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 (Vpmax ) at low intercellular [CO2 ] (ci ) with a sharp transition to a ci -saturated rate (Vmax ) due to co-limitation by ribulose-1:5-bisphosphate carboxylase/oxygenase (Rubisco) and regeneration of PEP. The response of A to ci has been widely used to determine these two parameters. Vmax and Vpmax depend on different enzymes but draw on a shared pool of leaf resources, such that resource distribution is optimized, and A maximized, when Vmax and Vpmax are co-limiting. We collected published A/ci curves in 49 C4 species and assessed variation in photosynthetic traits between phylogenetic groups, and as a function of atmospheric [CO2 ]. The balance of Vmax -Vpmax varied among evolutionary lineages and C4 subtypes. Operating A was strongly Vmax -limited, such that re-allocation of resources from Vpmax towards Vmax 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.