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

Abstract. 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 vapour–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 flux observations of the quaternary system, reveal distinct transport regimes defined by the mole flux ratio of H₂O and O₂ (F_w/F_o). Uphill O₂ diffusion occurs in the common regime where F_w/F_o»1, and the internal O₂ mole fraction increases toward its atmospheric value as relative humidity approaches 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.