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.