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 nutrientsolution of Hyponex in the greenhouse. Typical daily patternsof CO₂ fixation, production of malic acid and citric acid andpH of the leaf and root were determined. The rate of primaryfixation of CO₂ in the leaf increased markedly in the evening,remained high until well into the afternoon, then decreasedsharply. The pattern of production of organic acids resultedfrom the fact that the rate of uptake of CO₂ gas was highestat night and fell during the day. A high correlation betweenpH value and level of malic acid in the leaves was observed,and an exponential relationship appeared to exist between theseparameters. These rhythmic sequences were not observed in theroot. The results suggest that metabolism in the leaf had characteristicsof CAM (Crassulacean Acid Metabolism), while the root did not. (Received May 6, 1988; Accepted October 25, 1988)