Pinus virginiana seedlings were removed from the soil 15, 20, 25, 40, 50, 70, and 180 days after emergence. The roots of intact seedlings were placed on petri dish cultures of Pythium debaryanum. The upper centimeter of each root was embedded in paraffin 36 hours after inoculation, sectioned, and examined histologically. All cells exept the suberized endodermal cells and lignified vascular cells in 15- to 25-day-old roots were colonized with P. debaryanum mycelium. In roots older than 25 days, the incompletely suberized endodermis acted as a partial barrier temporarily restricting the mycelium to cells outside of the endodermis and to those directly inside of the nonsuberized portions of the endodermis. The amount of mycelium inside the endodermal layer was less in older roots where a larger number of endodermal cells were suberized. Mycelium was seldom observed penetrating the suberized endodermal cells. Nearly all tissue outside of the endodermis had been sloughed-off from 70- and 180-day-old roots ad the fully developed endodermis was the outermost layer Older roots were not infected except where secondary roots emerged.
Document Type: Journal Article
Assistant Professor of Plant Pathology, Botany Dept., Univ. of Maryland, College Park