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 bath 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.
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