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. Three 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 were from the south coast of England (Lullington), near a mountain summit (Great Dun Fell) and lowland Scotland (Bush). Ozone treatments were: charcoal and Purafil filtered air (CF); 35 nl l−1 for 24 h every day: 70 nl l−1 h for 7 h everyday: CF then three episodes each week of 70 nl l−1 for 7 h; and 35 nl l−1 continuously plus three 7 h episodes each week of 70 nl l−1.
The different ozone treatments resulted in different responses in each population. Ozone promoted senescence in the Great Dun Fell population but not in the others; it reduced root growth more in the Lullington population than in the others but those 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 ozone in the Lullington and Great Dun Fell populations. It is suggested that this might he a general stress response rather than being specifically related to ozone. Effects on stomatal conductance were similar to those previously reported and the converse of effects on seed production.
The relative responses of the populations varied according to the ozone treatment. Continuous exposure to 35 n1 l−l 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 n1 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-oxidant 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 allocation of resources to the shoot over the root, it is concluded that this is an over-simplification of the response. Even within a species there is a complex suite of responses that varies with the population and with ozone exposure. Describing a population as resistant or sensitive is also an over-simplification.