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From the mass of information on photoperiodic control of flower formation (2, 3), it appears that experimental and theoretical evidence supports the hypothesis of a balance, or competition, between two processes occurring in plant leaves which result in the formation of a flowering inhibitor under noninductive conditions and a flowering stimulus under inductive conditions. Particularly in reference to Kalanchoe blossfeldiana, a short day plant, Schwabe (10, 11) described some results consistent with the presence of an inhibitor of flowering in crude sap expressed from Kalanchoe leaves grown under noninductive conditions. We report here that this model for a dual action of photoperiodism can be extended to the control of an enzymic pathway, and hence the action may be understood at the metabolic level. The phrase CAM' defines a metabolic pathway which includes, (a) the synthesis of malate, through the coupled operation of P-enolpyruvate carboxylase EC 4.1 .1 .31 and malate dehydrogenase EC 1 .1 .1 . 37; (b) the decarboxylation of malate by malic enzyme EC 1. 1. 1. 40; and (c) the transamination reaction between oxaloacetate and glutamate catalyzed by aspartate aminotransferase EC 2.5. 1. 1. In young leaves of Kalanchoe blossfeldiana Tom Thumb, the activity of these enzymes is under photoperiodic and phytochrome control (8, 9). In long days or in short days with nights interrupted by red light, the pathway is not operative presumably due to the activity of P-enolpyruvate carboxylase being very small. In short days, there is a progressively rapid increase in the activity of all the enzymes of the pathway and a net accumulation of malate. Acid accumulation starts after a lag period of about 7 short days. It has been suggested that this lag time would correspond with a residual long day effect (7). In fact, Morel et al. (5) have shown that total activity of P-enolpyruvate carboxylase increases following two successive exponential functions and that the change in slope takes place after about 30 short days. We propose an interpretation according to which, (a) the 7th short day is a threshold for P-enolpyruvate carboxylase activity, which then becomes detectable in vitro and efficient in vivo, as shown by the changes in the amounts of malate; (b) the 30th short day would mark a critical low level of an inhibitory system produced in long days, the disappearance of this inhibitor progressively unmasking the increase in P-enolpyruvate carboxylase. Our results confirm these hypotheses.

Photoperiodism and Enzyme Activity: Balance between Inhibition and Induction of the Crassulacean Acid Metabolism