ECOPHYSIOLOGICAL STUDIES IN KALANCHOE-PORPHYROCALYX (BAKER) AND KALANCHOE-MINIATA (HILS ET BOJER), 2 SPECIES PERFORMING HIGHLY FLEXIBLE CAM

Preceding results, based on the determination of stable carbon isotope composition (delta(13)C) of leaf tissues from various Kalanchoe speci es, suggested a close coincidence between the photosynthetic flexibili ty of the species and their habitat, life form and taxonomic position within the genus. The ability to shift from C-3- to Crassulacean Acid Metabolism (CAM)-type of photosynthesis seemed to concern in particula r the more ancestral species in the genus and to be linked to epiphyti sm and changing climatic situations. For deeper insights into these in terrelationships, physiological studies in controlled conditions were carried out on K. miniata and K. porphyrocalyx. These two species diff er by their habitat preference and li:Fe form. Measurements were condu cted 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 'facultati ve' CAM plants, with very high flexibility in their photosynthetic beh aviour. The decrease in water availability seems to be a major factor triggering the shift from C-3 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 lea st for K. miniata, short-day treatment was found to be a further CAM-i nducing factor. The results are discussed in terms of their ecophysiol ogical significance under the environmental conditions of the sites wh ere the investigated species naturally grow.