Environmental control of crassulacean acid metabolism in Welwitschia mirabilis Hook. Fil. in its range of natural distribution in the Namib desert.

Within the area of its natural distribution in South West Africa, Welwitschia mirabilis has a less negative δ13C value than C3 plants and a more negative δ13C value than C4 species. This indicates that Welwitschia m. assimilates CO2 partially via CAM when growing in its natural habitat. The difference between the δ13C values of Welwitschia m. and of the C3 species is significant in the savanna, whereas it is only small and statistically not significant in the grassland zone. The proportion of CO2 fixed via CAM is largest in the coastal desert zone. There was no correlation between the δ13C values and the Cl- or ash content of the tissue. Thus, CAM in Welwitschia m. seems not to be induced by salt stress. There is no change in the δ13C values along the persistent Welwitschia m. leaf. The present data indicate that on a broad geographical scale in the area of distribution temperature regime, and water stress as a modifying factor, determine CAM in Welwitschia m. The ecological implications are discussed by comparing the behaviour of Welwitschia m. with other CAM, C3 and C4 species of the accompanying flora.