Diffusional conductances to CO₂ as a target for increasing photosynthesis and photosynthetic water-use efficiency

A key objective for sustainable agriculture and forestry is to breed plants with both high carbon gain and water-use efficiency (WUE). At the level of leaf physiology, this implies increasing net photosynthesis (A _N) relative to stomatal conductance (g _s). Here, we review evidence for CO₂ diffusional constraints on photosynthesis and WUE. Analyzing past observations for an extensive pool of crop and wild plant species that vary widely in mesophyll conductance to CO₂ (g _m), g _s, and foliage A _N, it was shown that both g _s and g _m limit A _N, although the relative importance of each of the two conductances depends on species and conditions. Based on Fick's law of diffusion, intrinsic WUE (the ratio A _N/g _s) should correlate on the ratio g _m/g _s, and not g _m itself. Such a correlation is indeed often observed in the data. However, since besides diffusion A _N also depends on photosynthetic capacity (i.e., V _c,max), this relationship is not always sustained. It was shown that only in a very few cases, genotype selection has resulted in simultaneous increases of both A _N and WUE. In fact, such a response has never been observed in genetically modified plants specifically engineered for either reduced g _s or enhanced g _m. Although increasing g _m alone would result in increasing photosynthesis, and potentially increasing WUE, in practice, higher WUE seems to be only achieved when there are no parallel changes in g _s. We conclude that for simultaneous improvement of A _N and WUE, genetic manipulation of g _m should avoid parallel changes in g _s, and we suggest that the appropriate trait for selection for enhanced WUE is increased g _m/g _s.