Determination of the Average Partial Pressure of CO₂ in Chloroplasts From Leaves of Several C₃ Plants

Abstract

Measurements of CO₂ and water vapour exchange by leaves were combined with measurements of carbon isotope composition (¹³C/¹²C) 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, p_I/p_a. 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, p_I/p_a, 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 gw was correlated with the chloroplast surface area appressing intercellular airspaces.