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Osmotic pressure of the sporangia of Phytophthora infestans during the process of zoosporogenesis was investigated. Sporangial suspensions were prepared using a hypotonic solution of inorganic ions, and the osmotic pressure was determined by observing plasmolysis of sporangia after 1hr incubation with a series of hypertonic sucrose solutions. Sporangial suspensions were incubated for 6hr at 22.5•Ž to mature the sporangia. Subsequently, the suspensions were incubated for 15min at 14•Ž to progress the sporangial conditions to the stage just ready for zoospore releasing. Before the low temperature treatment, osmotic pressure of the sporangia was av. 0.70OsM, and the plasmolyzed protoplasm was round in shape, indicating that the protoplasm was not yet divided. After the low temperature treatment, osmotic pressure tested was as low as 0.1OsM, and the plasmolyzed protoplasm was shaped like a bunch of grapes, indicating that the protoplasm was divided into zoosporial protoplasms. Thus, protoplasm division was accompanied by a drastic decrease in the osmotic pressure. This contradicts the conventional explanations which assumed increase in the internal pressure for the force to open the exit pore to release zoospores. Subsequently, the inhibitory effect of the hypertonic solutions of saccharides (xylose, glucose and sucrose) and polyethylene glycols (PEG400, PEG1000, PEG2000 and PEG4000) on zoosporogenesis was also investigated. The larger molecule tends to have more inhibitory effect on the zoosporogenesis. For example, 0.7M solution of xylose was required for complete inhibition, while 0.05 M was required in the case of PEG4000. These results indicate that the inhibitory effect may be attributed to the high viscosities of the solutions rather than the high osmotic potentials. (Received March 16, 1995; Accepted June 15, 1995)

Observation of the Division and Osmotic Pressure of Protoplasm on the Process of Zoosporogenesis of the Sporangia of Phytophthora infestans.