DETERMINATION OF THE AVERAGE PARTIAL-PRESSURE OF CO2 IN CHLOROPLASTS FROM LEAVES OF SEVERAL C3 PLANTS

Measurements of CO2 and water vapour exchange by leaves were combined with measurements of carbon isotope composition (C-13/C-12) of CO2 in the air passing over the leaf. Carbon isotope discrimination during CO2 uptake was determined from the difference in carbon isotope composition of the air leaving the leaf chamber with or without a leaf enclosed. Leaves of wheat plants grown with different nitrogen nutrition and leaves of several other species were examined. The measurements, made at different irradiances for a given leaf, showed that carbon isotope discrimination was strongly correlated with the rate of CO2 assimilation as well as the ratio of intercellular to ambient partial pressure of CO2, Pi/Pa. A function relating carbon isotope discrimination to the rate of CO2 assimilation was used to estimate the CO2 transfer conductance, g(w), from the substomatal cavities to the sites of carboxylation for individual leaves. The photosynthetic capacity correlated with the CO2 transfer conductance, g(w), and the average ratio of chloroplastic to intercellular partial pressure of CO2, Pc/Pi, was 0.7. This means that in general under high irradiance, the ratio of chloroplastic to ambient partial pressure of CO2 is about 0.5. In wheat, variation in g(w) was correlated with the chloroplast surface area appressing intercellular airspaces.