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.