Ozone flux to plasmalemma in barley and wheat is controlled by stomata rather than by direct reaction of ozone with cell wall ascorbate

Morning, midday and afternoon values of stomatal conductance and apoplastic and whole-leaf ascorbate concentrations in leaves of barley and wheat were measured on the 7(th) day of exposure of young seedlings to ozone enriched air (75-100 nL/L, 8 h d(-1)) in open-cop chambers. The aim of the study was to explore whether the contributions of these factors to the limitation of ozone flux to plasmalemma are different in these species. Until midday, ozone flux density to the ozone-exposed mesophyll cell surface was found to be negligibly (4 %) lower in barley compared with wheat. After midday, this difference increased to 12 %, mostly due to a more rapid afternoon decrease of stomatal conductance in barley. The diurnal decline in apoplastic ascorbate concentration in barley was less pronounced than in wheat. The differences in diurnal courses of apoplastic ascorbate were apparently not related to whole-leaf ascorbate levels, found to be stable during the day in both species. Due to the thin cell wall (0.12 mu m in barley and 0.10 mu m in wheat), only a minor part of the ozone flux entering the mesophyll cell surface was calculated to be detoxified in the direct reaction with apoplastic ascorbate (7-14 % in barley and 4-13 % in wheat), causing a 5-6 times lower contribution of this reaction to total ozone decay than the contribution of the stomata. The contribution of other direct apoplastic ozone scavengers is estimated to be even less than that of ascorbate. The overall resultant difference in calculated ozone fluxes to mesophyll plasmalemma in barley and wheat (5 % in the morning and 14 % in the afternoon) was statistically insignificant. An increase of this difference is suggested if singlet oxygen and/or hydroxyl radical are generated in the apoplast under ozone.