Implications of genotypic diversity and phenotypic plasticity in the ecophysiological success of CAM plants, examined by studies on the vegetation ofMadagascar

On the basis of delta C-13-values, genotypic diversity and phenotypic plast icity of CAM behaviour in plants of the Malagasy vegetation is surveyed. Th e study compares CAM patterns performed in the wild on the levels of genera (Kolanchoe [Crassulaceae], Angraecum [Orchidaceae], Lissochilus [Orchidace ae] and Rhipsalis [Cactaceae]), on the level of a family (Didiereaceae) and finally on the level of a common growth form, namely in leafless orchids. For Rhipsolis, also non-Malagasy species were included in the comparison. T he genus Kalanchoe; was found to be dominated by species representing the C AM-physiotype with CO, fixation taking place only during the night, whereas the CAM/C3- and the C3-physiotypes (with limited intrinsic CAM potential) were less frequent. The opposite holds true for Angroecum. In the genus Rhi psolis, in the Didiereacean family and in the leafless orchids only the CAM -physiotype is represented. The photosynthetic physiotypes of CAM plants we re found to be related to the environmental conditions of the habitat. That is, strong CAM performers are typically abundant in the dry climatic zones or at otherwise dry niches, species of the C3-physiotype (possibly with we ak intrinsic capability of CAM performance) are distributed at humid sites and those of the CAM/C3-physiotype occupy sites with medium and changing ex posure to stress. Phenotypic plasticity of CAM, as indicated by the intrasp ecific variability of the delta C-13-values, was lower in the CAM-physiotyp e compared with the CAM/C3-physiotype. Our data support the view that stron g stress leads to the dominance of highly adapted specialists among the CAM plants, with low phenotypic plasticity of the photosynthetic behaviour, wh ereas medium stress advances the unfolding of plastic CAM behaviour. Moreov er, the data suggest that genera comprising all three physiotypes (Koloncho e Angroecum) are dispersed all over Madagascar, whilst groups comprising on ly strong CAM performers are restricted to limited areas or special types o f habitats. This suggests that both genotypic diversity and phenotypic plas ticity are important factors for the ecophysiological success of CAM.