THE INFLUENCE OF DIFFERENT CO2 CONCENTRATIONS IN THE LIGHT AND THE DARK ON DIURNAL MALATE RHYTHM AND PHOSPHOENOLPYRUVATE CARBOXYLASE ACTIVITIES IN LEAVES OF LITTORELLA-UNIFLORA (L) ASCHERS

In short-term experiments, Littorella uniflora (L.) Aschers. was exposed to a sequence of different CO2 supply situations. A direct relationship between CO2 supply and Crassulacean acid metabolism (CAM) activity was shown. The results were confirmed by control experiments applying different constant CO2 supply situations for a period of 20 days. A diurnal difference in malate concentrations of approximately 25-mu-mol g-1 wwt was found when the CO2 concentration was approximately 22-mu-mol l-1 (= air equilibrium) in the light phase and approximately 60-mu-mol l-1 in the dark phase. The diurnal change in malate concentration increased to approximately 45-mu-mol g-1 wwt when the CO2 concentration was increased in the dark phase to approximately 1.4 mmol l-1. With high CO2 supply (approximately 1.4 mmol l-1) in the light, diurnal changes in malate concentration ceased immediately. Phosphoenolpyruvate carboxylase (PEPC) activity was found to be dependent on CO2 concentration. It decreased with increasing CO2 supply, regardless of whether the CO2 concentration was increased in the dark or in the light phase. In spite of this effect, decreasing PEPC activities did not induce a switch off of diurnal malate rhythm. So far, no decisive role of in vitro PEPC activity in the modulation of CAM has been found.