A new approach to measure gross CO2 fluxes in leaves. Gross CO2 assimilation, photorespiration, and mitochondrial respiration in the light in tomato under drought stress

We developed a new method using (CO2)-C-13 and mass spectrometry to elucida te the role of photsrespiration as an alternative electron dissipating path way under drought stress. This was achieved by experimentally distinguishin g between the CO2 fluxes into and out of the leaf. The method allows us to determine the rates of gross CO2 assimilation and gross CO2 evolution in ad dition to net CO2 uptake by attached leaves during steady-state photosynthe sis. Furthermore, a comparison between measurements under photorespiratory and non-photorespiratory conditions may give information about the contribu tion of photorespiration and mitochondrial respiration to the rate of gross CO2 evolution at photosynthetic steady state;In tomato (Lycopersicon escul entum Mill. cv Moneymaker) leaves, drought stress decreases the rates of ne t and gross CO2 uptake as well as CO2 release from photorespiration and mit ochondrial respiration in the light. However, the ratio of photorespiratory CO2 evolution to gross CO2 assimilation rises with water deficit. Also the contribution of re-assimilation of (photo) respiratory CO2 to gross CO2 as similation increases under drought.