VESSEL OCCLUSION AND SECRETORY ACTIVITIES OF VESSEL CONTACT CELLS IN RESISTANT OR SUSCEPTIBLE COTTON PLANTS INFECTED WITH FUSARIUM-OXYSPORUM F-SP VASINFECTUM

An ultrastructural and histochemical examination of secondarily infected and adjacent, uninfected vessels and associated contact cells was conducted in resistant and susceptible cotton plants following inoculation with Fusarium oxysporum f.sp. vasinfectum. Contact cells showed an increase in cytoplasmic content and activity. These contact cells produced lipoidal substances and other compounds that were apparently secreted into vessels through pits. The secretion products coated vessel walls and accumulated in vessel lumens in a variety of forms, from amorphous aggregates to vesicular structures with different sizes and shapes. The fungus in secondarily infected vessels was embedded in or coated by these secretion products. Accumulations of secretion products completely blocked the vessel lumens. The accumulation of the secretion products in vessel lumens and the development of the wall coating were more intensive in resistant cv Seabrook Sea Island than in susceptible Rowden. This more intensive secretory activity of contact cells in resistant plants resulted in the formation of barriers that may contribute to chemical inhibition and physical restriction of the fungus within vessels and prevention of its spread. © 1992.