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Interactions Between Glomus geosporum and Exposure of Soybeans to Ozone or Simulated Acid Rain in the Field. P. F. Brewer, Former graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27650, Present address: Tennessee Valley Authority, 1040 Chestnut Street Tower II, Chattanooga, TN 37401; A. S. Heagle, plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Phytopathology 73:1035-1040. Accepted for publication 9 February 1983. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-1035.

The effects of chronic exposure of soybean plants to ozone (O3) or to simulated acid rain on interactions between Glomus geosporum and soybean plants were studied at a field site near Raleigh, NC. In one experiment, plants grown in open-top field chambers were exposed to seasonal 7-hr (0930- 1630 hours EDT) per day mean O3 concentrations of 0.025, 0.049, or 0.079 ppm O3 for 139 days (from 12 days after emergence until plant harvest). In a concurrent experiment, plants were exposed twice per week for 13 weeks to simulated rain of pH 5.5, 4.0, 3.2, or 2.8. Ozone at 0.049 and 0.079 ppm caused foliar injury and growth reduction; O3 at 0.079 ppm also decreased pod yield. Soybeans infected by G. geosporum were less sensitive to adverse effects caused by 0.079 ppm O3; pod yield of mycorrhizal plants was reduced by 25%, whereas pod yield of nonmycorrhizal plants was reduced by 48%. Seasonal changes in soil P content indicated that only at 0.079 ppm O3 were mycorrhizal plants more efficient in P uptake than were nonmycorrhizal plants. G. geosporum produced 40% fewer chlamydospores per gram of root at 0.079 than at 0.025 ppm O3. In the concurrent experiment, neither rain acidity nor G. geosporum significantly affected foliar nutrient ion content, vegetative growth, or yield. However, G. geosporum produced 39% fewer chlamydospores per gram of root when plants received simulated rain of pH 2.8 than of pH 5.5.