Ecophysiological Studies on the Vegetation of Madagascar: A δ¹³C and δD Survey for Incidence of Crassulacean Acid Metabolism (Cam) Among Orchids From Montane Forests and Succulents from the Xerophytic Thorn-Bush

Abstract

Abstract The incidence of Crassulacean acid metabolism (CAM) in plants collected at various habitats in Madagascar was investigated by survey of carbon and hydrogen isotope ratios ((δ13C and δD values). In about 50% of the epiphytic orchids from evergreen higher and lower montane forests the δ13C values were indicative for CAM. The remainder of the species are presumably C3 plants. In all samples of malagasy epiphytic leafless orchids comprising 9 species, the δ13C values suggest extreme CAM with CO2 uptake proceeding entirely during the night. All terrestrial orchids collected in the lower montane forests obviously acquire external carbon by C3-photosynthesis, whereas Lissochilus decaryi, a terrestrial orchid from the semi-arid south of Madagascar and various other species of this genus are CAM plants. This is the first report of CAM occurrence in sympodial terrestrial orchids. Judged by the δ13C values, all succulents (mainly Didiereaceae, Euphorbiaceae, Crassulaceae and Asclepiadaceae) collected at the xerophytic thorn-bush of the semi-arid south perform pronounced CAM. Where it applies, our δ13C measurements in the thorn-bush succulents revealed values being practically identical with those found by K. Winter in samples of the same species collected at the same site nearly 10 years earlier. This shows extreme constancy over long duration of time in the mode of CAM performed by the succulents of the malagasy thorn-bush vegetation. Since the δ13C survey now comprises all 11 known species of the Didiereaceae, it is unequivocally clear that all members of this family are CAM plants. Most of the individuals of the species of the Didiereaceae grown in a glass-house had slightly more negative δ13C values compared with those grown at the natural stands suggesting some contribution of C3-photosynthesis to carbon acquisition under the evaporatively less demanding glass-house conditions (and perhaps higher CO2 concentrations in the gas phase). Despite of the fact that the hydrogen isotope composition of meteoric waters depends to a large extent on the altitude and temperature-climate of the site where the concerned plants grow, it was found that in samples obtained in the cooler higher evergreen montane forest as well as in the warmer lower evergreen montane forest and the lowland thorn-bush of the hot, semiarid south of Madagascar the δ values found in the organic matter (δDorg) were in the same range (between about - 10‰ to about - 90‰). This suggests that in our case the hydrogen isotope compositions of the meteoric waters were of minor importance in bringing about the δDorg values found in the plants.