Geographically separated populations of Plantago major
L. differ in ozone resistance, and this is correlated with
the exposure to ozone at the location of each population. In addition,
two populations of P. major have been
demonstrated to show an increase in ozone resistance after summers
when ozone concentrations were high.
However, if evolution of ozone resistance has occurred in the
field there must be appropriate heritable genetic
variation and this must be demonstrated.
In the present study, artificial selection for ozone resistance and
sensitivity was imposed on a resistant
(‘Lullington Heath’) and sensitive (‘Bush’)
population of P. major. Artificial selection is an
efficient means of demonstrating additive genetic variance for a character.
Selection was based on 2 wk growth in ozone (70 nl
O3 l−1 for 7 h d−1), and
selected lines were tested for ozone resistance under both short and
long term ozone exposures.
Changes in ozone resistance were demonstrated for each population.
On the basis of a 2-wk screening test,
selection from the initially sensitive Bush population led to a line
that was significantly more resistant to ozone,
but it was not possible to select a line with greater sensitivity
than the original population. Conversely, selection
from the initially resistant Lullington Heath population led to a
line with increased sensitivity but not to a line with
increased resistance. Differences in ozone resistance between the
selected lines were maintained over the long term
and were reflected in growth and seed production at the
final harvest. Net assimilation rate, stomatal conductance
and leaf pigments were measured. In both populations the lines selected
for sensitivity showed a greater effect of
ozone on net assimilation rate. Ozone had a minor effect on
chlorophyll content in the Lullington Heath sensitive
line but tended to increase carotenoid concentrations. However,
this effect on carotenoids was not related to ozone resistance.
The experiment demonstrated that ozone resistance in P.
major is heritable. The response to selection reported
here supports previous evidence from field collections of a rapid
evolution of ozone resistance.