Effect of Ozone on the Development of Vesicular-Arbuscular Mycorrhizae in Sugar Maple Saplings

We examined the effects of ozone on the incidence of vesicular-arbuscular mycorrhizae in the roots of sugar maple (Acer saccharum) seedlings using a modified grid-line intersect method. The presence of vesicles, arbuscules, hyphal coils and internal mycelium was recorded. As the concentration of ozone increased, the frequency of vesicles, hyphal coils and internal mycelium increased while that of arbuscules decreased. Total infection rates, however, remained unchanged with increasing ozone levels. It is suggested that the fungi are responding to stress in the plant by increasing the production of less energy demanding and less efficient organs for exchange of nutrients (coils), and by increasing the resources allocated to storage and future growth (vesicles). The increase in internal mycelium (most of which was probably not mycorrhizal) may represent an increase in susceptibility to infection by fungi other than vesicular-arbuscular mycorrhizal fungi.