Environmental factors have a greater influence on photosynthetic capacity in C₄ plants than biochemical subtypes or growth forms

Our understanding of how photosynthetic capacity varies among C₄ species and across growth and measurement conditions remains limited. We collated 1696 CO₂ response curves of net CO₂ assimilation rate (A/C_i curves) from C₄ species grown and measured at various environmental conditions and used these data to estimate the apparent maximum carboxylation activity of phosphoenolpyruvate carboxylase (V_pmaxA) and CO₂-saturated net photosynthetic rate (A_max), two key parameters describing photosynthetic capacity. We examined how V_pmaxA and A_max vary with species-specific traits, growth and measurement conditions. We found little systematic variation of V_pmaxA and A_max across the classical C₄ biochemical subtypes or growth forms, but showed that growth temperature and measurement conditions are major factors determining C₄ photosynthetic capacity. We found no evidence that common C₄ model species (e.g. maize, sorghum and Setaria viridis) differ in photosynthetic capacity from other C₄ species when grown in controlled environments. However, C₄ model species showed up to twice the photosynthetic capacity of other C₄ species when grown in the field. Our multivariate model accounts for 47–51% of the variation reported in V_pmaxA and A_max, and we argue that environmental conditions have a greater influence on C₄ photosynthetic capacity than biochemical subtypes or growth forms.