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.