Gradients of Intercellular CO2 Levels Across the Leaf Mesophyll

Most current photosynthesis models, and interpretations of many wholeleaf CO(2) gas exchange measurements, are based on the often unstated assumption that the partial pressure of CO(2) is nearly uniform throughout the airspaces of the leaf mesophyll. Here we present measurements of CO(2) gradients across amphistomatous leaves allowed to assimilate CO(2) through only one surface, thus simulating hypostomatous leaves. We studied five species: Eucalyptus pauciflora Sieb. ex Spreng., Brassica chinensis L., Gossypium hirsutum L., Phaseolus vulgaris L., and Spinacia oleracea L. For Eucalyptus, maximum CO(2) pressure differences across the leaf mesophyll were 73 and 160 microbar when the pressures outside the lower leaf surface were 310 and 590 microbar, respectively. Using an approximate theoretical calculation, we infer that if the CO(2) had been supplied equally at both surfaces then the respective mean intercellular CO(2) pressures would have been roughly 12 and 27 microbar less than the pressures in the substomatal cavities in these cases. For ambient CO(2) pressures near 320 microbar, the average and minimum pressure differences across the mesophyll were 45 and 13 microbar. The corresponding mean intercellular CO(2) pressures would then be roughly 8 and 2 microbar less than those in the substomatal cavities. Pressure differences were generally smaller for the four agricultural species than for Eucalyptus, but they were nevertheless larger than previously reported values.