Components of apoplastic ascorbate use in Betula pendula leaves exposed to CO2 and O3 enrichment

Here, the aim was to estimate loads imposed on the apoplastic ascorbate (ASC) pool by enzymatic and nonenzymatic reactions in Betula pendula exposed to doubled CO2 and O-3 concentrations in open-top chambers. Leaf apoplastic extracts were analysed for peroxidase and oxidase activities in vitro, using different substrates. Partial loads in vivo were deduced using measured kinetic constants and substituted-enzyme catalysis approaches. Ascorbate use in O-3 scavenging was calculated using measured stomatal conductances and ASC concentrations. Under elevated O-3, stomatal conductance and O-3 uptake were higher. O-3 fluxes to the plasmalemma were levelled off by higher apoplastic ASC concentrations. The effect of CO2 enrichment on ASC concentrations under elevated O-3 was minor. Under ambient O-3, the ascending hierarchy of ASC users was: peroxidases, O-3 scavenging, oxidases, coniferyl alcohol re-reduction. Under elevated O-3, ASC use in O-3 scavenging was higher than by oxidases. The redox state of ASC was not depressed by O-3; there was no leaf injury. The cell wall/plasmalemma/cytosol system in birch had sufficient capacity to maintain ASC redox status in the apoplast, without necessity to restrict O-3 uptake by stomatal closure.