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Effects of Atmospheric Gases and Light on Changes in Thickness of Oospore Walls and on Germinability of Oospores of Pythium ultimum. Leander F. Johnson, Professor, Department of Entomology and Plant Pathology, University of Tennessee, Knoxville 37901-1071; Phytopathology 78:435-439. Accepted for publication 12 October 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-435.

Water suspensions of thick-walled oospores of Pythium ultimum were exposed to atmospheres of air, CO2, O2, N2, or mixtures of air and CO2. Aeration or O2 was required for conversion to thin-walled oospores, but an atmosphere of 100% O2 was not more effective than air. Oospores in water suspensions did not convert when exposed to atmospheres of 100% CO2, and conversion was significantly inhibited in suspensions exposed to atmospheres containing 2.9% CO2. Germinability of oospores (determined by plating on a nutrient medium) previously exposed to atmospheres of 100% CO2 for 8 days was not appreciably affected. Conversion was inhibited in water suspensions adjusted to pH 6.0 or lower. Exposure to CO2 atmospheres lowered the pH of the suspensions, but increased CO2 atmospheric concentrations progressively inhibited conversion even in buffered solutions (pH 6.26.8). Exposure to light was required for conversion. At low light levels, linear increases in percentages of oospores converted were related to logarithmic increases in light quantity. Germination of thin-walled oospores on a nutrient medium was not affected by light. Evidence is presented that both thick- and thin-walled oospores are exogenously dormant.