AGE-SPECIFIC CHANGES OF ACIDITY, PHOSPHOENOLPYRUVATE CARBOXYLASE, RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE, ABSCISIC-ACID AND LEAF WATER POTENTIAL IN MESEMBRYANTHEMUM-NODIFLORUM/

Age-induced changes in 1) nocturnal and diurnal acidity fluctuations t hat coincide with the ongoing environmental conditions, 2) the build u p of abscisic acid (ABA) in plant roots and leaves during sunrise, mid day, and sunset in all growing stages, 3) the changes in phosphoenolpy ruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase/ox ygenase (RuBPCO) activities as key enzymes of the photosynthetic pathw ays of C-3 and CAM, 4) leaf water potential (psi(1)), and 5) K-m and V -max for PEPC to express its activity and affinity, were studied in Me sembryanthemum nodiflorum during transition from C-3 to CAM mode of CO 2 fixation. The acidity during sunset in mature stage was higher than in earlier stages and reflected the impact of environmental conditions on physiological and metabolic changes. Moreover, the higher acidity during sunrise and sunset was observed during the senescence than the mature stage; this might be due to CO2 release and oxygen intake durin g senescence induced ethylene formation that lead to increased malic a cid formation. The ABA concentration was high in M. nodiflorum leaves, but stomatal closure was insensitive to elevated ABA concentrations r ecorded. V-max of PEPC, K-m, and the affinity of PEPC during later sta ges indicated the ability of PEPC to fix CO2 taking up at night in CAM cycle of M. nodiflorum. Less affinity during sunrise indicated inhibi tory effect of malate on PEPC during the release of CO2. The second pe ak of PEPC activity before sunset caused CO2 fixation. The RuBPCO was inactive at night. Slight increase in ABA during sunset, and night dro p in air temperature and increase in relative humidity reduced markedl y transpiration rate without decreasing psi(1).