The ability of apoplastic ascorbate to protect poplar leaves against ambient ozone concentrations: a quantitative approach

Shoots of a sensitive (Populus nigra 'Brandaris') and a more tolerant (Populus euramericana 'Robusta') poplar clones were exposed for 30 days to Filtered Air or ambient O-3-concentrations in fumigation cabinets. At regular intervals were determined: gas exchange of the leaves, the internal air space (V-air) and apoplastic water volume (V-apo) and the reduced (ASA) and oxidized (DHA) ascorbate concentration in the apoplast and in the mesophyll cells. The apoplastic ASA-concentration was 0.2 mM at the start of the experiment for both cultivars, while the effective cell wall thickness, estimated from V-apo, varied from 0.3 to 0.6 mum. Model calculations revealed that only 30% of the O-3 Molecules entering the apoplast was intercepted at these values. The O-3-treatment induced a decline in stomatal conductance, an increase in V-apo and in the apoplastic ASA-concentration. As a result the estimated O-3-flux to the cell membrane strongly declined. However, these responses occurred after the O-3-induced reduction in photosynthesis. Moreover, they did not prevent early senescence of the leaves at a prolonged exposure. Therefore, it is concluded that the increase in apoplastic ASA-concentration was rather a general stress reaction of the affected poplar leaf than a (specific) defence reaction induced by O-3. Our results suggest that other factors than the scavenging efficiency of apoplastic ASA were responsible for the difference in O-3 sensitivity between both poplar cultivars. (C) 2001 Elsevier Science Ltd. All rights reserved.