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 (O₃). 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 O₃ concentrations in mid-afternoon or the maximum stomatal O₃ 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 O₃ system confirmed that the diurnal variations of the ASC_apo were induced more by the daily variations of O₃ concentrations rather than the cumulative O₃ effects. In conclusion, the O₃-stress detoxification should be a dynamic variable rather than a fixed threshold as assumed in the stomatal flux-based O₃ dose metrics. - Highlights: • Significant diurnal changes of ascorbate in the apoplast (ASC_apo) was found. • The ascorbate in leaf tissues remained stable throughout the day. • The variations in ASC_apo are mainly induced by daily O₃ concentration. • The variations in ASC_apo mismatch with O₃ concentration or stomatal O₃ uptake. • The stomatal flux-based O₃ dose should be a variable rather than a fixed threshold. - Leaf apoplastic ascorbate content exhibits large diurnal fluctuations making the stomatal flux-based O₃ dose metrics as a variable rather than a fixed threshold.