Abstract
Barrier zone formation was studied in annual shoots and small branches of Ulmus americana L., Prunus pensylvanica L.f., and Populus balsamifera L. following inoculation with Ophiostoma ulmi (Buism.) Nannf. Ultrastructural observations showed that electron-opaque compounds, which usually were identified as containing phenolics by previous histochemical tests, were often present within cells of this tissue. Many lipidic globules were also observed within some barrier zone cells of Populus balsamifera. Additional wall layers, suberized or not, were frequently observed in these barrier zone cells. When histochemical tests for suberin at the light microscope level were strongly positive, the lamellation typical of suberin was evident in transmission electron microscopy. Gum canal formation in Prunus pensylvanica was found to be initiated and the canals widened predominantly by a lysogenous process. Key words: Dutch elm disease, nonhost plants, Ophiostoma ulmi, Ulmus americana, ultrastructure, gum canals, suberized layers.
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