INSITU STUDIES OF CRASSULACEAN ACID METABOLISM IN KALANCHOE-BEHARENSIS DELCASTILLO,DRAKE, A PLANT OF THE SEMIARID SOUTHERN REGION OF MADAGASCAR

The first comprehensive field study has been conducted of crassulacean acid metabolism in a Kalanchoe species, namely K. beharensis Drake De l Castillo, a plant of the flora of the semi-arid southern part of Mad agascar. The study includes measurements of diel patterns of CO2 excha nge and transpiration, and of fluctuations in organic acid levels, PEP carboxylase properties and water relations. In contrast to the other Kalanchoe species studied so far, the CAM behaviour of K. beharensis i s very uniform. Regardless of whether the plants were naturally drough ted for several months or had received 6 wk of daily watering, CO2 upt ake was quantitatively the same and occurred entirely during the night . Watering also had no effect on the amplitude of the diel fluctuation of titratable acidity. The fluctuating citrate level was somewhat hig her in the droughted plants. The data on PEP carboxylase indicate a di el change between an active night-form and a less active day-form of t he enzyme known already from other CAM plants studied in the laborator y. Experiments with leaves kept in situ artificially darkened during t he day revealed that the onset of deacidification was under the contro l of an external stimulus, presumably of temperature. In the CAM perfo rming leaves, neither water status nor seasons were associated with si gnificant differences in leaf water potential, osmotic pressure and tu rgor pressure. The lower amplitude of titratable acidity found during the dry season is presumably more due to suboptimal night temperatures during this time than to plant water relations. Altogether, the study shows that K. beharensis is capable of maintaining the full CAM perfo rmance throughout the entire arid seasons, and thus avoids CAM idling. It is assumed that this advantageous behaviour is based on the capabi lity to maintain appropriate water status in the photosynthetically ac tive leaves even during long-lasting drought. This capability is due, on the one hand, to minimization of transpiration by the extreme CAM m ode, accompanied by very low cuticular transpiration, and on the other hand due to water transfer from the older leaves which then wilt and are shed.