EVOLUTIONARY AND ECOLOGICAL ASPECTS OF PHOTOSYNTHETIC PATHWAY VARIATION

C4 and CAM photosynthesis are evolutionarily derived from C3 photosynt hesis. The morphological and biochemical modifications necessary to ac hieve either C4 or CAM photosynthesis are thought to have independentl y arisen numerous times within different higher plant taxa. It is thou ght that C4 photosynthesis evolved in response to the low atmospheric CO2 Concentrations that arose sometime after the end of the Cretaceous . LoW CO2 concentrations result in significant increases in photorespi ration of C3 plants, reducing productivity; both C3-C4 intermediate an d C4 Plants exhibit reduced photorespiration rates. In contrast, it ma y be argued that CAM arose either in response to selection of increase d water-use efficiency or for increased carbon gain. Globally, all thr ee pathways are widely distributed today, with a tendency toward ecolo gical adaptation Of C4 plants into warm, monsoonal climates and CAM pl ants into water-limited habitats. In an anthropogenically altered CO2 environment, C4 plants may lose their competitive advantage over C3 pl ants.