Ecological selection pressures for C4photosynthesis in the grasses.

Grasses using the C4photosynthetic pathway dominate grasslands and savannahs of warm regions, and account for half of the species in this ecologically and economically important plant family. The C4pathway increases the potential for high rates of photosynthesis, particularly at high irradiance, and raises water-use efficiency compared with the C3type. It is therefore classically viewed as an adaptation to open, arid conditions. Here, we test this adaptive hypothesis using the comparative method, analysing habitat data for 117 genera of grasses, representing 15 C4lineages. The evidence from our three complementary analyses is consistent with the hypothesis that evolutionary selection for C4photosynthesis requires open environments, but we find an equal likelihood of C4evolutionary origins in mesic, arid and saline habitats. However, once the pathway has arisen, evolutionary transitions into arid habitats occur at higher rates in C4than C3clades. Extant C4genera therefore occupy a wider range of drier habitats than their C3counterparts because the C4pathway represents a pre-adaptation to arid conditions. Our analyses warn against evolutionary inferences based solely upon the high occurrence of extant C4species in dry habitats, and provide a novel interpretation of this classic ecological association. [ABSTRACT FROM AUTHOR]