Retrospective analysis of biochemical limitations to photosynthesis in 49 species: C₄ crops appear still adapted to pre-industrial atmospheric [CO₂]

Leaf CO₂ uptake (A) in C₄ photosynthesis is limited by the maximum apparent rate of PEPc carboxylation (V_pmax) at low intercellular [CO₂] (c_i) with a sharp transition to a c_i-saturated rate (V_max) due to co-limitation by ribulose-1:5-bisphosphate carboxylase/oxygenase (Rubisco) and regeneration of PEP. The response of A to c_i has been widely used to determine these two parameters. V_max and V_pmax depend on different enzymes but draw on a shared pool of leaf resources, such that resource distribution is optimized, and A maximized, when V_max and V_pmax are co-limiting. We collected published A/c_i curves in 49 C₄ species and assessed variation in photosynthetic traits between phylogenetic groups, and as a function of atmospheric [CO₂]. The balance of V_max-V_pmax varied among evolutionary lineages and C₄ subtypes. Operating A was strongly V_max-limited, such that re-allocation of resources from V_pmax towards V_max was predicted to improve A by 12% in C₄ crops. This would not require additional inputs but rather altered partitioning of existing leaf nutrients, resulting in increased water and nutrient-use efficiency. Optimal partitioning was achieved only in plants grown at pre-industrial atmospheric [CO₂], suggesting C₄ crops have not adjusted to the rapid increase in atmospheric [CO₂] of the past few decades.