Impacts of ozone on Plantago major: apoplastic and symplastic antioxidant status

The aim of this work was to examine the correspondence between apoplastic/symplastic antioxidant status and previously reported plant age-related shifts in the ozone (O₃ ) resistance of Plantago major L. Seed-grown plants were fumigated in duplicate controlled environment chambers with charcoal/Purafil?-filtered air (CFA) or CFA plus 70 nmol mol^-1 O₃ for 7 h d^-1 over a 42 d period. Measurements of stomatal conductance and antioxidants were made after 14, 28 and 42 d fumigation, on leaves at an equivalent stage of development (youngest fully expanded leaf, measured c. 9 d after emergence). Ozone exposure resulted in a similar decline in stomatal conductance across plant ages, indicating that increases in O₃ resistance with plant age were mediated through changes in the tolerance of leaf tissue rather than enhanced pollutant exclusion. Leaf apoplastic washing fluid was found to contain 'unspecific' peroxidase, ascorbate peroxidase, superoxide dismutase and ascorbate, but not glutathione and the enzymes required to facilitate the regeneration of ascorbate from its oxidized forms. A weak induction in the activity of certain symplastic antioxidants was found after 14 d O₃ fumigation, despite a lack of visible symptoms of injury, but shifts in symplastic antioxidant enzyme activity were not consistent with previously observed increases in resistance to O₃ with plant age. By contrast, changes in 'unspecific' peroxidase activity and in the small pool of ascorbate in the leaf apoplast were found to accompany age-related shifts in O₃ resistance. It is concluded that constituents of the leaf apoplast may constitute a potentially important front line defence against O₃ .