title: Mesophyll conductance to CO2: current knowledge and future prospects.
abstract: During photosynthesis, CO2 moves from the atmosphere (C(a)) surrounding the leaf to the sub-stomatal internal cavities (C(i)) through stomata, and from there to the site of carboxylation inside the chloroplast stroma (C(c)) through the leaf mesophyll. The latter CO2 diffusion component is called mesophyll conductance (g(m)), and can be divided in at least three components, that is, conductance through intercellular air spaces (g(ias)), through cell wall (g(w)) and through the liquid phase inside cells (g(liq)). A large body of evidence has accumulated in the past two decades indicating that g(m) is sufficiently small as to significantly decrease C(c) relative to C(i), therefore limiting photosynthesis. Moreover, g(m) is not constant, and it changes among species and in response to environmental factors. In addition, there is now evidence that g(liq) and, in some cases, g(w), are the main determinants of g(m). Mesophyll conductance is very dynamic, changing in response to environmental variables as rapid or even faster than stomatal conductance (i.e. within seconds to minutes). A revision of current knowledge on g(m) is presented. Firstly, a historical perspective is given, highlighting the founding works and methods, followed by a re-examination of the range of variation of g(m) among plant species and functional groups, and a revision of the responses of g(m) to different external (biotic and abiotic) and internal (developmental, structural and metabolic) factors. The possible physiological bases for g(m), including aquaporins and carbonic anhydrases, are discussed. Possible ecological implications for variable g(m) are indicated, and the errors induced by neglecting g(m) when interpreting photosynthesis and carbon isotope discrimination models are highlighted. Finally, a series of research priorities for the near future are proposed.
authors.affiliation: Grup de Recerca en Biologia de les Plantes en Condicions Mediterrànies, Departament de Biologia, Universitat de les Illes Balears, Carretera de Valldemossa Km 7.5, 07122 Palma de Mallorca, Balears, Spain. jaume.flexas@uib.es
authors.forename: Jaume
authors.initials: J
authors.lastname: Flexas
authors.forename: Miquel
authors.initials: M
authors.lastname: Ribas-Carbó
authors.forename: Antonio
authors.initials: A
authors.lastname: Diaz-Espejo
authors.forename: Jeroni
authors.initials: J
authors.lastname: Galmés
authors.forename: Hipólito
authors.initials: H
authors.lastname: Medrano
mesh_terms: D002245:Carbon Dioxide / Q000378:metabolism*; D010788:Photosynthesis / Q000502:physiology*; D018515:Plant Leaves / Q000166:cytology*; D018521:Plant Physiological Phenomena
chemical_list: D002245:Carbon Dioxide