The first comprehensive field study has been conducted of crassulacean acid metabolism in a Kalanchoë species, namely K. beharensis Drake Del Castillo, a plant of the flora of the semi-arid southern part of Madagascar. The study includes measurements of diel patterns of CO2 exchange and transpiration, and of fluctuations in organic acid levels, PEP carboxylase properties and water relations.
In contrast to the other Kalanchoë species studied so far, the CAM behaviour of K. beharensis is very uniform. Regardless of whether the plants were naturally droughted for several months or had received 6 wk of daily watering, CO2 uptake 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 higher in the droughted plants. The data on PEP carboxylase indicate a diel change between an active night-form and a less active day-form of the enzyme known already from other CAM plants studied in the laboratory.
Experiments with leaves kept in situ artificially darkened during the day revealed that the onset of deacidification was under the control of an external stimulus, presumably of temperature. In the CAM performing leaves, neither water status nor seasons were associated with significant differences in leaf water potential, osmotic pressure and turgor 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 performance throughout the entire arid seasons, and thus avoids CAM idling. It is assumed that this advantageous behaviour is based on the capability to maintain appropriate water status in the photosyntheticaily active leaves even during long-lasting drought. This capability is due, on the one hand, to minimization of transpiration by the extreme CAM mode, 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.
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