THE EFFECTS OF DIFFERENT OZONE EXPOSURES ON 3 CONTRASTING POPULATIONSOF PLANTAGO-MAJOR

Plantago major grows throughout Britain in a range of ozone climates. Because populations have been shown to differ in ozone resistance, the aim of the experiment was to compare the reaction of populations from contrasting ozone climates to different types of ozone exposure. Thre e populations were grown under controlled conditions in five different ozone treatments (including controls for 10 wk. Development, growth, stomatal conductance and seed production were recorded. Populations we re from the south coast of England (Lullington), near a mountain summi t (Great Dun Fell) and lowland Scotland (Bush). Ozone treatments were: charcoal and Purafil filtered air (CF); 35 nl l(-1) for 24 h every da y; 70 nl l(-1) h for 7 h every day; CF then three episodes each week o f 70 nl l(-1) for 7 h; and 35 nl l(-1) continuously plus three 7 h epi sodes each week of 70 nl l(-1). The different ozone treatments resulte d in different responses in each population. Ozone promoted senescence in the Great Dun Fell population but not in the others; it reduced ro ot growth more in the Lullington population than in the others but tho se from Lullington and Great Dun Fell maintained seed production to a much greater extent than the Bush population. The reproductive effort (number of seeds g(-1) of vegetative weight) actually increased in ozo ne in the Lullington and Great Dun Fell populations. It is suggested t hat this might be a general stress response rather than being specific ally related to ozone. Effects on stomatal conductance were similar to those previously reported and the converse of effects on seed product ion. The relative responses of the populations varied according to the ozone treatment. Continuous exposure to 35 nl l(-1) reduced leaf size only in the Great Dun Fell population, but seed output was reduced in the Bush population. In some cases, giving 3-d episodes of 70 nl l(-1 ) had a greater effect than giving the dose every day but the effects varied with the population. This greater effect was considered to be a result of the time it takes for a plant to develop maximum anti-oxida nt defence, which is lost when the ozone decreases after the episode. A plant exposed to episodes might have to re-induce defence with each exposure. Although it is reported frequently that ozone favours alloca tion of resources to the shoot over the root, it is concluded that thi s is an over-simplification of the response. Even within a species the re is a complex suite of responses that varies with the population and with ozone exposure. Describing a population as resistant or sensitiv e is also an over-simplification.