Crassulacean acid metabolism (CAM) is a variant of carbon dioxide assimilation, occurring in succulent plants, and characterised by a diurnal fluctuation of malic acid accumulation during the night and decrease during the day. In some species capable of performing CAM, photoperiodic treatments can enhance or even induce the CAM pathway (Queiroz, 1970). In Kalanchoë blossfeldiana cv. Tom Thumb, malate formation is promoted by SD. For plants grown in LD, malate formation was only seen after 7 SD treatment (Brulfert et al., 1975) and maximum malate production was found in continuous SD. When plants were transferred back into non-inductive
conditions after 7 or 10 SD the level of malate remained unchanged, suggesting that CAM metabolism had been 'induced' in these plants (Fig. 13.10). Similar results were obtained for the activity of phosphoenolpyruvate (PEP) carboxylase, the enzyme responsible for the production of malate via the synthesis of oxaloacetate by the dark fixation of carbon dioxide. The non-inductive treatment consisted of a SD + 30 min NB with R light which was FR-reversible. Thus the perception of daylength for modulating the CAM metabolism system appeared to be similar to the photoperiodic floral induction response rather than a direct effect on photosynthesis. The photoperiodic sensitivity of Kalanchoe blossfeldiana for CAM is most apparent in young seedlings; leaf ageing under LD eventually leads to CAM and the promotive effect of SD lessens as the plants age (Brulfert et al., 1982). Although CAM is associated with daily rhythms in malate, PEP carboxylase, carbon dioxide fixation, cellular pH and other properties, it is not understood how the induction of CAM by photoperiod is achieved. CAM is also triggered by water stress in Kalanchoe blossfeldiana and increases in cellular ABA, which would be expected in drought-stressed tissues, cause an early increase in PEP carboxylase mRNA transcripts (Taybi et al., 1995). It is possible that the photosynthetic behaviour of the leaves depends on the availability of external carbon as governed by stomatal behaviour, although the link with photoperiod for this mode of regulation has not been made.
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