THE INTERRELATIONSHIP BETWEEN CHANGES IN PEPC ACTIVITY AND ORGANIC-ACID ACCUMULATION DURING THE C3-CAM SHIFT IN MESEMBRYANTHEMUM-CRYSTALLINUM

Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) activity and diurn al changes in malic (Delta-malate) and citric acid (Delta-citrate) con tents were determined in salt-treated and continuously well-watered pl ants of the facultatively halophytic ephemeral Mesembryanthemum crysta llinum for a period of approximately 50 days. Twenty-two days after ge rmination, PEPC activity (measured at pH 7 and pH 8) began to increase slightly in salt-stressed and in control plants. This change coincide d with a step-wise (approximate to 160%) rise in the ratio of PEPC act ivity at pH 7 to pH 8 (PEPC-7/8) and with the onset of a low but signi ficant Delta-citrate. After 35 days a second, sudden, three to four-fo ld augmentation in maximum PEPC activity was observed in plants from a ll sets. PEPC-7/8 increased by nearly 40% to 0.8 within 4 days and a p ronounced Delta-malate appeared for the first time. The appearance of Delta-malate was independent of the duration of the salt treatment and occurred, though to a much lesser extent, also in control plants. The length of the lag-phases between the occurrence of Delta-citrate and Delta-malate, and the two st eps of PEPC activity increase, decreased with increase of plant age at the beginning of the salt treatment. Thi s coincidence pointed out the causal interrelationship between these e vents. The low initial rise in PEPC activity allowed a nocturnal accum ulation of citric acid. An accumulation of malic acid required a high PEPC activity and a high PEPC-7/8. Relief from salt stress resulted in a drastically decreased PEPC activity without preventing nighttime ma lic acid accumulation. PEPC-7/8 also declined slightly to nearly the l evel attained by the control plants at the same rime. PEPC-7/8 tended to increase continuously in well-watered plants during the later stage s of the experiment.