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

Leaf CO uptake (A) in C photosynthesis is limited by the maximum apparent rate of PEPc carboxylation (V ) at low intercellular [CO ] (c ) with a sharp transition to a c -saturated rate (V ) due to co-limitation by ribulose-1:5-bisphosphate carboxylase/oxygenase (Rubisco) and regeneration of PEP. The response of A to c has been widely used to determine these two parameters. V and V depend on different enzymes but draw on a shared pool of leaf resources, such that resource distribution is optimized, and A maximized, when V and V are co-limiting. We collected published A/c curves in 49 C species and assessed variation in photosynthetic traits between phylogenetic groups, and as a function of atmospheric [CO ]. The balance of V -V varied among evolutionary lineages and C subtypes. Operating A was strongly V -limited, such that re-allocation of resources from V towards V was predicted to improve A by 12% in C 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 [CO ], suggesting C crops have not adjusted to the rapid increase in atmospheric [CO ] of the past few decades.