Developmental Control of CAM

It is well known that the capacity for CAM increases with leaf age (Wolf, 1960 Avadhani et al., 1971 Jones, 1975 Osmond, 1978). In Mesembryanthemum crys-tallinum, induced CAM appears first in mature leaves and only later in younger leaves as they develop (Winter and Liittge, 1976 von Willert et al., 1976a Mooney et al., 1974 and Chap.6.2.2.3). As indicated earlier in this chapter, Osmond (1978) has outlined that induction of CAM by low water potentials or by photoperiod may reflect an acceleration of normal developmental processes by the above factors. From this point of view it can be argued that a distinction between constitutive and facultative CAM plants (see Chap. 3.4) might be without relevance. 

Furthermore, it should be mentioned that many of the results obtained by Queiroz s group on CAM control by photoperiods in K. blossfeldiana are based on investigations of the upper, young leaves and not the mature ones which represent the majority on the plant. Thus, it must be asked if the very interesting induction phenomenon is relevant to CAM of the whole adult K. blossfeldiana plant. 

The available data strongly indicate a dependence of CAM activity on environmental signals (see also Chaps. 5 and 6). Photoperiod, thermoperiod, and hydroperiod all influence and control CAM in a complex manner. It seems reasonable to conclude that these environmental signals can reversibly shift certain CAM plants from the CAM type to the C3 type of photosynthesis, and also can regulate the degree of CAM activity when featuring CAM. 

To a large extent the shifts in metabolic options in CAM plants are a direct function of water status and it is this, of course, which seems to be the adaptive significance of CAM. Nevertheless, it seems clear, particularly in the case of the photoperiodic studies, that factors other than water status are important. 

These responses to photo-, thermo-, and hydroperiods tend to give a distinct seasonal asepct to CAM. Although no generalizations can be made with respect to seasonal patterns, there are quite definite species-specific patterns. The best 

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