RESUMO
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.
