RESPONSE OF 2 WHITE CLOVER CLONES TO PEANUT STUNT VIRUS AND OZONE

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 day-1 for 111 days. The exposures were proportional to ambient O3 and resulted in 12-h day-1 mean concentrations of 26, 45, 64, and 76 nL L-1 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 L-1) was 20% less than for NC-S plants in charcoal-filtered air chambers (26 nL L-1). Shoot weight of NC-R was not significantly affected by any of the 03 treatments. The clone X 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.