title: Diffusional conductances to CO2 as a target for increasing photosynthesis and photosynthetic water-use efficiency.
abstract: 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.
authors.affiliation: Research Group on Plant Biology under Mediterranean Conditions, Departament de Biologia, Universitat de les Illes Balears, Carretera de Valldemossa Km 7.5, 07122, Palma de Mallorca, Illes Balears, Spain, jaume.flexas@uib.es.
authors.forename: Jaume
authors.initials: J
authors.lastname: Flexas
authors.forename: Ulo
authors.initials: U
authors.lastname: Niinemets
authors.forename: Alexander
authors.initials: A
authors.lastname: Gallé
authors.forename: Margaret M
authors.initials: MM
authors.lastname: Barbour
authors.forename: Mauro
authors.initials: M
authors.lastname: Centritto
authors.forename: Antonio
authors.initials: A
authors.lastname: Diaz-Espejo
authors.forename: Cyril
authors.initials: C
authors.lastname: Douthe
authors.forename: Jeroni
authors.initials: J
authors.lastname: Galmés
authors.forename: Miquel
authors.initials: M
authors.lastname: Ribas-Carbo
authors.forename: Pedro L
authors.initials: PL
authors.lastname: Rodriguez
authors.forename: Francesc
authors.initials: F
authors.lastname: Rosselló
authors.forename: Raju
authors.initials: R
authors.lastname: Soolanayakanahally
authors.forename: Magdalena
authors.initials: M
authors.lastname: Tomas
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Wright
authors.forename: Graham D
authors.initials: GD
authors.lastname: Farquhar
authors.forename: Hipólito
authors.initials: H
authors.lastname: Medrano
mesh_terms: D000040:Abscisic Acid / Q000494:pharmacology; D017360:Arabidopsis / Q000187:drug effects / Q000235:genetics / Q000502:physiology; D002245:Carbon Dioxide / Q000378:metabolism*; D004058:Diffusion / Q000187:drug effects; D005838:Genotype; D058503:Mesophyll Cells / Q000187:drug effects / Q000502:physiology; D010788:Photosynthesis / Q000187:drug effects; D054046:Plant Stomata / Q000187:drug effects / Q000502:physiology; D010944:Plants / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D030821:Plants, Genetically Modified; D013045:Species Specificity; D027843:Vitis / Q000187:drug effects / Q000502:physiology; D014867:Water / Q000378:metabolism*
chemical_list: D014867:Water; D002245:Carbon Dioxide; D000040:Abscisic Acid