Mesophyll conductance to CO 2 : current knowledge and future prospects

During photosynthesis, CO2 moves from the atmosphere (Ca) surrounding the leaf to the sub-stomatal internal cavities (Ci) through stomata, and from there to the site of carboxylation inside the chloroplast stroma (Cc) through the leaf mesophyll. The latter CO2 diffusion component is called mesophyll conductance (gm), and can be divided in at least three components, that is, conductance through intercellular air spaces (gias), through cell wall (gw) and through the liquid phase inside cells (gliq). A large body of evidence has accumulated in the past two decades indicating that gm is sufficiently small as to significantly decrease Cc relative to Ci, therefore limiting photosynthesis. Moreover, gm is not constant, and it changes among species and in response to environmental factors. In addition, there is now evidence that gliq and, in some cases, gw, are the main determinants of gm. 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 gm 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 gm among plant species and functional groups, and a revision of the responses of gm to different external (biotic and abiotic) and internal (developmental, structural and metabolic) factors. The possible physiological bases for gm, including aquaporins and carbonic anhydrases, are discussed. Possible ecological implications for variable gm are indicated, and the errors induced by neglecting gm when interpreting photosynthesis and carbon isotope discrimination models are highlighted. Finally, a series of research priorities for the near future are proposed. Key-words: AN-Ci curves; aquaporins; carbonic anhydrase; CO2 diffusion conductance; isotope discrimination; leaf internal diffusion conductance; photosynthesis. INTRODUCTION: THE CONCEPT OF MESOPHYLL CONDUCTANCE TO CO2 AND THE EVOLUTION OF ITS PERCEPTION IN PLANT SCIENCE During photosynthesis, CO2 has to move from the atmosphere surrounding the leaf across a boundary layer in the air above the foliage surface to the sub-stomatal internal cavities through the stomata (Fig. 1a), and from there to the site of carboxylation inside the stroma through the leaf mesophyll (Fig. 1b). From Fick’s first law of diffusion, the net photosynthetic flux at steady state (AN) can be expressed as: AN = gs (Ca Ci) = gm (Ci Cc), where gs and gm are the stomatal and mesophyll conductance to CO2 diffusion, and Ca, Ci and Cc are the CO2 concentrations in the atmosphere, in the sub-stomatal internal cavity and in the chloroplast stroma, respectively (Long & Bernacchi 2003). While this two-dimensional view of CO2 diffusion is mostly valid when referring to the stomata, it is rather simplistic when referring to the mesophyll, and it may be inadequate to describe a system where there are CO2 sinks and sources distributed along the pathway (Parkhurst 1994). However, although there have been attempts to develop three-dimensional models for CO2 mesophyll diffusion (Parkhurst 1994; Aalto & Juurola 2002; Juurola et al. 2005), at present, they are far from being of general use. Nevertheless, as pointed out by Parkhurst (1994), ‘representing internal diffusion limitations with single numbers seems preferable to neglecting them completely’. Therefore, the present review intends to update the current knowledge on mesophyll conductance from a two-dimensional perspective, considering only vertical CO2 diffusion through leaf mesophyll. Lateral CO2 diffusion inside the mesophyll can also be important for photosynthesis in some cases (Morison et al. 2005; Pieruschka, Schurr & Jahnke 2005), although this aspect has already been reviewed recently (Morison & Lawson 2007). In Gaastra’s (1959) pioneer work on leaf photosynthesis, mesophyll conductance (and its inverse, mesophyll resistance) was defined essentially as a diffusion component of the photosynthesis pathway, and it was considered to be an important factor in determining leaf photosynthesis (actually more important than stomatal conductance). Later, Correspondence: J. Flexas. Fax: +34 971 173184; e-mail: jaume.flexas@uib.es Plant, Cell and Environment (2008) 31, 602–621 doi: 10.1111/j.1365-3040.2007. 01757.x © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd 602 Troughton & Slatyer (1969) extended the use of the term to a ‘mixed’ diffusion-biochemical component, that is, to refer to the initial slope of AN versus Ci relationship. Following Gaastra’s (1959) assumption that CO2 concentration inside the chloroplast was near zero (i.e. at the CO2 compensation concentration), Jones & Slatyer (1972b) proposed a method to separate the transport and carboxylation components of what continued to be called mesophyll or intracellular conductance. Using this method, earlier conclusions by Gaastra that mesophyll transport resistance was a limiting factor for photosynthesis were confirmed (Jones & Slatyer 1972a,b; Samsuddin & Impens 1979), and it was suggested, for the first time, that the leaf internal resistance to CO2 transfer could be variable and could respond, for instance, to water stress (Jones 1973). The nomenclature was further complicated by Samsuddin & Impens (1979), who termed internal resistance to the ‘mixed’ diffusion-biochemical term, while restoring the use of the term mesophyll resistance to refer to the transfer component alone. Such a semantic confusion continues nowadays, and several more recent papers still call ‘mesophyll conductance’ or ‘internal conductance’ to the initial slope of AN versus Ci relationship (e.g. Moldau & Kull 1993; Laisk & Loreto 1996; Eichelmann et al. 2004a,b; Béjaoui et al. 2006). We propose, as already did Farquhar & Sharkey (1982) and Parkhurst (1994), to restrict the term mesophyll conductance to the diffusion of CO2 through leaf mesophyll, both through intercellular air spaces (gias), cell wall (gw) and the intracellular liquid pathway (gliq). That is, mesophyll conductance should be viewed as synonymous of ‘leaf internal diffusion conductance’, while terms such as ‘apparent carboxylation efficiency’ or ‘apparent carboxylation conductance’ may be more adequate to refer to the initial slope of the AN versus Ci relationship. With the introduction of the most commonly used leaf photosynthesis model by Farquhar, von Caemmerer & Berry (1980), the early assumption that CO2 concentration in the chloroplasts was close to zero or to the compensation point was rejected, as was later being confirmed by direct measurements of Ci (Lauer & Boyer 1992). Thereafter, most gas exchange studies have usually assumed that gm is large and constant, that is, that Ci = Cc. However, different studies in the late 80s and early 90s have already suggested that Cc is significantly less than Ci, although not close to zero. Evans et al. (1986) reached this conclusion by comparing online carbon isotope discrimination of photosynthesizing leaves with the theoretical discrimination that would be expected if Ci = Cc. Evans (1983) and Evans & Terashima (1988) reached the same conclusion by comparing the initial slope of the AN versus Ci curve in wheat and spinach leaves with the activity of ribulose 1·5-bisphosphate carboxylase/oxygenase (Rubisco) as determined in vitro. With the advent of pulse-amplitude fluorometers, the comparison of chlorophyll fluorescence with gas exchange measurements also indicated that Cc was lower than Ci, and that their difference increased under conditions of water stress or salinity (Bongi & Loreto 1989; Cornic et al. 1989; Di Marco et al. 1990). The water stress-induced decrease of Cc below Ci was independently demonstrated by measuring leaf O discrimination (Renou et al. 1990), and was lately confirmed several times by both chlorophyll fluorescence and leaf O discrimination (Tourneux & Peltier 1995; Sánchez-Rodríguez, Pérez & Martínez-Carrasco 1999). METHODOLOGIAL ADVANCES IN DETERMINING MESOPHYLL CONDUCTANCE AND ITS COMPONENTS The pioneer studies described in the previous section includes the development of several methods that have become usual to estimate mesophyll conductance to CO2.A detailed description of the principles and procedures of these methods has recently been reviewed by Warren (2006a). Therefore, the present review will only mention the existing methods, providing the original and extended references for readers interested in a more detailed description. cw