EFFECTS OF DIFFERENT NITROGEN-SOURCES ON THE GROWTH OF SCOTS PINE-SEEDLINGS AND THE ULTRASTRUCTURE AND DEVELOPMENT OF THEIR MYCORRHIZAE

Scots pine (Pinus sylvestris L.) seedlings were grown in a natural forest humus and exposed to three different N sources, NaNO3, ammonium acetate, and urea formaldehyde, each at four levels corresponding to 0, 50, 150, and 300 kg N/ha. Application of all levels of the three N sources caused increased shoot growth, but response was especially prominent in the nitrate treatments. At the higher concentrations of ammonium N and urea formaldehyde, a decline in root growth was observed together with a decreased root/shoot ratio with increasing levels of all the N sources. The numbers of mycorrhizal rootlets decreased with increasing concentration of nitrate, but the ammonium N and urea formaldehyde treatments did not significantly affect the total number of mycorrhizae counted against root length. Ultrastructural observations revealed a developing cellular injury pattern in the fungal tissues of Cenococcum geophilum and unidentified brown dichotomous mycorrhizae of seedlings exposed to all N sources and levels. Changes were first detected in the sheath tissue and were characterized by the formation of dark accumulations in fungal vacuoles, decrease in numbers of cytoplasmic storage bodies, swelling of mitochondria, and dark staining and disintegration of the cytoplasm. Increased severity of the changes was correlated with increasing N application level, being most severe in the ammonium acetate treatment These observations suggest that high N levels originating from industrial fallout and N fertilization could be harmful to Scots pine mycorrhizae.