CAM: not so much a curiosity, more a lesson in integrated management

Abstract

The strikingly diverse life-forms of CAM plants belie the physical constraints to diffusion imposed by succulent leaves, stems and cladodes, which are overcome by subtle biochemical and molecular interactions for regulating CO2 fixation and carbon metabolism. With PEPc operating at night, the high substrate affinity will help to overcome low stomatal and mesophyll conductances and lead to an effective drawdown of CO2 into the mesophyll during Phase I of CAM. However, as PEPc activity often extends into the light period for several hours after dawn (Phase II) or before dusk (Phase IV), there could be direct competition with Rubisco. In part, this is prevented by the slow Rubisco activation and gradual increase in activity which is sustained under the elevated CO2 during decarboxylation (Phase III), which in turn we will show is related to expression of Rubisco activase. There are energetic implications for the concurrent operation of PEPc and Rubisco, but we will show that the quantum efficiency is also regulated across this transition, even at the low light intensities available close to dawn and dusk. The co-ordination of these carboxylation processes leads to a balance between carbon supply needed to fuel the daily CAM cycle and export for growth and reproduction, moderated by responses to changing environmental conditions. Ultimately, the plasticity of metabolism, with the potential for acclimation across day and night, demonstrates that CAM is more than just a simple CO2 concentrating mechanism, providing the potential to enhance CO2 uptake across a 24 h period should extreme conditions prevail.