No evidence of homeostatic regulation of leaf temperature in Eucalyptus parramattensis trees: integration of CO 2 flux and oxygen isotope methodologies.

Thermoregulation of leaf temperature (T-leaf) may foster metabolic homeostasis in plants, but the degree to whichT(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 degrees C; by contrast, leaf biophysical theory suggests a strong dependence ofT(leaf)on environmental drivers. Can this apparent disparity be reconciled? We continuously measuredT(leaf)and whole-crown net CO(2)uptake forEucalyptus parramattensistrees growing in field conditions in whole-tree chambers under ambient and +3 degrees C warming conditions, and calculated assimilation-weighted leaf temperature (TL-AW) across 265 d, varying in air temperature (T-air) from -1 to 45 degrees C. We compared these data toT(L-AW)derived from wood cellulose delta O-18. T(leaf)exhibited substantial variation driven byT(air), light intensity, and vapor pressure deficit, andT(leaf)was strongly linearly correlated withT(air)with a slope of c. 1.0.T(L-AW)values calculated from cellulose delta O-18 vs crown fluxes were remarkably consistent; both varied seasonally and in response to the warming treatment, tracking variation inT(air). The leaves studied here were nearly poikilothermic, with no evidence of thermoregulation ofT(leaf)towards a homeostatic value. Importantly, this work supports the use of cellulose delta O-18 to inferT(L-AW), but does not support the concept of strong homeothermic regulation ofT(leaf)