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.