VIEW ARTICLE

Disease Detection and Losses

Effects of Long-Term Ozone Exposure and Soil Moisture Deficit on Growth of a Ladino Clover-Tall Fescue Pasture. Allen S. Heagle, Plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, North Carolina State University, Raleigh 27695; Joanne Rebbeck(2), S. R. Shafer(3), Udo Blum(4), and W. W. Heck(5). (2)Graduate research assistant, Department of Botany, North Carolina State University, Raleigh 27695; (3)Plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Departments of Plant Pathology and Soil Science, North Carolina State University, Raleigh 27695; (4)Physiological ecologist, Department of Botany, North Carolina State University, Raleigh 27695; (5)Plant physiologist, Agricultural Research Service, U.S. Department of Agriculture, Department of Botany, North Carolina State University, Raleigh 27695. Phytopathology 79:128-136. Accepted for publication 29 July 1988. 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, 1989. DOI: 10.1094/Phyto-79-128.

Most field studies relating seasonal ozone (O₃) exposure to crop yield have been performed in the absence of plant moisture stress. Loss estimates from such studies may be too large if moisture stress, which occurs during most growing seasons, decreases plant sensitivity to O₃. Thus, we examined the response of a mixture of ladino clover (Trifolium repens L. 'Regal') and tall fescue (Festuca arundinacea Schreb. 'Kentucky 31') to chronic doses of O₃ at two soil-moisture levels over two growing seasons. The mixture was seeded on 15 September 1983 and exposed to six levels of O₃ in open-top chambers for 12-hr day^-1 from 26 April to 18 October in 1984 and from 13 April to 22 October in 1985. The six seasonal 12-hr day^-1 mean O₃ concentrations ranged from 0.025 to 0.092 μL L^-1. The soil-moisture treatments, obtained by differential irrigation, were well-watered or water-stressed. A soil-moisture deficit occurred intermittently in water-stressed plots during both seasons. Shoots were harvested when plants reached a height of 20-25 cm. Total forage (clover and fescue) yield in the water-stressed plots (O₃ levels combined) was 12% less than that in the well-watered plots in 1984 and 14% less in 1985. Clover was much more sensitive than fescue to O₃. Ozone-induced suppression of clover growth was accompanied by an increase in fescue growth, and these effects increased as the O₃ level increased. There was a statistically significant interaction between soil moisture and plant response to O₃ only in 1985. This effect probably occurred because clover growth exceeded fescue growth only in the charcoal-filtered air (CF) well-watered treatment. There were no significant effects of soil moisture on response to O₃ when CF was eliminated from the analysis of variance. Over the two seasons, estimated effects of ambient levels of O₃ (2-yr seasonal 12-hr day^-1 mean of 0.046 μLL^-1) were a 10% decrease in total forage yield, a 19% decrease in clover yield, and a 19% increase in fescue yield (compared to 0.028 μL L^-1 of O₃). The decrease in total forage yield and decreased quality caused by decreased growth of clover suggest a need for ladino clover lines that are tolerant to O₃.