Effects of elevated atmospheric CO2 concentration on leaf 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.