Gradients of Intercellular CO₂ Levels Across the Leaf Mesophyll

Abstract

Most current photosynthesis models, and interpretations of many wholeleaf CO2 gas exchange measurements, are based on the often unstated assumption that the partial pressure of CO2 is nearly uniform throughout the airspaces of the leaf mesophyll. Here we present measurements of CO2 gradients across amphistomatous leaves allowed to assimilate CO2 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 CO2 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 CO2 had been supplied equally at both surfaces then the respective mean intercellular CO2 pressures would have been roughly 12 and 27 microbar less than the pressures in the substomatal cavities in these cases. For ambient CO2 pressures near 320 microbar, the average and minimum pressure differences across the mesophyll were 45 and 13 microbar. The corresponding mean intercellular CO2 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.there. Because no net CO2 exchange occurred through that side, the CO2 pressure measured in that cuvette would be close to the pressure in the substomatal chambers. The other chamber was operated normally, and pi was calculated for that side using the methods of Caemmerer and Farquhar (1) (a list of abbreviations used in this paper can be found in Table I). The maximum differences in CO2 pressure across the mesophyll were 10 to 14 ,ubar. These results suggest that in a 'one-sided' leaf having internal structure similar to that in cotton and cocklebur, pi should vary by no more than about 14 ,ubar across the mesophyll.Mott and O'Leary (4) performed a modified version of the Sharkey experiment using open gas exchange systems on both sides of sunflower and cocklebur leaves. They operated the system normally on one side, and adjusted the ambient CO, level at the other side until no net gas exchange occurred there. This technique allowed them to maintain small and equal positive pressures in both chambers. This method prevented room air (often high in CO2) from leaking into either chamber, and also removed any pressure difference that might drive a bulk flow of air through the leaf.In sunflower, Mott and O'Leary (4) found no measurable differences in the estimates of c; at the two surfaces. In cocklebur, c, at the surface not exchanging CO2 was always lower than c; at the other surface. The difference increased with increase in ambient CO2 concentration at the 'fed' surface. For example (K Mott, personal communication, 1985), when the upper and lower surfaces o...

Diffusion of CO2and other gases inside leaves

“…Reiati\ely large A^,.^ differences were also found by Parkhurst et al (1988) using Mott & O'Leary's methods with twelve lea\'es of five species. When />" at the fed side was at normal atmospheric levels near 32 Pa, we found p^^ differences up to 2'3 Pa in Phaseolus vulgaris, 3 Pa in Brassica chinensis, 3-6 Pa in G. hirsutum, 3-7 Pa in Spinacia oleracea, and 7 3 Pa in Eucalyptus paucifiora.…”

“…For this reason, 'p^^'' will replace '/>,' in this paper (cf. Parkhurst et a/., 1988;Lloyd et al, 1992). Also, 'Pi,,.,' is the partial pressure at particular points within the intercellular airspace, and '?^' will represent the assimilation-weighted mean CO., pressure m the mesophyll, i.e.…”

“…This reduction indicates a timitation caused by cross-mesophyll diffusion to the sites with higher illumination levels, but must underestimate that limitation because of the assumption of 1-D diffusion. Indeed, Parkhurst et al (1988) measured a 20-25 °o difference in one leaf of Eucalyptus pauciflora. (Gutschick's calculations appear to have been made with diffusivity £),," -7 X 10"" m"-s"\ about half the values usually quoted.…”

Gas-Exchange Properties of Salt-Stressed Olive (Olea europea L.) Leaves

“…Parkhurst et al (14) reported a relationship between rd, the diffusivity of CO2 in the air (Dd), and the thickness of the leaf (td). This relationship can be reduced to rd = 2td/Dd in hypostomatous leaves.…”

“…To this aim we have carried out measurements of quantum yield, Chl fluorescence, CO2 and 02 responses of photosynthesis and have calculated 02 effects on photosynthesis. Changes in CO2 permeability inside the leaves were also considered since in schlerophyllous leaves the increase in leaf thickness could produce a substantial variation in the resistance to C02 diffusion (14).…”