ORIGIN AND SIGNIFICANCE OF THE DISCHARGE VESICLE IN PHYTOPHTHORA

During zoospore release from sporangia, all Phytophthora species studied developed a discharge (germination) vesicle into which 30 to 70% of the total number of zoospores were released before the vesicle burst. Vesicles of P. cactorum, P. Palmivora, and P. cinnamomi reached maximal diameters of 26, 40, and 46 μm, respectively, when sporangia were placed in double-distilled water for zoospore release. The mean velocities of surface area enlargement of the vesicles represent ratios of 1:1.7:4.6, comparing P. cactorum with P. palmivora and with P. cinnamomi. For sporangia of P. cactorum, the surface area of the vesicle enlarged at a constant rate, whereas the increase in vesicle volume proceeded with an acceleration of about 79 μm3/second2. When sporangia produced by P. cactorum grown in either bean meal broth or a chloride mineral solution were transferred to double-distilled water they developed discharge vesicles that enlarged in surface area at velocities of 224 and 145 μm2/second, respectively, and had turgor pressures of about -8.0 and -4.8 bar, respectively. During zoospore release in P. palmivora, the outer sporangial wall and an amorphous cover layer were ruptured, and the papillar material (vesiculogen) was forced through the exit pore to form a continuous and plastic vesicle wall consisting of thin fibrillar elements of undetermined lengths (EM). The opening of the sporangium of P. palmivora, the enlargement of the vesicle, and subsequent zoospore release could be retarded or stopped in any stage by transferring germinating sporangia to CaCl2 or PEG solutions with osmotic potentials between -2.5 and -3.5 bar. During plasmolytic treatments, the papillae of sporangia of P. cactorum and P. palmivora kept their original shape, whereas the apices of P. cinnamoni sporangia collapsed. © 1979 Academic Press, Inc. All rights reserved.