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 wa s to explore whether the contributions of these factors to the limitation o f ozone flux to plasmalemma are different in these species. Until midday, o zone 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 decreas e of stomatal conductance in barley. The diurnal decline in apoplastic asco rbate concentration in barley was less pronounced than in wheat. The differ ences in diurnal courses of apoplastic ascorbate were apparently not relate d 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 w heat), only a minor part of the ozone flux entering the mesophyll cell surf ace 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 o f 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 a nd wheat (5 % in the morning and 14 % in the afternoon) was statistically i nsignificant. An increase of this difference is suggested if singlet oxygen and/or hydroxyl radical are generated in the apoplast under ozone.