Ecophysiological studies in Kalanchoë porphyrocalyx (Baker) and K. miniata (Hils et Bojer), two species performing highly flexible CAM.

Preceding results, based on the determination of stable carbon isotope composition (δ13C) of leaf tissues from various Kalanchoë species, suggested a close coincidence between the photosynthetic flexibility of the species and their habitat, life form and taxonomic position within the genus. The ability to shift from C3-to Crassulacean Acid Metabolism (CAM)-type of photosynthesis seemed to concern in particular the more ancestral species in the genus and to be linked to epiphytism and changing climatic situations. For deeper insights into these interrelationships, physiological studies in controlled conditions were carried out on K. miniata and K. porphyrocalyx. These two species differ by their habitat preference and life form. Measurements were conducted on CO2 exchange patterns, day/night fluctuation of malate content in the leaves and 'capacity' of phosphoenolpyruvate carboxylase (PEPC). The results show that the 2 species can be considered as 'facultative' CAM plants, with very high flexibility in their photosynthetic behaviour. The decrease in water availability seems to be a major factor triggering the shift from C3 to the CAM mode. In K. miniata, 21 days of drought depressed CO2 uptake to the level of CAM idling whereas in K. porphyrocalyx, CO2 exchange was considerably more resistant. At least for K. miniata, short-day treatment was found to be a further CAM-inducing factor. The results are discussed in terms of their ecophysiological significance under the environmental conditions of the sites where the investigated species naturally grow.