RAPID TRIGGERING OF MALATE ACCUMULATION IN THE C-3 CAM INTERMEDIATE PLANT SEDUM-TELEPHIUM - RELATIONSHIP WITH WATER STATUS AND PHOSPHOENOLPYRUVATE CARBOXYLASE/

Sedum telephium is a C-3/CAM intermediate plant in which expression of CAM is caused by water deficit. The timing of the C-3-CAM switch and its relationship with water status and phosphoenolpyruvate (PEP) carbo xylase activity have been investigated. Water deficit was provided by application of polyethylene glycol (PEG) solutions so that roots were exposed to water potentials from 0 to -2.0 MPa below that of the nutri ent solution. The response of the plants was measured during the first dark period after PEG addition and 7 d later. Malic acid accumulation was triggered during the first dark period at root water potentials o f -0.3 MPa or less. This corresponded with very small decreases in lea f water potential and relative water content. The capacity of PEP carb oxylase was not altered at any water potential during the first dark p eriod. After 7 d the capacity of PEP carboxylase progressively increas ed as water potential declined to -0.4 MPa. At this, and more negative , water potentials it was 5-fold higher than in well-watered leaves, M alic acid fluctuations increased with decreasing PEG water potential b elow a threshold of -0.1 MPa. Malic acid levels at the end of the ligh t period were progressively lower as water potential decreased. NAD- a nd NADP-malic enzyme activity were not affected by low water potential . Leaves detached from well-watered plants in the middle of the light period and kept hydrated did not accumulate malic acid during the foll owing dark period. Allowing the leaves to lose 10% of their water cont ent induced malic acid accumulation during the same time. Conversely, leaves detached from longterm droughted plants (which had malate fluct uations and a PEP carboxylase capacity 5-fold higher than well-watered plants) accumulated malate during the night if maintained at the same low hydration state (82% RWC), whereas malic acid accumulation was pr omptly reduced if they were rehydrated. Malic acid accumulation could therefore be rapidly altered by changing the hydration state of the le aves. The shortterm rehydration treatments did not alter PEP carboxyla se capacity. However, alteration of leaf hydration affected the appare nt K-m (PEP) of PEP carboxylase extracted 1 h before the end of the da rk period. The K-m was increased by rehydration and decreased by dehyd ration. Sensitivity to feedback inhibition by malate was not affected by hydration state and was high for PEP carboxylase from well-watered leaves and lower for PEP carboxylase from long-term droughted leaves. Taken together, the responses of intact plants and detached leaves sho w that malic acid accumulation can be triggered very rapidly by small water deficits in the leaves. The extent of night-time malic acid accu mulation is independent of PEP carboxylase capacity. However, a change in the hydration state of the leaves can rapidly alter the affinity o f PEP carboxylase for PEP. The regulation of malic acid accumulation i n relation to the drought-induced triggering of CAM is discussed.