Facultative CAM photosynthesis (crassulacean acid metabolism) in four species of Calandrinia, ephemeral succulents of arid Australia
2017
Holtum, Joseph A. M. | Hancock, Lillian P. | Edwards, Erika J. | Winter, Klaus
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.
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