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 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 CO ₂ 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 δ ¹⁸ O. 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 δ ¹⁸ O 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 δ ¹⁸ O to infer T _L-AW , but does not support the concept of strong homeothermic regulation of T _leaf .
(© 2020 The Authors. New Phytologist © 2020 New Phytologist Foundation.)