C2 Photosynthesis across Scales

Stomata patterning, leaf structure, gas exchange, and ultimately species biogeography are inherently connected and have been observed to differ in plants by photosynthetic type. Patterns distinguishing C3 and C4 plants across scales are elucidated well in the literature. However, how trait shifts occur along the C3 to C4 evolutionary continuum are less clear. This review focuses on the role of C2 species as C3-C4 evolutionary intermediates that help to transition important traits from typical C3 to C4 phenotypes across scales. We show that the C2 phenotype is remarkably diverse and not always intermediate between C3 and C4 phenotypes. We highlight that studies of C2 species in the literature commonly lump C3-C4 evolutionary intermediates, stable state C2 species, and C3 × C4 hybrid species together and we argue that these different C2 types should be distinguished, when possible, and researched independently to help explain the broad diversity of C2 phenotypes and their global biogeography.