Temperature Response of Mesophyll Conductance. Implications for the Determination of Rubisco Enzyme Kinetics and for Limitations to Photosynthesis in Vivo

CO2 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 CO2 is not saturating, as in C3 leaves in the present atmosphere. Little is known about the processes that determine the magnitude ofg m . The process dominatingg m is uncertain, though carbonic anhydrase, aquaporins, and the diffusivity of CO2 in water have all been suggested. The response ofg 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 (Q10) of approximately 2.2 for g m , suggesting control by a protein-facilitated process because the Q10for diffusion of CO2 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 CO2 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 byg m . Despite an exponential rise with temperature, g m does not keep pace with increased capacity for CO2 uptake at the site of Rubisco. The fraction of the total limitations to CO2uptake within the leaf attributable tog m rose from 0.10 at 10°C to 0.22 at 40°C. This shows that transfer of CO2 from the intercellular air space to Rubisco is a very substantial limitation on photosynthesis, especially at high temperature.