Carbon Isotope Discrimination measured Concurrently with Gas Exchange to Investigate CO₂ Diffusion in Leaves of Higher Plants

Abstract

Conventional gas-exchange techniques that measure the stomatal conductance and rate of CO₂ assimilation of leaves were combined with measurements of the carbon isotope composition of CO₂ in air passing over a leaf. Isotopic discrimination during uptake was determined from the difference in the carbon isotope composition of air entering and leaving the leaf chamber. Isotopic discrimination measured over the short term correlated strongly with that determined from combusted leaf material. Environmental conditions were manipulated to alter the relative influences of stomatal conductance and carboxylation on the discrimination of carbon isotopes by intact leaves. With C₃ plants, discrimination increased as the gradient in partial pressure of CO₂ across the stomata decreased. For C₄ plants there was little change in discrimination despite substantial changes in the diffusion gradient across the sto- mata. These results are consistent with, and provide the first direct experimental support for, theoretical equations describing discrimination during photosynthesis.

Despite uncertainties about various processes affecting carbon isotope composition, the resistance to the transfer of CO₂ from the intercellular airspaces to the sites of carboxylation in the mesophyll chloroplasts was estimated using this technique. For wheat the estimated resistance was 1.2-2.4 m² s bar mol ^-1.