The taxonomic distribution of C4 photosynthesis in Amaranthaceae sensu stricto

C-4 photosynthesis evolved multiple times in the Amaranthaceae s.s., but the C-4 evolutionary lineages are unclear because the photosynthetic pathway is unknown for most species of the family. To clarify the distribution of C-4 photosynthesis in the Amaranthaceae, we determined carbon isotope ratios of 607 species and mapped these onto a phylogeny determined from matK/trnK sequences. Approximately 28% of the Amaranthaceae species use the C-4 pathway. C-4 species occur in 10 genera-Aerva, Amaranthus, Blutaparon, Alternanthera, Froelichia, Lithophila, Guilleminea, Gomphrena, Gossypianthus, and Tidestromia. Aerva, Alternanthera, and Gomphrena contain both C-3 and C-4 species. In Aerva, 25% of the sampled species are C-4. In Alternanthera, 19.5% are C-4, while 89% of the Gomphrena species are C-4. Integration of isotope and matK/trnK data indicated C-4 photosynthesis evolved five times in the Amaranthaceae, specifically in Aerva, Alternanthera, Amaranthus, Tidestromia, and a lineage containing Froelichia, Blutaparon, Guilleminea, Gomphrena pro parte, and Lithophila. Aerva and Gomphrena are both polyphyletic with C-3 and C-4 species belonging to distinct clades. Alternanthera appears to be monophyletic with C-4 photosynthesis originating in a terminal sublineage of procumbent herbs. Alpine C-4 species were also identified in Alternanthera, Amaranthns, and Gomphrena, including one species (Gomphrena rneyeniana) from 4600 m a.s.1.