Carbon Isotope Composition of the Halophyte Disphyma clavellatum (Haw.) Chinnock (Aizoaceae), as Affected by Salinity

The δ13C values of the leaves of Disphyma clavellatum (Aizoaceae) systematically became less negative as the salinity in the root environment increased from 0 to 500 mol m-3 NaCl. The maximum shift of δ13C was from -26.1‰ to -20.0‰. Similar increases in salinity did not, however, result either in a change in the diurnal pattern of net CO2 assimilation or in an appreciable increase in the fluctuations of titratable acidity. It is suggested that salinity induced the observed shift in δ13C values, not by effecting change in the mode of carboxylation towards that of crassulacean acid metabolism (CAM), but by affecting the degree of limitation of CO2 assimilation by diffusion processes. It is concluded therefore that, in D. clavellatum, CAM is not induced by salinity, as it is in Mesembryanthemum crystallinum.