No abstract

CO 2 transfer conductance from the intercellular airspaces of the leaf into the chloroplast, defined as mesophyll conductance (g m ), is finite. Therefore, it will limit photosynthesis when CO 2 is not saturating, as in C3 leaves in the present atmosphere. Little is known about the processes that determine the magnitude of g m . The process dominating g m is uncertain, though carbonic anhydrase, aquaporins, and the diffusivity of CO 2 in water have all been suggested. The response of g m to temperature (10°C-40°C) in mature leaves of tobacco (Nicotiana tabacum L. cv W38) was determined using measurements of leaf carbon dioxide and water vapor exchange, coupled with modulated chlorophyll fluorescence. These measurements revealed a temperature coefficient (Q 10 ) of approximately 2.2 for g m , suggesting control by a protein-facilitated process because the Q 10 for diffusion of CO 2 in water is about 1.25. Further, g m values are maximal at 35°C to 37.5°C, again suggesting a protein-facilitated process, but with a lower energy of deactivation than Rubisco. Using the temperature response of g m to calculate CO 2 at Rubisco, the kinetic parameters of Rubisco were calculated in vivo from 10°C to 40°C. Using these parameters, we determined the limitation imposed on photosynthesis by g m . Despite an exponential rise with temperature, g m does not keep pace with increased capacity for CO 2 uptake at the site of Rubisco. The fraction of the total limitations to CO 2 uptake within the leaf attributable to g m rose from 0.10 at 10°C to 0.22 at 40°C. This shows that transfer of CO 2 from the intercellular air space to Rubisco is a very substantial limitation on photosynthesis, especially at high temperature.In C3 plants, the diffusion of CO 2 from the atmosphere to the active site of Rubisco follows a complex pathway involving as many as eight discrete conductance components (Nobel, 1999). Most commonly, this pathway is simplified into three main components: boundary layer, stomatal conductance, and mesophyll conductance (g m ; Farquhar and Sharkey, 1982). Boundary layer conductance depends on several leaf physical and environmental properties, in particular, size, surface structures, stomatal location, and air movement around the leaf, whereas stomatal conductance is primarily influenced by stomatal pore numbers and dimensions. The flexible and dynamic qualities of the stomatal pores provide the leaf with physiological control of CO 2 influx and water efflux (Farquhar and Sharkey, 1982). Estimates of boundary layer and stomatal conductances to CO 2 are based on water vapor released from the leaf because water and CO 2 share the same gaseous diffusion pathway (e.g. von Caemmerer and Farquhar, 1981). As a result, it has long been known that limitations of diffusion through the stomata and boundary layer are purely physical (Penman and Schofield, 1951). g m , defined as the conductance of CO 2 transfer from the intercellular leaf airspaces to the site of carboxylation, was initially assumed large enough to have a negligible...

Abbreviations: t, Rubisco specificity factor (dimensionless); G*, CO 2 compensation point in the absence of dark respiration (mmol mol -2 ); DH a , energy of activation (kJ mol -1 ); A, net rate of CO 2 uptake per unit leaf area (mmol m -2 s -1 ); c, scaling constant (dimensionless); C i , intercellular CO 2 concentration (mmol mol -1 ); K c , Michaelis constant for CO 2 (mmol mol -1 ); K o , Michaelis constant for O 2 (mmol mol -1 ); R, molar gas constant (kJ K -1 mol -1 ); R d , Mitochondrial respiration rate in the light (mmol m -2 s -1 ); T k , Leaf absolute temperature (K); v c , carboxylation velocity (mmol m -2 s -1 ), V c,max , maximum RuBP saturated rate of carboxylation (mmol m -2 s -1 ); v o , oxygenation velocity (mmol m -2 s -1 ); V o,max , maximum RuBP saturated rate of oxygenation (mmol m -2 s -1 ); W c , RuBP saturated rate of carboxylation (mmol m -2 s -1 ); PPFD, photosynthetically active photon flux density (mmol m -2 s -1 ).