A New Approach to Measure Gross CO(2) Fluxes in Leaves. Gross CO(2) Assimilation, Photorespiration, and Mitochondrial Respiration in the Light in Tomato under Drought Stress

We developed a new method using (13)CO(2) and mass spectrometry to elucidate the role of photorespiration as an alternative electron dissipating pathway under drought stress. This was achieved by experimentally distinguishing between the CO(2) fluxes into and out of the leaf. The method allows us to determine the rates of gross CO(2) assimilation and gross CO(2) evolution in addition to net CO(2) uptake by attached leaves during steady-state photosynthesis. Furthermore, a comparison between measurements under photorespiratory and non-photorespiratory conditions may give information about the contribution of photorespiration and mitochondrial respiration to the rate of gross CO(2) evolution at photosynthetic steady state. In tomato (Lycopersicon esculentum Mill. cv Moneymaker) leaves, drought stress decreases the rates of net and gross CO(2) uptake as well as CO(2) release from photorespiration and mitochondrial respiration in the light. However, the ratio of photorespiratory CO(2) evolution to gross CO(2) assimilation rises with water deficit. Also the contribution of re-assimilation of (photo) respiratory CO(2) to gross CO(2) assimilation increases under drought.