The ecophysiology and global biology of c₄ photosynthesis

The appearance of the C⁴photosynthetic pathway in the Earth’s flora represents one of the most impressive and curious examples of evolutionary diversification and biogeographic expansion in the history of life (Ehleringer and Monson, 1983). This complex pathway, involving novel patterns of biochemical compartmentation and anatomical design, has evolved with independent but convergent patterns approximately fifty times during the relatively short geological span of 12–15 million years (Kellogg, 1999; Monson, 1999; Sage, 2004; Christin et al., 2007). The appearance of C₄photosynthesis has changed the nature of photosynthetic productivity and ecosystem structure on Earth, both regionally and globally. Grassland ecosystems emerged in southwestern Asia, Africa and North America during the mid- to late-Miocene (5–10 Ma) and continued through the Pliocene, (~3Ma), with many of these systems dominated by C₄species (Cerling, 1999; Beerling and Osborne, 2006). During the appearance of C⁴ grasslands, the trophic structures of grazed ecosystems were completely revised, resulting in the emergence of novel mammalian lineages (Cerling et al., 1993; Wang et al., 1994; MacFadden and Cerling, 1996; Ehleringer et al., 1997). Arguably, there is not a better example in the history of life to illustrate the tightly integrated nature of evolutionary novelty and ecological impact, as that shown in C⁴ photosynthesis. Clearly, C⁴ photosynthesis, though present in only 8,000 of the estimated 250,000 higher plant species, deserves a significant role in the discussion of plant biology.