INHIBITION OF PHOTOSYNTHESIS IN DUCKWEED BY ELEVATED CO2 CONCENTRATION IS RAPID AND IS NOT OFFSET BY A TEMPERATURE-INDUCED INCREASE IN METABOLIC-RATE

The rates of net photosynthesis (P(N)), respiration and growth of Lemn a gibba L. were measured as functions of time across ranges of tempera ture, irradiance and carbon dioxide concentrations. P(N) on an area ba sis increased with temperature up to 30-degrees-C but decreased dramat ically with a few hours of exposure to elevated CO2, when reported on a dry mass basis. Reductions in the apparent quantum efficiency, photo synthetic capacity and the affinity of ribulose- 1,5-bisphosphate carb oxylase/oxygenase for CO2 were observed for plants grown at elevated C O2. Starch concentration was not significantly affected by elevated CO 2. Although elevated temperature increased metabolic activity, it only partially alleviated the inhibition of P(N). L. gibba exhibits a char acteristic C3-type response to elevated CO2 and the methodology descri bed is useful for further elucidating the mechanism of photosynthetic acclimation to elevated CO2.