Novel ozone flux metrics incorporating the detoxification process in the apoplast: An application to Chinese winter wheat.

A modified Ball-Berry-Leuning model of stomatal conductance was applied to data from fully open-air ozone (O)-enrichment experiments with winter wheat (Triticum aestivum L.). The O fluxes reaching both surface of cell wall (F) and plasmalemma (F) were estimated considering apoplastic ascorbate, a major scavenger of O. The difference (D) between F and F was defined as detoxification capacity of O by reaction with ascorbate in the leaf apoplast (ASC). The accumulated stomatal O flux above D nmol O m s (AF) and the accumulated F (AF) were calculated over the optimal integration period covering the whole reproductive development of wheat, and used to derive OAF yield-response relationships in comparison with POD (phytotoxic O dose above a threshold of Y nmol m s) and AOT40 (accumulated O dose over a threshold of 40 ppb). There was a good agreement between the observed and modeled values of ASC and stomatal conductance. AF and AF performed better than POD and AOT40 in terms of R and intercept. However, the AF metric was more suitable for assessing grain yield loss due to lower sensitivity of the regression slope to variations in the input parameters, compared with AF. The average critical level (CL) of four cultivars for 5% grain-yield reduction was 1.53 mmol m using POD and 2.81 mmol m using AF, with the latter being well above the POD-derived value for European cultivars (1.3 mmol m). The minimum hourly averaged O concentration contributed to CLs was below 20 ppb according to AF, a value that is lower than that suggested by POD (≈27 ppb). O flux-response relationships and CLs on the basis of quantified detoxification capacity shall facilitate the understanding of the different degrees of susceptibility to O among species or cultivars, and improve the assessments of O impacts on plants.