature (Tair) from −1 to 45°C. We compared these data to TL-AW derived from wood cellulose δ18O. Tleaf exhibited substantial variation driven by Tair, light intensity, and vapor pressure deficit, and Tleaf was strongly linearly correlated with Tair with a slope of c. 1.0. TL-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 Tair. The leaves studied here were nearly poikilothermic, with no evidence of thermoregulation of Tleaf towards a homeostatic value. Importantly, this work supports the use of cellulose δ18O to infer TL-AW, but does not support the concept of strong homeothermic regulation of Tleaf.