No evidence of homeostatic regulation of leaf temperature in <i>Eucalyptus parramattensis</i> trees: integration of CO<sub>2</sub> flux and oxygen isotope methodologies
John E. Drake, Richard R. Harwood, Angelica Vårhammar, Margaret M. Barbour, Peter B. Reich, Craig V. M. Barton, Mark G. Tjoelker
Abstract
Summary 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