A portrait of the C4 photosynthetic family on the 50th anniversary of its discovery: species number, evolutionary lineages, and Hall of Fame

Fifty years ago, the C₄ photosynthetic pathway was first characterized. In the subsequent five decades, much has been learned about C₄ plants, such that it is now possible to place nearly all C₄ species into their respective evolutionary lineages. Sixty-one independent lineages of C₄ photosynthesis are identified, with additional, ancillary C₄ origins possible in 12 of these principal lineages. The lineages produced ~8100 C₄ species (5044 grasses, 1322 sedges, and 1777 eudicots). Using midpoints of stem and crown node dates in their respective phylogenies, the oldest and most speciose C₄ lineage is the grass lineage Chloridoideae, estimated to be near 30 million years old. Most C₄ lineages are estimated to be younger than 15 million years. Older C₄ lineages tend to be more speciose, while those younger than 7 million years have <43 species each. To further highlight C₄ photosynthesis for a 50th anniversary snapshot, a Hall of Fame comprised of the 40 most significant C₄ species is presented. Over the next 50 years, preservation of the Earth's C₄ diversity is a concern, largely because of habitat loss due to elevated CO₂ effects, invasive species, and expanded agricultural activities. Ironically, some members of the C₄ Hall of Fame are leading threats to the natural C₄ flora due to their association with human activities on landscapes where most C₄ plants occur.