Technical note: New insights into stomatal oxygen transport viewed as a multicomponent diffusion process

We investigate oxygen (O2) transport through stomata, focusing on its interaction with water vapour (H2O) flux. The dominant H2O flux exerts a drag force on other gases, a well-studied effect in the ternary air–water vapor–carbon dioxide (CO2) system but unexplored for O2 transport. This study aims to: (1) apply the Stefan-Maxwell equations to a quaternary system of H2O, O2, CO2, and N2; (2) identify conditions where O2 transport from stomata to the atmosphere occurs against its mole fraction gradient (“uphill”); and (3) derive an expression linking the O2 mole fraction in sub-stomatal air spaces (xoi) to that in the atmosphere (xoa) based on atmospheric relative humidity. Our theoretical results, constrained by typical values from previous flux observations of this quaternary system, reveal distinct transport regimes defined by the mole flux ratio of H2O and O2 (Fw/Fo). Uphill O2 diffusion occurs in the common regime where Fw/Fo≫1, and internal O2 mole fraction increases towards its atmospheric value as relative humidity tends to 100 %. These theoretical results offer a framework for interpreting laboratory and field experiments on stomatal O2 exchange under stagnant atmospheric or low Reynolds number conditions and can support the development of more physically accurate models of leaf–atmosphere oxygen exchange.