Climate change and the evolution of C(4) photosynthesis.

Plants assimilate carbon by one of three photosynthetic pathways, commonly called the C(3), C(4), and CAM pathways. The C(4) photosynthetic pathway, found only among the angiosperms, represents a modification of C(3) metabolism that is most effective at low concentrations of CO(2). Today, C(4) plants are most common in hot, open ecosystems, and it is commonly felt that they evolved under these conditions. However, high light and high temperature, by themselves, are not sufficient to favor the evolution of C(4) photosynthesis at atmospheric CO(2) levels significantly above the current ambient values. A review of evidence suggests that C(4) plants evolved in response to a reduction in atmospheric CO(2) levels that began during the Cretaceous and continued until the Miocene.