Facultative CAM photosynthesis (crassulacean acid metabolism) in four species of Calandrinia, ephemeral succulents of arid Australia.

Crassulacean acid metabolism (CAM) was demonstrated in four small endemic Australian terrestrial succulents from the genus Calandrinia (Montiaceae) viz. C. creethiae, C. pentavalvis, C. quadrivalvis and C. reticulata. CAM was substantiated by measurements of CO2 gas-exchange and nocturnal acidification. In all species, the expression of CAM was overwhelmingly facultative in that nocturnal H+ accumulation was greatest in droughted plants and zero, or close to zero, in plants that were well-watered, including plants that had been droughted and were subsequently rewatered, i.e. the inducible component was proven to be reversible. Gas-exchange measurements complemented the determinations of acidity. In all species, net CO2 uptake was restricted to the light in well-watered plants, and cessation of watering was followed by a progressive reduction of CO2 uptake in the light and a reduction in nocturnal CO2 efflux. In C. creethiae, C. pentavalvis and C. reticulata net CO2 assimilation was eventually observed in the dark, whereas in C. quadrivalvis nocturnal CO2 exchange approached the compensation point but did not transition to net CO2 gain. Following rewatering, all species returned to their original well-watered CO2 exchange pattern of net CO2 uptake restricted solely to the light. In addition to facultative CAM, C. quadrivalvis and C. reticulata exhibited an extremely small constitutive CAM component as demonstrated by the nocturnal accumulation in well-watered plants of small amounts of acidity and by the curved pattern of the nocturnal course of CO2 efflux. It is suggested that low-level CAM and facultative CAM are more common within the Australian succulent flora, and perhaps the world succulent flora, than has been previously assumed.