Diurnal and seasonal variation in light and dark respiration in field-grown Eucalyptus pauciflora.

Respiration from vegetation is a substantial part of the global carbon cycle and the responses of plant respiration to daily and seasonal fluctuations in temperature and light must be incorporated in models of terrestrial respiration to accurately predict these CO2 fluxes. We investigated how leaf respiration (R) responded to changes in leaf temperature (T(leaf)) and irradiance in field-grown saplings of an evergreen tree (Eucalyptus pauciflora Sieb. ex Spreng). Seasonal shifts in the thermal sensitivity of leaf R in the dark (R(dark)) and in the light (R(light)) were assessed by allowing T(leaf) to vary over the day in field-grown leaves over a year. The Q10 of R (i.e., the relative increase in R for a 10 °C increase in T(leaf)) was similar for R(light) and R(dark) and had a value of ∼ 2.5; there was little seasonal change in the Q10 of either R(light) or R(dark), indicating that we may be able to use similar functions to model short-term temperature responses of R in the dark and in the light. Overall, rates of R(light) were lower than those of R(dark), and the ratio of R(light)/R(dark) tended to increase with rising T(leaf), such that light suppression of R was reduced at high T(leaf) values, in contrast to earlier work with this species. Our results suggest we cannot assume that R(light)/R(dark) decreases with increasing T(leaf) on daily timescales, and highlights the need for a better mechanistic understanding of what regulates light suppression of R in leaves.