Diurnal variation of apoplastic ascorbate in winter wheat leaves in relation to ozone detoxification

Besides stomatal closure, biological detoxification is an important protection mechanism for plants against ozone (O-3). This study investigated the diurnal changes of ascorbate (a major detoxification agent) in the apoplast and leaf tissues of winter wheat grown under ambient air field conditions. Results showed the reduced ascorbate in the apoplast (ASC(apo)) exhibited a peak in late morning or midday, mismatching with either the maximum external O-3 concentrations in mid-afternoon or the maximum stomatal O-3 uptake between late morning and mid-afternoon. In contrast, the ASC in leaf tissues remained stable throughout the day. The investigations conducted in a Free-Air Concentration Elevation of O-3 system confirmed that the diurnal variations of the ASC(apo) were induced more by the daily variations of O-3 concentrations rather than the cumulative O-3 effects. In conclusion, the O-3-stress detoxification should be a dynamic variable rather than a fixed threshold as assumed in the stomatal flux-based O-3 dose metrics. (C) 2015 Elsevier Ltd. All rights reserved.