The effects of osmotic water potentials on Aphanomyces euteiches during zoosporogenesis

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Abstract
Zoosporogenesis in Aphanomyces euteiches was inhibited by increasing the osmotic water potential (ψπ) of the wash solution used to induce asexual reproduction. Mycelial protoplasm did not differentiate into primary spores when exposed to osmotica of glucose, mannose, fructose, sorbitol, sucrose, KCl, CaCl2, and polyethylene glycol (PEG) 4000 of ψπ ≤ −5.0 bars. Primary spores were formed in these osmotica at ψπ ≥ −4.0 bars, but they were not extruded from the sporangia until the ψπ ≥ −3.0 bars. Active zoospores were observed only at ψπ ≥ −2.0 bars and were most abundant at ψπ ≥ −0.5 bars. Osmotica of mannitol, i-erythritol, glycerol, and i-inositol did not completely suppress zoospore formation until the ψπ ≤ −7.0 bars, and primary spore cleavage occurred at ψπ as low as −11.0 bars. PEG-6000 and PEG-20 000 prevented primary spore formation at ψπ below −3.0 bars and −0.5 bars, respectively. The ψπ and turgor pressure (ψp) of mycelia exposed to various water potentials were also determined and found not to differ greatly between the various osmotica. The ψπ of vegetative mycelia was found to be about −8.0 bars, while that activated to form primary spores in the basal salt wash solution was −3.0 bars. As the ψπ of the solution decreased, so did the ψπ of the mycelia until the two were equal to about −14.0 bars, thus creating a zero ψp of the mycelia. Mechanisms for asexual spore formation and discharge are also discussed.

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