CHARACTERISTICS OF ADHESION PADS FORMED DURING IMBIBITION AND GERMINATION OF UREDINIOSPORES OF UROMYCES-VICIAE-FABAE ON HOST AND SYNTHETIC SURFACES

To imbibe and germinate urediniospores require free water or near satu rated atmospheres in which both adsorption of water is maximal and cap illary condensation pads form. The rate of spore germination is influe nced by the amount of free water available. During imbibition, spores swell and capillary pads form beneath spores in contact with the subst ratum. These pads greatly increase the area of contact with the substr atum. Pad area fluctuates and may depend on the balance between the ra te of capillary condensation and the rate of uptake of fluid by spores . When spores, on leaves or cellulose acetate, are misted with water t hey imbibe rapidly and germinate. Adhesion pads which form under fully imbibed, germinating spores are morphologically similar to capillary pads but they differ in composition. On synthetic surfaces, during imb ibition of spores and under microscopic glass beads, pads are composed principally of volatile capillary condensation and are removed when s amples are freeze-dried. On host surfaces, when spores are fully imbib ed and germinating, pads contain non-volatile soluble materials derive d from both pathogen and host. These substances are not removed by ext ended freeze-drying.