A RAPID CHANGE IN OZONE RESISTANCE OF PLANTAGO-MAJOR AFTER SUMMERS WITH HIGH OZONE CONCENTRATIONS

Ozone concentrations in Britain vary greatly from year to year. In the mid-1980s they were relatively low but in 1989 and 1990 the number of hours when concentrations exceeded 60 nl l(-1) was 3-4 times greater than in the preceding 2 yr. Previous experiments suggested that this m ight have resulted in an increase in ozone resistance of Plantago majo r L. populations, so a comparison was made of seed-grown plants collec ted from the same three sites in Derbyshire/South Yorkshire in the 198 0s and 1991. Ozone resistance was measured by exposure to charcoal-fil tered air or air containing 70 nl l(-1) O-3 (7 h d(-1)) and expressed as R%, the mean relative growth rate in ozone expressed as R% of that in charcoal-filtered air. Ozone resistance changed significantly in tw o of the populations (ISP and Totley) between the years 1985 and 1991. There was no significant change in the resistance of the Scaftworth p opulation between 1988 and 1991 when expressed as R% but there were si gnificant physiological differences between the collections. The chang es in R% were accompanied by differences in the response of the stomat a to ozone and in effects on dark respiration, but not in assimilatory capacity. This is the first report of a change in ozone resistance in populations over a short period of time. It is not known if the 1991 plants were the descendants of the 1985/88 plants or whether there had been an invasion by new genotypes from outside the area. The role of other environmental factors in determining ozone resistance, and the i mplications of the data for the definition and mapping of critical lev els are both discussed. It is predicted that if the reported changes w ere caused by ozone, the region where future changes in ozone resistan ce are most likely to occur is that between the Midlands and the north of England.