EFFECTS OF ABRUPT OR PROGRESSIVE DROUGHT ON THE PHOTOSYNTHETIC MODE OF CRASSULA-SIEBERIANA CULTIVATED UNDER DIFFERENT DAYLENGTHS

Well-watered plants of Crassula sieberiana were shown to perform C3-type of photosynthesis independently of photoperiodic regimes. Abrupt drought conditions shifted the plants to CAM or CAM-like Situations according to C02-exchange patterns, levels and diel oscillation of malate pools and phosphoenolpyruvate carboxylase capacity (EC 4.1.1.31 PEPC). Progressively increased levels of drought under long days resulted, at each step, in reversible increase in PEPC extractable activity, independently of other CAM criteria (CO2 exchange patterns and diel oscillation of malate). Immunotitration experiments showed that the increase in PEPC was due to de novo synthesis of the enzyme. No correlation could be found between PEPC synthesis and water parameters of the leaves. The primary triggering element for the observed changes remains to be discovered.