|Portis Jr, Archie|
|Von Caemmerer, S|
Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/31/2002
Publication Date: 12/31/2002
Citation: BERNACCHI, C.J., PORTIS JR, A.R., NAKANO, H., VON CAEMMERER, S., LONG, S.P. TEMPERATURE RESPONSE OF MESOPHYLL CONDUCTANCE, IMPLICATIONS FOR THE DETERMINATION OF RUBISCO ENZYME KINETICS AND FOR LIMITATIONS TO PHOTOSYNTHESIS IN VIVO. PLANT PHYSIOLOGY. 2002. v. 130. p. 1992-1998. Interpretive Summary: Predicting the environmental responses of leaf photosynthesis is central to many models attempting to assess the impacts of global climate change. One factor potentialy limiting photosynthesis is the rate of CO2 transfer from the intercellular airspaces of the leaf into the chloroplast, defined as mesophyll conductance. In this study, the temperature response of mesophyll conductance was determned by two diffeent methods, yielding similar results. Mesophyll conductance was observed to increasingly limit photosynthesis at higher temperatures. The results provide an improved ability to model leaf photosynthesis at various temperatures, which is necessary for predicting global carbon uptake in response to global climate change.
Technical Abstract: CO2 transfer conductance from the intercellular airspaces of the leaf into the chloroplast, defined as mesophyll conductance, is finite and thus limits the rate of photosynthesis. Little is known about the processes that determine the magnitude of mesophyll conductance. Although a role has been suggested for carbonic anhydrase and aquaporins, most studies have assumed that diffusion of CO2 in the liquid phase determines mesophyll conductance. In this study, the temperature response of mesopnyll conductance from 10 to 30 degrees C in mature leaves of wild-type tobacco (Nicotiana tabacum L. cv W38)is 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, suggesting that mesophyll conductance may be controlled by an enzyme-catalyzed process rather than by simple diffusion. Further, mesophyll conductance values are maximal slightly above 35 degrees C and then decrease, again suggesting mesophyll conductance is an enzyme dominated process, but with a lower energy of deactivation than Rubiwsco. Using the temperature response of mesophyll conductance to calculate CO2 at Rubisco, the kinetic parameters of Rubisco were calculated in vivo from 10 to 40 degrees C. Using these parameters, it was possible to determine the limitation imposed on photosynthesis by mesophyll conductance. The fractional limitation rises from 0.10 at 10 degrees C to 0.22 at 40 degrees C. This shows that despite the exponential rise over this temperature range, mesophyll conductance does not keep pace with increased capacity for CO2 uptake at the site of Rubisco. It also shows that transfer of CO2 from the intercellular air space to Rubisco is a very substantial limitation on photosynthesis, especially at high temeprature.