Evolutionary physiology: the extent of C4 and CAM photosynthesis in the genera Anacampseros and Grahamia of the Portulacaceae

The Portulacaceae is one of the few terrestrial plant families known to have both C4 and Crassulacean acid metabolism (CAM) species. There may be multiple origins of the evolution of CAM within the Portulacaceae but the only clear evidence of C4 photosynthesis is found in members of the genus Portulaca. In the Portulaca, CAM succulent tissue is overlaid with the C4 tissue in a unique fashion where both pathways are operating simultaneously. Earlier reports have shown that the clade containing the genera Anacampseros and Grahamia may also contain C4 photosynthetic species similar to the Portulaca, which would indicate multiple origins of C4 photosynthesis within the family. The aim of the present study was to ascertain the true photosynthetic nature of these genera. An initial survey of the carbon isotope composition of the Anacampseros ranged from –12.6& to –24.0&, indicating very little CAM activity in some species, with other values close to the C4 range. Anacampseros (¼Grahamia) australiana which had been previously identified as a C4 species had a carbon isotope composition value of –24.0&, which is more indicative of a C3 species with a slight contribution of CAM activity. Other Anacampseros species with C4-like values have been shown to be CAM plants. The initial isotope analysis of the Grahamia species gave values in the range of –27.1& to –23.6&, placing the Grahamia species well towards the C3 photosynthetic range. Further physiological studies indicated increased night-time CO2 uptake with imposition of water stress, associated with a large diurnal acid fluctuation and a marked increased phosphoenolpyruvate carboxylase activity. This showed that the Grahamia species are actually facultative CAM plants despite their C3-like carbon isotope values. The results indicate that the Grahamia and Anacampseros species do not utilize the C4 photosynthetic pathway. This is the first to identify that the Grahamia species are facultative CAM plants where CAM can be induced by water stress. This work supports earlier physiological work that indicates that this clade containing Anacampseros and Grahamia species comprises predominantly facultative CAM plants. This report suggests there may be only one clade which contains C4 photosynthetic members with CAM-like characteristics.