Stomatal mechanism as the basis of the evolution of CAM and C4 photosynthesis

The similarities between the component reactions of the presently known variants of photosynthetic carbon metabolism (crassulacean acid metabolism, the acid metabolism of Tillandsia usneoides, aquatic acid metabolism, and C4 photosynthesis) when considered along with their widely scattered taxonomic distribution strongly suggest polyphyletic origins resulting from evolutionary modification of a common, universally distributed metabolic sequence. The synthesis and consumption of four‐carbon acids in the cation‐balancing reactions involved in the regulation of stomatal aperture appear to exhibit all of the characteristics likely to be displayed by such a metabolic progenitor. The present status of the proposal that the expression of aspects of stomatal metabolism in photosynthetic mesophyll cells represents the basis for the evolution of the variant of photosynthetic carbon metabolism is discussed. The prospects of experimental approaches which may yield information relevant to the proposal are also explored.