Effects of elevated atmospheric CO₂ concentration on leaf dark respiration of Xanthium strumarium in light and in darkness

Leaf dark respiration (R) is an important component of plant carbon balance, but the effects of rising atmospheric CO₂ on leaf R during illumination are largely unknown. We studied the effects of elevated CO₂ on leaf R in light (R_L) and in darkness (R_D) in Xanthium strumarium at different developmental stages. Leaf R_L was estimated by using the Kok method, whereas leaf R_D was measured as the rate of CO₂ efflux at zero light. Leaf R_L and R_D were significantly higher at elevated than at ambient CO₂ throughout the growing period. Elevated CO₂ increased the ratio of leaf R_L to net photosynthesis at saturated light (A_max) when plants were young and also after flowering, but the ratio of leaf R_D to A_max was unaffected by CO₂ levels. Leaf R_N was significantly higher at the beginning but significantly lower at the end of the growing period in elevated CO₂-grown plants. The ratio of leaf R_L to R_D was used to estimate the effect of light on leaf R during the day. We found that light inhibited leaf R at both CO₂ concentrations but to a lesser degree for elevated (17-24%) than for ambient (29-35%) CO₂-grown plants, presumably because elevated CO₂-grown plants had a higher demand for energy and carbon skeletons than ambient CO₂-grown plants in light. Our results suggest that using the CO₂ efflux rate, determined by shading leaves during the day, as a measure for leaf R is likely to underestimate carbon loss from elevated CO₂-grown plants.