On the evolutionary pathways resulting in C4 photosynthesis and crassulacean acid metabolism (CAM)

This chapter describes C3-C4 intermediate photosynthesis and crassulacean acid metabolism (CAM)-like function in an evolutionary framework that can be used to evaluate some possible paths that may have been taken during the evolution of fully-expressed C4 photosynthesis and CAM. In the case of C4 photosynthesis, it is proposed that the initial evolutionary stages were driven by selection for reduced photorespiration rates and the influence of such reduction on the net CO2 assimilation rate. The assimilation of respired CO2 during the night, with assimilation of atmospheric CO2 during the day (known as CAM-cycling)is proposed as a precursor to the evolution of fully-expressed CAM in some species. Thus, C4 photosynthesis and CAM may have both originated from mechanisms that improved the plant's carbon balance by assimilating internally generated CO2. The adaptive significance of CAM-cycling is thought to be water conservation, a role that is consistent with the adaptive significance of fully expressed CAM. CAM-cycling may have evolved from the organic-acid metabolism associated with ion uptake in plants (particularly in the guard cells of stomata), or from the organic-acid metabolism characteristic of aquatic plants.