Comparison of photosynthetic properties of field-grown pitaya between in summer and winter conditions.

Diurnal changes in the contents of organic acids, carbohydrates, and activities of two enzymes, phosphoenolpyruvate carboxylase (PEPC) and malate dehydrogenase (oxaloacetate-decarboxylating, NADP+) (ME), were compared between summer and winter conditions in a crassulacean acid metabolism (CAM) plant, pitaya. Preliminary experiments showed that pitaya’s PEPC required Mn2+ but not Mg2+, and exhibited maximum activity at pH 7.0, in contrast to the PEPCs of some other plants. Analyses of sap revealed that sucrose was the loading sugar in pitaya. In summer, the diurnal changes in the contents of organic acids and starch in pitaya were typical of starch-using CAM plants. In addition, no phosphoenolpyruvate carboxykinase (PEPCK) activity was detected, and high levels of starch, but not of hexose, accumulated. These results indicate that pitaya belongs to the MEstarch type among the four kinds of CAM plants. In winter, the levels of these substances (including sucrose) decreased, and there were no clear CAM-type diurnal changes. In particular, we did not detect malate accumulation during the night, degradation of malate during the day, or starch accumulation during the day. This may have resulted from decreased enzyme activities and lack of C3 photosynthesis during the day. Thus, since pitaya displays a reduced capacity for CO2 fixation in winter, it cannot store nutrients during this period.