An elucidatory model of oxygen’s partial pressure inside substomatal cavities

Abstract. A parsimonious model based on Dalton’s law reveals substomatal cavities to be dilute in oxygen (O₂), despite photosynthetic O₂ production. Transpiration elevates the partial pressure of water vapour but counteractively depresses those of dry air’s components – proportionally including O₂ – preserving cavity pressurization that is negligible as regards air composition. Suppression of O₂ by humidification overwhelms photosynthetic enrichment, reducing the O₂ molar fraction inside cool/warm leaves by hundreds/thousands of ppm. This elucidates the mechanisms that realize O₂ transport: diffusion cannot account for up-gradient conveyance of O₂ from dilute cavities, through stomata to the more aerobic atmosphere. Rather, leaf O₂ emissions depend on non-diffusive transport via mass flow in the form of “stomatal jets” forced by cavity pressurization, which is not negligible in the context of driving viscous flow. Jet expulsion overcomes massive inward O₂ diffusion to force net O₂ emission. At very high leaf temperatures, jets also influence transport of water vapour and carbon dioxide, physically decoupling their exchanges and reducing water-use efficiency, independent of stomatal regulation.