Photoperiodism and Crassulacean acid metabolism : III. Different characteristics of the photoperiod-sensitive and non-sensitive isoforms of phosphoenolpyruvate carboxylase and Crassulacean acid metabolism operation.

Sensitivity to glucose-6-P, inorganic phosphate and malate, Km phosphoenolpyruvate (PEP), and the effect of pH were comparatively investigated for phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) extracted along the day-night cycle from leaves of Kalanchoe blossfeldiana v. Poelln. Tom Thumb, grown under different photoperiodic conditions. Under long days, sensitivity to effectors is weak and varies only slightly during the 24h cycle, together with small variations of Km (PEP), ranging from 1.2 to 1.8 mM. The response-curve to pH shows two peaks for pH 7.4 and 8.4. Transfer of the plants to short days established an increase in the sensitivity of the enzyme to the effectors together with the appearance of a day-night variation of this effect, maximum during the day. A clear diurnal oscillation of Km (PEP) is observed from 3.1 mM at the beginning of day (09.00 h) to 0.9 mM at midnight (00.00 h), at pH 7.4. These results complement previous electrophoretic and immunological data by affording enzyme kinetic evidence that short days induce a PEP carboxylase form responsible for full-Crassulacean acid metabolism (CAM) operation which is kinetically different from the photoperiod-sensitive, C3-like form prevailing in young leaves under long days. The results indicate that diurnal enzymic rhythms would improve the efficiency of the metabolic regulatory mechanisms and act as a coordinating factor for the daily and seasonal adaptive operation of CAM.