Light inhibition of leaf respiration in field-grown Eucalyptus saligna in whole-tree chambers under elevated atmospheric CO₂ and summer drought

SUMMARY: We investigated whether the degree of light inhibition of leaf respiration (R) differs among large Eucalyptus saligna grown in whole-tree chambers and exposed to present and future atmospheric [CO₂] and summer drought. Associated with month-to-month changes in temperature were concomitant changes in R in the light (R_light) and darkness (R_dark), with both processes being more temperature dependent in well-watered trees than under drought. Overall rates of R_light and R_dark were not significantly affected by [CO₂]. By contrast, overall rates of R_dark (averaged across both [CO₂]) were ca. 25% lower under drought than in well-watered trees. During summer, the degree of light inhibition of leaf R was greater in droughted (ca. 80% inhibition) than well-watered trees (ca. 50% inhibition). Notwithstanding these treatment differences, an overall positive relationship was observed between R_light and R_dark when data from all months/treatments were combined (R²=0.8). Variations in R_light were also positively correlated with rates of Rubisco activity and nitrogen concentration. Light inhibition resulted in a marked decrease in the proportion of light-saturated photosynthesis respired (i.e. reduced R/A_sat). Collectively, these results highlight the need to account for light inhibition when assessing impacts of global change drivers on the carbon economy of tree canopies. We investigated whether rates of leaf respiration in the light (R_light) and darkness (R_dark) differ among large Eucalyptus saligna grown in whole-tree chambers and exposed to present and future atmospheric [CO₂] and summer drought. Although both processes were unaffected by [CO₂], marked effects of drought were found, with light inhibition of leaf R (which decreased the proportion of light-saturated photosynthesis respired) being greatest in drought-stressed trees.