Computational analysis of the productivity potential of CAM.

There is considerable interest in transferring crassulacean acid metabolism (CAM) to C ₃ crops to improve their water-use efficiency. However, because the CAM biochemical cycle is energetically costly, it is unclear what impact this would have on yield. Using diel flux balance analysis of the CAM and C ₃ leaf metabolic networks, we show that energy consumption is three-fold higher in CAM at night. However, this additional cost of CAM can be entirely offset by the carbon-concentrating effect of malate decarboxylation behind closed stomata during the day. Depending on the resultant rates of the carboxylase and oxygenase activities of rubisco, the productivity of the PEPCK-CAM subtype is 74-100% of the C ₃ network. We conclude that CAM does not impose a significant productivity penalty and that engineering CAM into C ₃ crops is likely to lead to a major increase in water-use efficiency without substantially affecting yield.