Diurnal Rhythm and Characteristics of Photosynthesis and Respiration in the Leaf and Root of a Phalaenopsis Plant

A hybrid Phalaenopsis plant was grown hydroponically on a nutrient solution of Hyponex in the greenhouse. Typical daily patterns of CO2 fixation, production of malic acid and citric acid and pH of the leaf and root were determined. The rate of primary fixation of CO2 in the leaf increased markedly in the evening, remained high until well into the afternoon, then decreased sharply. The pattern of production of organic acids resulted from the fact that the rate of uptake of CO2 gas was highest at night and fell during the day. A high correlation between pH value and level of malic acid in the leaves was observed, and an exponential relationship appeared to exist between these parameters. These rhythmic sequences were not observed in the root. The results suggest that metabolism in the leaf had characteristics of CAM (Crassulacean Acid Metabolism), while the root did not.