Besides the major 250 kD PEPCase isoform, a 300 kD minor PEPCaseform was also found in Mesembryanthemum cordifolium leaves (Fig. 1 ). Byusing SDS PAGE it was found that bothisoforms were homodiners. ’The 250 kDform consisted of two 100 kD subunits ,and the 300 kD form is a dimer of two110 kD subunits (Figs. 2,3). ’The discrepency between molectilar weights ofthe native enzymes and their subunitswas a result of the different molecularweight determinating methods. There fore, there is no heterodimer of PEPCase in this plant. A 1 080 kD PEPCase form was found in both saltstressed and well-watered leaf samples.This PEPCase is an octamer, and therewas no diurnal fluctuation in its abundance.
After salt stress, the sensitivitytoward the inhibitory action of L-malateof PEPCase from Mesembryanthemum cordifolium showed diurnal fluctuation(Fig. 1a, b). The day form PEPCasewas more sensitive to the inhibition byL-malate than the night form (Fig.fo). However, there was no differencein molecular weight between the dayform and the night form of PEPCase(Fig. 2). Both salt stress and droughtelevated the leaf ABA level dramatically (Fig. 3A ). The ABA level indrought-stressed leaves decreasedmarkedly after the plants were rewatered (Fig. 3A), but the ABA level insalt-stressed leaves remained unchangedafter soil salts were thoroughly washedout and the plants were well watered(Fig. 3A). External ABA treatment increased the leaf PEPChse activity anddecreased the leaf water content (Fig.3B). Externally applied 6-BA had similar effects on the plants as the ABA(Fig. 4 ). We concluded that: 1. Theday/night difference in the regulatoryaction of CAM form PEPCase was notcaused by any change in the saute of aggregation of the enzyme; 2. The effectSo f ABA and 6-BA on leaf PEPCase level are most likely mediated by leaf water loss they induced.
