The variable fluorescence of leaves from Kalachoi daigrenwntiana and pineapple, Anaws comnosus, both CAM plants, was found to change over a 24-hour cycle and to exhibit high temperature-dependent maxima during the night period. The time course of the induced fluorescence was correlated with malic acid accumulation but not with other aspects of CAM such as with the nature of the decarboxylation pathway or with stomatal movements. The variable fluorescences of sunflower (Helanthus awuws L.) and corn (Zea mays L.) leaves were compared with the CAM plants diurnafly, both plants also exhibit high fluorescence maxima during the night period. We conclude that the assembly of the photosystems in the Light is a primary process in photosynthesis induction and may be influenced by other celiular metabolic processes, specifically in the case of CAM leaves by malic acid accumulaton.events associated with CAM CO2 metabolism and organic acid metabolism (13,14). We are not aware of other investigations on variable fluorescence induction during the night phase of a higher plant's diurnal cycle, so diurnal curves for a C3 and a C4 plant also were measured and compared with CAM curves.We have chosen the two CAM species as representing two different biochemical pathways of CAM. Kalanchoe utilizes malic enzyme for decarboxylating malic acid and pyruvate Pi dikinase in the regenerative phase of glycolysis; pineapple uses PEP4 carboxykinase for decarboxylation and PEP production. Furthermore, the two plants vary in the source of sugars as substrate for glycolysis during malic acid production. Pineapple utilizes fructose and glucose (cytosolic) plus glucans while Kalanchoe utilizes starch (chloroplastic) as their respective sources of carbon for producing the nighttime CO2 acceptor PEP (3, 13). MATERIALS AND METHODSPreviously darkened leaves display fluorescence which varies in intensity during continuous excitation with weak actinic light. This phenomenon first described five decades ago (11) has been studied more recently in relation to water potential, frost resistance, and photoinhibition of photosynthesis in leaves (6,20) as well as more extensively in laboratory studies on the coupling of photochemical and metabolic processes in chloroplasts and cells (2,17, 23, 24). Although the time course of this fluorescence has certain consistent features (for reviews, see Clayton [5]