Quantifying determinants of ozone detoxification by apoplastic ascorbate in peach (Prunus persica) leaves using a model of ozone transport and reaction

Ascorbate in leaf apoplast (ASCₐₚₒ) reacts with ozone (O₃) and thereby reduces O₃ flux reaching plasmalemma (Fₚₗ). Some studies have shown significant protection of cells from O₃ by ASCₐₚₒ, while others have questioned its efficacy. Hypothesizing that the protection by ASCₐₚₒ depends on other variables, we quantified determinants of O₃ detoxification with a model of O₃ transport and reaction in apoplast. The model determines ascorbic acid concentration in apoplast (AAₐₚₒ) using measured values of O₃ concentration (cₒ), leaf tissue ascorbic acid concentration (AAₗₑₐf), cell wall thickness (L₃), apoplastic pH (pHₐₚₒ), and stomatal conductance (Gₛw). We compared the measured and model‐estimated AAₐₚₒ in leaves of peach (Prunus persica) grown in open‐top chambers under non‐filtered air (NF) and elevated (EO₃: NF + 80 ppb) O₃ concentrations. The estimated AAₐₚₒ in individual leaves agreed well with the measured values (R² = .91). Analyses of the simulation results yielded the following findings: (a) The efficacy of O₃ reduction with ASCₐₚₒ as quantified by fractional reduction (ϕ₃) of O₃ flux at the surface of plasmalemma (Fₚₗ) was lowered from 70% in NF to 40% in EO₃ due to the reduction of L₃. The EO₃ reduced AAₐₚₒ, but the lower Gₛw and L₃ in EO₃ increased AAₐₚₒ resulting in no significant change in AAₐₚₒ due to EO₃. ϕ₃ can be calculated with measured values of AAₐₚₒ and L₃, and Fₚₗ can be estimated with the measurement‐based ϕ₃. (b) When c₀ is increased, Fₚₗ increased curvilinearly with the increase of Fₛₜ: nominal O₃ flux via stomatal diffusion, exhibiting apparent threshold on Fₛₜ. The deviation of Fₚₗ from Fₛₜ became greater when L₃, pHₐₚₒ, and AAₗₑₐf were increased. The quantification of ϕ₃ and Fₚₗ using leaf traits shall facilitate the understanding of the mechanisms of differential plant sensitivity to O₃ and improve quantification of the O₃ impacts on plants.