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

Abstract

Ascorbate in leaf apoplast (ASC_apo ) reacts with ozone (O₃ ) and thereby reduces O₃ flux reaching plasmalemma (F_pl ). Some studies have shown significant protection of cells from O₃ by ASC_apo , while others have questioned its efficacy. Hypothesizing that the protection by ASC_apo 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 (AA_apo ) using measured values of O₃ concentration (c_o ), leaf tissue ascorbic acid concentration (AA_leaf ), cell wall thickness (L₃ ), apoplastic pH (pH_apo ), and stomatal conductance (G_sw ). We compared the measured and model-estimated AA_apo 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 AA_apo 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 ASC_apo as quantified by fractional reduction (ϕ₃ ) of O₃ flux at the surface of plasmalemma (F_pl ) was lowered from 70% in NF to 40% in EO₃ due to the reduction of L₃ . The EO₃ reduced AA_apo , but the lower G_sw and L₃ in EO₃ increased AA_apo resulting in no significant change in AA_apo due to EO₃ . ϕ₃ can be calculated with measured values of AA_apo and L₃ , and F_pl can be estimated with the measurement-based ϕ₃ . (b) When c₀ is increased, F_pl increased curvilinearly with the increase of F_st : nominal O₃ flux via stomatal diffusion, exhibiting apparent threshold on F_st . The deviation of F_pl from F_st became greater when L₃ , pH_apo , and AA_leaf were increased. The quantification of ϕ₃ and F_pl 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.

Keywords: apoplast; ascorbate; ascorbic acid; cell wall; detoxification; ozone flux; pH; peach; stomatal conductance.