EFFECT OF SEVERE WATER-STRESS ON ASPECTS OF CRASSULACEAN ACID METABOLISM IN XEROSICYOS

Xerosicyos danguyi H. Humb. (Cucurbitaceae) is a Crassulacean acid met abolism (CAM) species native to Madagascar. Previously, it was shown t hat when grown under good water conditions, it is a typical CAM plant, but when water stressed, it shifts to a dampened form of CAM, termed CAM-idling, in which stomata are closed day and night but with a conti nued, low diurnal organic acid fluctuation. We have now studied the ki netics of some metabolic features of the shift from CAM to CAM-idling under severe water stress and the recovery upon rewatering. When water is withheld, there is a steady decrease in relative water content (RW C), reaching about 50%, at which point the water potential decreases p recipitously from about -2 or -3 bars to -12 bars. Abscisic acid (ABA) increases sharply at about 75% RWC. Stomata close, which limits CO2 u ptake, and there is a dampened diurnal organic acid fluctuation typica l of CAM-idling. Throughout an extended stress period to 50% RWC, ther e is no change in chlorophyll, protein, and ribulose bisphosphate carb oxylase activity compared with the well-watered plants. Despite the fa ct that the tissue was already in CAM, the stress is accompanied by an increase in phosphoenolpyruvate carboxylase (PEPc) mRNA, extractable PEPc activity, and PEPc protein (such that the specific activity remai ned approximately constant) and a decrease in the apparent K(m)(PEP). It is not known if the changes in K(m)(PEP) in response to drought are related to or are separate from the increases in PEPc protein and mRN A. The changes in K(m)(PEP) could be in response to the decreased endo genous levels of organic acids, but evidently are not an assay artifac t. The increases in PEPc protein and mRNA appear to be related to the water-stress treatment and may result from the increased concentration of ABA or the decreased levels of endogenous organic acids. When rewa tered, the metabolism quickly returns to the well-watered control typi cal of CAM.