A WHITE CLOVER SYSTEM TO ESTIMATE EFFECTS OF TROPOSPHERIC OZONE ON PLANTS

An ozone-sensitive (NC-S) and an ozone-resistant (NC-R) clone of white clover (Trifolium repens L.) were tested to determine the feasibility of using them to indicate concentrations of tropospheric ozone (O3) and potential effects of O3 On plants. Plants of each clone were exposed daily in open-top field chambers to O3 concentrations ranging from 0.5 to 1.5 times the ambient concentrations for three summer seasons near Raleigh, NC. Foliar injury, foliar chlorophyll, and forage production of both clones were related directly to the O3 concentration. Ozone routinely injured leaves, suppressed foliar chlorophyll, and decreased growth of NC-S more than that of NC-R. Forage production was highly variable over a wide range of weather conditions, but the relative forage production rate of both clones under such conditions was similar and the seasonal O3 response relationship between NC-S and NC-R was relatively stable. The level of response of NC-S to O3 routinely increased from growth period to growth period suggesting an effect of previous exposure. More work is needed to calibrate the system over a wider range of ambient O3 levels and climatic conditions.