Adaptation of a white clover population to ozone stress

White clover (Trifolium repens L.) 'Regal' and tall fescue (Festuca arundinacea Schreb.) 'Kentucky 31' were grown together in a field and exposed for two seasons in open-top chambers to six ozone (O₃ ) regimes ranging from 0.59 to 1.95 times the ambient O₃ concentration. Plants that survived were propagated clonally and used in the present study to determine whether selection for resistance or sensitivity to O₃ had occurred. Relative foliar sensitivity of surviving clones to various short-term O₃ , exposure regimes was determined with and without infection by several viruses. In tests of all surviving clones with viruses present, higher percentages of clones that survived two seasons at the high O₃ levels were resistant to short-term exposure to O₃ , than were those that survived exposure to the low O₃ treatments. Only one of the 33 clones that survived exposure to charcoal-filtered air (059 treatment) was O₃ -resistant while 19 of the 30 clones surviving the 1.95 treatment were O₃ -resistant. Conversely, eight clones that survived the 0.59 treatment were sensitive to O₃ while none of those that survived the l.95 treatment were sensitive. The results indicate that selection pressure in the presence of O₃ stress was for resistance to O₃ . Various combinations of five common viruses of clover were present among the surviving clones. Shoot-rip meristem culture was used to free one O₃ -resistant and one O₃ -sensitive clone from at) viruses. The relative O₃ sensitivity of these two clones was not affected by viruses. Further testing is required to determine the relationships between relative foliar sensitivity to short-term O₃ exposure and relative sensitivity to growth effects caused by long-term exposure.