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 Q(10) of R (i.e., the relative increase in R for a 10 degrees C increase in T-leaf) was similar for R-light and R-dark and had a value of similar to 2.5; there was little seasonal change in the Q(10) 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.