Intercellular Diffusion Limits to CO 2 Uptake in Leaves

Abstract

We studied plants of five species with hypostomatous leaves, and six with amphistomatous leaves, to determine the extent to which gaseous diffusion of CO2 among the mesophyll cells limits photosynthetic carbon assimilation. In helox (air with nitrogen replaced by helium), the diffusivities of CO2 and water vapor are 2.3 times higher than in air. For fixed estimated CO2 pressure at the evaporating surfaces of the leaf (p'), assimilation rates in helox ranged up to 27% higher than in air for the hypostomatous leaves, and up to 7% higher in the amphistomatous ones. Thus, intercellular diffusion must be considered as one of the processes limiting photosynthesis, especially for hypostomatous leaves. A corollary is that CO2 pressure should not be treated as uniform through the mesophyll in many leaves. To analyze our helox data, we had to reformulate the usual gas-exchange equation used to estimate CO2 pressure at the evaporating surfaces of the leaf; the new equation is applicable to any gas mixture for which the diffusivities of CO2 and H20 are known. Finally, we describe a diffusion-biochemistry model for CO2 assimilation that demonstrates the plausibility of our experimental results. Modelers and experimentalists studying carbon assimilation by plant leaves commonly assume that the partial pressure pi (or concentration, ci) of CO2 is uniform throughout the intercellular air spaces in the leaf mesophyll (Table I provides a list of symbols and abbreviations used in this article). A corollary of this assumption is that CO2 diffusion in the air spaces occurs with negligible 'resistance,' and presents negligible limitation to CO2 assimilation. This article provides experimental evidence that internal airspace diffusion can substantially limit assimilation in some leaves, especially hypostomatous ones. This implies that Pi varies considerably with distance from the stomata in such leaves. The assumption of uniform pi has not always been made; early resistance-based models (e.g. 6, 7) sometimes included terms for an 'intercellular-diffusion-resistance' component. Public and Environmental Affairs. questionable resistance analogies. These models predict differences of up to 10 Pa in CO2 pressure from the substomatal cavity to the adaxial surface of hypostomatous leaves under some conditions. Parkhurst et al. (17) measured gradients similar to those predicted by the models, in amphistomatous ('amphi') leaves that simulated hypostomatous ('hypo') leaves because they were fed CO2 from only one surface. In addition to calculating CO2 gradients in leaves, Parkhurst (16) used his 1986 model to estimate that carbon assimilation would increase by 24% in Arbutus menziesii if the diffusion coefficient in the intercellular air spaces of its thick, sclero-phyllous leaves were increased, hypothetically, by a factor of one million. (This change can be interpreted as effectively eliminating any internal diffusion limitation, or 'resistance.' For a given process or pathway to be completely nonlimiting, its related 'conductance' must be essentially infinite.) This result suggests quite directly that intercellular diffusion limits photosynthetic carbon assimilation to an important extent in at least some leaves. After hearing of this result, John R. Evans (personal communication , 1986) suggested that the calculations could be tested experimentally by measuring CO2 assimilation in leaves placed in a helium-oxygen atmosphere instead of normal air. We report here the results of experiments based on that suggestion. The experiments work because CO2 diffuses more rapidly through the smaller He atoms than through the larger N2 molecules. (The diffusivity of CO2 is about 3.6 times greater in pure He than in pure N2-see "Appendix 2;" the presence of 21% 02 reduces the difference to a factor of about 2.3, but even so, the effect is large enough to provide much useful information.) We also describe a model that shows our results to be theoretically reasonable. MATERIALS AND METHODS Plant Material Plants were grown in either controlled-environment greenhouses or controlled-environment growth chambers; they were watered and fertilized as necessary. For this initial survey, we studied one leaf of each of six amphi plants, and of seven hypo plants. We determined these categories by direct microscopic examination. The amphi plants were cocklebur (Xan-thium strumarium L.), broad bean (Viciafaba L.), maize (Zea mays L.), ragweed (Ambrosia cordifolia Gray [Payne]) grown 1024