The effect of leaf age and salt stress on malate accumulation and phosphoenolpyruvate carboxylase activity in Mesembryanthemum crystallinum

Abstract

Prior to treatment with 300 mM NaCl Mesembryanthemum crystallinum leaves of different ages have only small differences in phosphoenolpyruvate (PEP) carboxylase activity and do not accumulate malate at night, indicating that they behave like C 3-plants. After 12 days of NaCl treatment the mature leaves exhibit a 20-fold increase in PEP carboxylase activity and a significant accumulation of malate during the night. These features are characteristics of crassulacean acid metabolism (CAM) plants. The PEP carboxylase activity in the youngest leaf did not increase until the leaf reached a certain stage of development. Since the youngest leaves always showed the lowest PEP carboxylase activity and never accumulated malate at night they do not contribute to a CAM. The results provide evidence for the existence of two regulatory processes in Mesembryanthemum crystallinum both causing a change from C 3 to CAM. One process is under environmental control (NaCl concentration) and occurs only in mature leaves. The second process is an ontogenetic response and is associated with maturation.