Evolutionary and Ecological Aspects of Photosynthetic Pathway Variation

C4 and CAM photosynthesis are evolutionarily derived from C3 photosynthesis. The morphological and biochemical modifications necessary to achieve either C4 or CAM photosynthesis are thought to have independently arisen numerous times within different higher plant taxa. It is thought 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 photorespiration of C3 plants, reducing productivity; both C3-C4 intermediate and C4 plants exhibit reduced photorespiration rates. In contrast, it may be argued that CAM arose either in response to selection of increased water-use efficiency or for increased carbon gain. Globally, all three pathways are widely distributed today, with a tendency toward ecological adaptation of C4 plants into warm, monsoonal climates and CAM plants into water-limited habitats. In an anthropogenically altered CQ2 environment, C4 plants may lose their competitive advantage over C3 plants. 411