PROPERTIES OF PHOSPHOENOLPYRUVATE CARBOXYLASE AND CARBOHYDRATE ACCUMULATION IN THE C3-CAM INTERMEDIATE SEDUM-TELEPHIUM L GROWN UNDER DIFFERENT LIGHT AND WATERING REGIMES

A comparison of the activity and properties of the enzyme phosphoenolpyruvate carboxylase (PEPC) was made for plants of Sedum telephium L. grown under low (70-mu-mol m-2 s-1) or high (500-mu-mol m-2 s-1) PPFD and subjected to varying degrees of water stress. Under well-watered conditions only plants grown under high PPFD accumulated titratable acidity overnight and the extractable activity of PEPC was almost 2-fold higher in these plants than in plants grown under low PPFD. Increasing drought stress resulted in a substantial increase in the activity of PEPC extracted both during the light and dark periods and a decrease in the sensitivity to inhibition by malic acid. The magnitude of these changes was determined by the severity and duration of drought and by light intensity. A comparison of the kinetic properties of PEPC from severely droughted plants revealed that plants droughted under high PPFD had a lower K(m) for PEP than plants under low PPFD. Addition of 2.0 mol m-3 malate resulted in an increase in the K(m) for PEP, with plants droughted under low PPFD having a significantly higher K(m) in the presence of malic acid compared to those under high PPFD. Response to the activator glc-6-P, which lowered the K(m) for PEP, also varied between plants grown under the two light regimes. Under well-watered conditions PEPC extracted from plants under high PPFD was more sensitive to activation by glc-6-P than those under low PPFD. After the severe drought treatment, however, the K(m) for PEP in the presence of glc-6-P was similar for enzyme extracted from plants grown under both light regimes. Soluble sugars and starch were depleted overnight and were both possible sources of substrate for PEPC. With increasing drought, however, the depletion of starch relative to soluble sugars increased under both light regimes. The properties of PEPC and the characteristics of carbohydrate accumulation/depletion are discussed in relation to the regulation of CAM in S. telephium grown under different light and watering regimes.