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 that coincide with the ongoing environmental conditions, 2) the build up of abscisic acid (ABA) in plant roots and leaves during sunrise, midday, and sunset in all growing stages, 3) the changes in phosphoenolpyruvate carboxylase (PEPC) and ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCO) activities as key enzymes of the photosynthetic pathways of C3 and CAM, 4) leaf water potential (ψ1), and 5) Km and Vmax for PEPC to express its activity and affinity, were studied in Mesembryanthemum nodiflorum during transition from C3 to CAM mode of CO2 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 during senescence induced ethylene formation that lead to increased malic acid formation. The ABA concentration was high in M. nodiflorum leaves, but stomatal closure was insensitive to elevated ABA concentrations recorded. Vmax of PEPC, Km, and the affinity of PEPC during later stages indicated the ability of PEPC to fix CO2 taking up at night in CAM cycle of M. nodiflorum. Less affinity during sunrise indicated inhibitory effect of malate on PEPC during the release of CO2. The second peak of PEPC activity before sunset caused CO2 fixation. The RuBPCO was inactive at night. Slight increase in ABA during sunset, and night drop in air temperature and increase in relative humidity reduced markedly transpiration rate without decreasing ψ1.