Uncertainties and limitations of using carbon-13 and oxygen-18 leaf isotope exchange to estimate the temperature response of mesophyll CO2 conductance in C3 plants

The internal CO2 gradient imposed by mesophyll conductance (g(m)) reduces substrate availability for C-3 photosynthesis. With several assumptions, estimates of g(m) can be made from coupled leaf gas exchange with isoflux analysis of carbon increment C-13-g(m) and oxygen in CO2, coupled with transpired water (H2O) increment O-18-g(m) to partition g(m) into its biochemical and anatomical components. However, these assumptions require validation under changing leaf temperatures. To test these assumptions, we measured and modeled the temperature response (15-40 degrees C) of increment C-13-g(m) and increment O-18-g(m) along with leaf biochemistry in the C-3 grass Panicum bisulcatum, which has naturally low carbonic anhydrase activity. Our study suggests that assumptions regarding the extent of isotopic equilibrium (theta) between CO2 and H2O at the site of exchange, and that the isotopic composition of the H2O at the sites of evaporation (delta w-e18) and at the site of exchange (delta w-ce18) are similar, may lead to errors in estimating the increment O-18-g(m) temperature response. The input parameters for increment C-13-g(m) appear to be less sensitive to temperature. However, this needs to be tested in species with diverse carbonic anhydrase activity. Additional information on the temperature dependency of cytosolic and chloroplastic pH may clarify uncertainties used for increment O-18-g(m) under changing leaf temperatures.