The photosynthetic plasticity of crassulacean acid metabolism: an evolutionary innovation for sustainable productivity in a changing world

The photosynthetic specialization of crassulacean acid metabolism (CAM) has evolved many times in response to selective pressures imposed by water limitation. Integration of circadian and metabolite control over nocturnal C-4 and daytime C-3 carboxylation processes in CAM plants provides plasticity for optimizing carbon gain and water use by extending or curtailing the period of net CO2 uptake over any 24-h period. Photosynthetic plasticity underpins the ecological diversity of CAM species and contributes to the potential for high biomass production in water-limited habitats. Perceived evolutionary constraints on the dynamic range of CO2 acquisition strategies in CAM species can be reconciled with functional anatomical requirements and the metabolic costs of maintaining the enzymatic machinery required for C-3 and C-4 carboxylation processes. Succulence is highlighted as a key trait for maximizing biomass productivity in water-limited habitats by serving to buffer water availability, by maximizing the magnitude of nocturnal CO2 uptake and by extending the duration of C-4 carboxylation beyond the night period. Examples are discussed where an understanding of the diverse metabolic and ecological manifestations of CAM can be exploited for the sustainable productivity of economically and ecologically important species.