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 expressionof CAM is caused by water deficit. The timing of the C₃-CAMswitch and its relationship with water status and phosphoenolpyruvate(PEP) carboxylase activity have been investigated. Water deficitwas provided by application of polyethylene glycol (PEG) solutionsso that roots were exposed to water potentials from 0 to –2.0 MPa below that of the nutrient solution. The response ofthe plants was measured during the first dark period after PEGaddition and 7 d later. Malic acid accumulation was triggeredduring the first dark period at root water potentials of –0.3MPa or less. This corresponded with very small decreases inleaf water potential and relative water content. The capacityof PEP carboxylase was not altered at any water potential duringthe first dark period. After 7 d the capacity of PEP carboxylaseprogressively increased as water potential declined to –0.4MPa. At this, and more negative, water potentials it was 5-foldhigher than in well-watered leaves. Malic acid fluctuationsincreased with decreasing PEG water potential below a thresholdof –0.1 MPa. Malic acid levels at the end of the lightperiod were progressively lower as water potential decreased.NAD- and NADP-malic enzyme activity were not affected by lowwater potential. Leaves detached from well-watered plants in the middle of thelight period and kept hydrated did not accumulate malic acidduring the following dark period. Allowing the leaves to lose10% of their water content induced malic acid accumulation duringthe same time. Conversely, leaves detached from long-term droughtedplants (which had malate fluctuations and a PEP carboxylasecapacity 5-fold higher than well-watered plants) accumulatedmalate during the night if maintained at the same low hydrationstate (82%RWC), whereas malic acid accumulation was promptlyreduced if they were rehydrated. Malic acid accumulation couldtherefore be rapidly altered by changing the hydration stateof the leaves. The short-term rehydration treatments did notalter PEP carboxylase capacity. However, alteration of leafhydration affected the apparent K_m (PEP) of PEP carboxylaseextracted 1 h before the end of the dark period. The K_m wasincreased by rehydration and decreased by dehydration. Sensitivityto feedback inhibition by malate was not affected by hydrationstate and was high for PEP carboxylase from well-watered leavesand lower for PEP carboxylase from long-term droughted leaves. Taken together, the responses of intact plants and detachedleaves show that malic acid accumulation can be triggered veryrapidly by small water deficits in the leaves. The extent ofnight-time malic acid accumulation is independent of PEP carboxylasecapacity. However, a change in the hydration state of the leavescan rapidly alter the affinity of PEP carboxylase for PEP. Theregulation of malic acid accumulation in relation to the drought-inducedtriggering of CAM is discussed. Key words: Crassulacean acid metabolism, water stress, Sedum telephium, phosphoenolpyruvate carboxylase (PEP carboxylase), malic enzyme