THE EFFECT OF TEMPERATURE AND LIGHT ON GAS-EXCHANGE AND ACID ACCUMULATION IN THE C3-CAM PLANT CLUSIA-MINOR L

Gas exchange and organic acid accumulation of the C3-CAM intermediate Clusia minor L. were investigated in response to various day/night temperatures and two light regimes (low and high PAR). For both light levels equal day/night temperatures between 20-degrees-C and 30-degrees-C caused a typical C3 gas exchange pattern with all CO2 uptake occurring during daylight hours. A day/night temperature of 15-degrees-C caused a negative CO2 balance over a 24 h period for low-PAR-grown plants while high-PAR-grown plants showed a CAM gas exchange pattern with most CO2 uptake taking place during the dark period. However, there was always a considerable night-time accumulation of malic acid which increased when the night-time temperature was lowered and had its maximum (54 mmol m-2) at day/night temperature of 30/15-degrees-C. A significant amount of malic acid accumulation (23 mmol m-2) in low-PAR-grown plants was observed only at 30/15-degrees-C. Recycling of respiratory CO2 in terms of malic acid accumulation reached between 2.0 and 21.5 mmol m-2 for high-PAR-grown plants while there was no significant recycling for low-PAR-grown plants. Both low and high-PAR-grown plants showed considerable night-time accumulation of citric acid. Indeed under several temperature regimes low-PAR-grown plants showed day/night changes in citric acid levels whereas malic acid levels remained approximately constant or slightly decreased. It is hypothesized that low and high-PAR-grown plants have different requirements for citrate. In high-PAR-grown plants, the breakdown of citrate prevents photoinhibition by increasing internal CO2 levels, whereas in low-PAR-grown plants the night-time accumulation of citric acid may function as an energy and carbon saving mechanism.