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 (AN) relative to stomatal conductance (gs). 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 (gm), gs, and foliage AN, it was shown that both gs and gm limit AN, 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 AN/gs) should correlate on the ratio gm/gs, and not gm itself. Such a correlation is indeed often observed in the data. However, since besides diffusion AN also depends on photosynthetic capacity (i.e., Vc,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 AN and WUE. In fact, such a response has never been observed in genetically modified plants specifically engineered for either reduced gs or enhanced gm. Although increasing gm 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 gs. We conclude that for simultaneous improvement of AN and WUE, genetic manipulation of gm should avoid parallel changes in gs, and we suggest that the appropriate trait for selection for enhanced WUE is increased gm/gs.