COSTANZO, NADINE, & PAUL WIDDEN.
Biology Department, Concordia University, 1455 De Maisonneuve Blvd. West, Montréal, Québec, CANADA. H3G 1M8.
Sugar maple has a Paris series V-A mycorrhiza which forms inter- and
intracellular vesicles and a well- developed system of intracellular coils from
which composite arbuscules form. It has been demonstrated that the relative
numbers of vesicles, arbuscules and coils formed are dependent on the
conditions under which the trees are growing. Stress, in the form of ozone,
has been shown to reduce the formation of arbuscules and result in higher
numbers of coils and vesicles. Using 2 year old seedlings grown in open
topped chambers, we tested the hypothesis that the effects of multiple stresses
on the mycorrhizal system would be additive. Seedlings were grown in chambers
at 0, 50, 100, 150, 200 or 300 ppb ozone for a single growing season. Within
each chamber, half of the seedlings were shaded with shading cloth to reduce
light by 80%, and half each group had their leaf area reduced to 50% in
mid-June by cutting the leaves in half.
As predicted, increasing levels of ozone resulted in reduced levels of
mycorrhizal colonization, but, the coils and vesicles were not as sensitive to
increased ozone as were the arbuscules. However, contrary to expectations, the
added stress of shading and defoliation did not significantly modify the
response of the mycorrhizal system to ozone. It appears that sugar maples
can compensate effectively for shading and herbivory during a single growing
season and longer-term studies are needed on this system.