Ascorbate in the leaf apoplast is a factor mediating ozone resistance in Plantago major

The role of ascorbate in mediating ozone resistance was examined in Plantag o major L. Seedlings of eleven populations which exhibited differential res istance to ozone were fumigated in controlled environment chambers with cha rcoal/Purafil(R)-filtered air (CFA) or CFA plus 15 nmol.mol(-1) ozone overn ight rising to a maximum between 12:00-16:00 hours of 75 nmol.mol(-1) for 1 4 d. Measurements of ascorbate content were made on apoplastic and symplast ic extracts. Populations differed in their constitutive level of ascorbate in youngest fully expanded leaves, and regression analysis revealed a signi ficant correlation between ascorbate content in ozone-treated leaves and th e ozone resistance of the populations. The relationship was stronger using apoplastic ascorbate levels than with corresponding symplastic measurements . The ascorbate content of the youngest fully expanded leaf of an ozone sen sitive population was increased by foliar application of ascorbate. No sign ificant difference in stomatal conductance was found between control and as corbate-treated plants. Following spraying, plants were fumigated with 400 nmol.mol(-1) ozone for 7 h. In control plants, ozone exposure resulted in e xtensive visible leaf damage (20-70% at the end of the fumigation period) a nd decreased rates of CO2 assimilation (-57%). However, ascorbate treatment prevented the appearance of visible injury, and ameliorated the decline in photosynthesis induced by ozone (-26%). Modelled data estimating the exten t of protection afforded by apoplastic ascorbate against ozone supported th e experimental observations. The results suggested that although apoplastic ascorbate plays an important role, other factors must also contribute to t he mediation of ozone resistance in P. major. (C) 2000 Editions scientifiqu es et medicales Elsevier SAS.