Impacts of drought on leaf respiration in darkness and light in Eucalyptus saligna exposed to industrial-age atmospheric CO₂ and growth temperature

Our study assessed the impact of a wide range of industrial-age climate scenarios on leaf respiration (R) in Eucalyptus saligna. Well-watered or sustained drought-treated plants were grown in glasshouses differing in atmospheric CO₂ concentration ([CO₂]) (280, 400 and 640μll^-1) and temperature (26 and 30°C). Rates of R in darkness (R_dark) and light (R_light), photosynthesis (A) and related leaf traits (mass area relationships, and nitrogen, phosphorus, starch and sugar concentrations) were measured. Light inhibited R in all cases (R_light<R_dark) (well-watered: 40%; drought-treated: 73%). Growth [CO₂] and temperature had little impact on area-based rates of R_dark or R_light, with R_light exhibiting minimal thermal acclimation. By contrast, sustained drought resulted in reduced R_dark, R_light and A, with the inhibitory effect of drought on A and R_light (c. 50-70%) greater than that on R_dark (c. 15%). Drought effects were fully reversible after watering. Variability in R_light appeared to be dependent on the underlying rate of R_dark and associated Rubisco activity. Collectively, our data suggest that there is an asynchronous response of leaf carbon metabolism to drought, and a tighter coupling between R_light and A than between R_dark and A, under both past and future climate scenarios. These findings have important implications for ecosystem/global models seeking to predict carbon cycling.