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 (ASC(apo)) reacts with ozone (O-3) and thereby reduces O-3 flux reaching plasmalemma (F-pl). Some studies have shown significant protection of cells from O-3 by ASC(apo), while others have questioned its efficacy. Hypothesizing that the protection by ASC(apo) depends on other variables, we quantified determinants of O-3 detoxification with a model of O-3 transport and reaction in apoplast. The model determines ascorbic acid concentration in apoplast (AA(apo)) using measured values of O-3 concentration (c(o)), leaf tissue ascorbic acid concentration (AA(leaf)), cell wall thickness (L-3), 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) O-3 concentrations. The estimated AA(apo) in individual leaves agreed well with the measured values (R-2 = .91). Analyses of the simulation results yielded the following findings: (a) The efficacy of O-3 reduction with ASC(apo) as quantified by fractional reduction (phi(3)) of O-3 flux at the surface of plasmalemma (F-pl) was lowered from 70% in NF to 40% in EO3 due to the reduction of L-3. The EO3 reduced AA(apo), but the lower G(sw) and L-3 in EO3 increased AA(apo) resulting in no significant change in AA(apo) due to EO3. phi(3) can be calculated with measured values of AA(apo) and L-3, and F-pl can be estimated with the measurement-based phi(3). (b) When c(0) is increased, F-pl increased curvilinearly with the increase of F-st: nominal O-3 flux via stomatal diffusion, exhibiting apparent threshold on F-st. The deviation of F-pl from F-st became greater when L-3, pH(apo), and AA(leaf) were increased. The quantification of phi(3) and F-pl using leaf traits shall facilitate the understanding of the mechanisms of differential plant sensitivity to O-3 and improve quantification of the O-3 impacts on plants.