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Response of Two White Clover Clones to Peanut Stunt Virus and Ozone. Allen S. Heagle, Plant pathologist, USDA-ARS, Department of Plant Pathology, North Carolina State University, Raleigh 27695; Michael R. McLaughlin(2), Joseph E. Miller(3), and Ronald L. Joyner(4). (2)Plant pathologist, USDA-ARS, Crop Science Research Laboratory, Mississippi State, MS 39762; (3)Plant physiologist, USDA-ARS, Department of Crop Science; and (4)Research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Phytopathology 82:254-258. Accepted for publication 10 September 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992. DOI: 10.1094/Phyto-82-254.

Effects of ozone (O3) and peanut stunt virus (PSV) on two clones of white clover (Trifolium repens) were measured in open-top field chambers. An O3-resistant clone (NC-R) and an O3-sensitive clone (NC-S), with and without PSV infection, were exposed to O3 for 12-h day1 for 111 days. The exposures were proportional to ambient O3 and resulted in 12-h day1 mean concentrations of 26, 45, 64, and 76 nL L1 for the 111 days of exposure. Plant shoots were harvested five times to measure effects of O3 and PSV on foliar injury, foliar chlorophyll, and shoot dry weight. Infection by PSV caused foliar chlorosis, which tended to be more severe on NC-S than on NC-R. PSV infection suppressed shoot dry weight accumulation of NC-R by 23% and of NC-S by 18%. O3 also caused foliar chlorosis and suppressed shoot dry weight accumulation, and the severity of the effects increased with increased O3 dose. Seasonal shoot weight of NC-S plants exposed in nonfiltered air chambers to ambient concentrations of O3 (45 nL L1) was 20% less than for NC-S plants in charcoal-filtered air chambers (26 nL L1). Shoot weight of NC-R was not significantly affected by any of the O3 treatments. The clone O3 interaction was significant for all measures for each harvest except for the first harvest. Although the O3 concentrations remained relatively constant, the differences between NC-S and NC-R shoot weight became greater as the season progressed. There were no significant interactions between O3 and PSV for any of the response measures.