On the nature of facultative and constitutive CAM:: environmental and developmental control of CAM expression during early growth of Clusia, Kalanchoe, and Opuntia

The capacity to induce crassulacean acid metabolism developmentally ( constitutive CAM) and to up-regulate CAM expression in response to drought stress (facultative CAM) was studied in whole shoots of seven species by measuring net CO2 gas exchange for up to 120 day-night cycles during early growth. In Clusia rosea, CAM was largely induced developmentally. Well-watered seedlings began their life cycle as C-3 plants and developed net dark CO2 fixation indicative of CAM after the initiation of the fourth leaf pair following the cotyledons. Thereafter, CAM activity increased progressively and drought stress led to only small additional, reversible increases in dark CO2 fixation. In contrast, CAM expression was overwhelmingly under environmental control in seedlings and mature plants of Clusia pratensis. C-3-type CO2 exchange was maintained under well-watered conditions, but upon drought stress, CO2 exchange shifted, in a fully reversible manner, to a CAM-type pattern. Clusia minor showed CO2 exchange reponses intermediate to those of C. rosea and C. pratensis. Clusia cretosa operated in the C3 mode at all times. Notably, reversible stress-induced increases of dark CO2 fixation were also observed during the developmental progression to pronounced CAM in young Kalanchoe daigremontiana and Kalanchoe pinnata, two species considered constitutive CAM species. Drought-induced up-regulation of CAM was even detected in young cladodes of a cactus, Opuntia ficus-indica, an archetypal constitutive CAM species. Evidently, the de. ning characteristics of constitutive and facultative CAM are shared, to variable degrees, by all CAM species.