Rapid triggering of malate accumulation in the C3/CAM intermediate plant Sedum telephium: relationship with water status and phosphoenolpyruvate carboxylase

Sedum telephium is a C3/CAM intermediate plant in which expression of CAM is caused by water deficit. The timing of the C3-CAM switch and its relationship with water status and phosphoenolpyruvate (PEP) carboxylase 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 nutrient 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 of −0.3 MPa or less. This corresponded with very small decreases in leaf water potential and relative water content. The capacity of PEP carboxylase was not altered at any water potential during the first dark period. After 7 d the capacity of PEP carboxylase progressively increased 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. Malic acid fluctuations increased with decreasing PEG water potential below a threshold of −0.1 MPa. Malic acid levels at the end of the light period were progressively lower as water potential decreased. NAD- and 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 following dark period. Allowing the leaves to lose 10% of their water content induced malic acid accumulation during the same time. Conversely, leaves detached from long-term droughted plants (which had malate fluctuations 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 promptly reduced if they were rehydrated. Malic acid accumulation could therefore be rapidly altered by changing the hydration state of the leaves. The short-term rehydration treatments did not alter PEP carboxylase capacity. However, alteration of leaf hydration affected the apparent Km (PEP) of PEP carboxylase extracted 1 h before the end of the dark period. The Km was increased by rehydration and decreased by dehydration. 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 show that malic acid accumulation can be triggered very rapidly by small water deficits in the leaves. The extent of night-time malic acid accumulation is independent of PEP carboxylase capacity. However, a change in the hydration state of the leaves can rapidly alter the affinity of PEP carboxylase for PEP. The regulation of malic acid accumulation in relation to the drought-induced triggering of CAM is discussed.