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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

Sonawane, Balasaheb V., Cousins, Asaph B.
Thenew phytologist 2019 v.222 no.1 pp. 122-131
C3 photosynthesis, C3 plants, Panicum, carbon, carbon dioxide, carbonate dehydratase, enzyme activity, evaporation, gas exchange, grasses, isotopes, leaves, mesophyll, oxygen, pH, temperature, uncertainty
The internal CO₂ gradient imposed by mesophyll conductance (gₘ) reduces substrate availability for C₃ photosynthesis. With several assumptions, estimates of gₘ can be made from coupled leaf gas exchange with isoflux analysis of carbon ∆¹³C‐gₘ and oxygen in CO₂, coupled with transpired water (H₂O) ∆¹⁸O‐gₘ to partition gₘ 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°C) of ∆¹³C‐gₘ and ∆¹⁸O‐gₘ along with leaf biochemistry in the C₃ grass Panicum bisulcatum, which has naturally low carbonic anhydrase activity. Our study suggests that assumptions regarding the extent of isotopic equilibrium (θ) between CO₂ and H₂O at the site of exchange, and that the isotopic composition of the H₂O at the sites of evaporation (δw−e18) and at the site of exchange (δw−ce18) are similar, may lead to errors in estimating the ∆¹⁸O‐gₘ temperature response. The input parameters for ∆¹³C‐gₘ 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 ∆¹⁸O‐gₘ under changing leaf temperatures.