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

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