Effects of Long-Term Ozone Exposure and Soil Moisture Deficit on Growth of a Ladino Clover-Tall Fescue Pasture

Abstract

Most field studies relating seasonal ozone (O3) exposure to crop yield have been prerformed in the abaence 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 .mu.L 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 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 .mu.LL-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 .mu.L 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.