ULTRASTRUCTURE OF THE VASCULAR-RESPONSES OF COTTON TO VERTICILLIUM-DAHLIAE

Ultrastructural examination of cotton plants infected with Verticilliu m dahliae showed responses in the vascular tissue similar to those rep orted earlier in cotton infected with Fusarium oxysporum f.sp. vasinfe ctum. Contact cells reacted vigorously to infection, but the nature of the response depended on the location of the cells. The majority of t he contact cells adjacent to initially infected vessels showed a reorg anization of the cytoplasm with the formation of an apposition layer, a partitioning of the central vacuole into smaller vacuoles, and the a ppearance of osmiophilic material that was secreted through the plasma lemma and deposited in the apposition layer and in the pit regions of the vessels. The remainder of the contact cells adjacent to initially infected vessels contained a degenerated cytoplasm but no fungal hypha e. Contact cells adjacent to secondarily infected vessels responded wi th increased cytoplasmic activity and the direct secretion of osmiophi lic material into the vessel lumens where it accumulated to form aggre gates of various sizes and shapes that occluded the vessels. The fungu s in these vessels was coated and embedded in this secreted material.