SummaryThe 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 CO 2 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 CO 2 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 CO 2 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.

Succulence and leaf thickness are important anatomical traits in CAM plants, resulting from the presence of large vacuoles to store organic acids accumulated overnight. A higher degree of succulence can result in a reduction in intercellular air space which constrains internal conductance to CO2. Thus, succulence presents a trade-off between the optimal anatomy for CAM and the internal structure ideal for direct C3 photosynthesis. This study examined how plasticity for the reversible engagement of CAM in the genus Clusia could be accommodated by leaf anatomical traits that could facilitate high nocturnal PEPC activity without compromising the direct day-time uptake of CO2 via Rubisco. Nine species of Clusia ranging from constitutive C3 through C3/CAM intermediates to constitutive CAM were compared in terms of leaf gas exchange, succulence, specific leaf area, and a range of leaf anatomical traits (% intercellular air space (IAS), length of mesophyll surface exposed to IAS per unit area, cell size, stomatal density/size). Relative abundances of PEPC and Rubisco proteins in different leaf tissues of a C3 and a CAM-performing species of Clusia were determined using immunogold labelling. The results indicate that the relatively well-aerated spongy mesophyll of Clusia helps to optimize direct C3-mediated CO2 fixation, whilst enlarged palisade cells accommodate the potential for C4 carboxylation and nocturnal storage of organic acids. The findings provide insight on the optimal leaf anatomy that could accommodate the bioengineering of inducible CAM into C3 crops as a means of improving water use efficiency without incurring detrimental consequences for direct C3-mediated photosynthesis.

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