Spatial variation in ozone resistance of British populations of Plantago major L.

summary The relative ozone resistance of samples from 28 British Plantago major L. populations, grown from seed, was measured by exposure to charcoal–filtered air or air containing 70 nl I −1 O 3 (7 h d −1 ) over 2 wk. Mean relative growth rate ( R ) and the allometric root/shoot coefficient ( K ) were measured. Ozone resistance was expressed as O res = R ozone / R control × 100. There were substantial differences between the response of the populations to O 3 , and the degree of resistance was statistically related to O 3 exposure indices and other environmental factors, notably the amount of summer sunshine. This implies that ozone concentrations in parts of Britain are sufficiently high to promote evolution of resistance but alternative explanations are also discussed. Overall, it is concluded that although our results tend to support the hypothesis that O 3 resistance in P. major is related primarily to the O 3 climate, the question of causation can only be established unequivocally by further experiments. Establishing whether there has been a change in gene frequency needs repeated observation of populations over time. The significance of spatial variation in ozone resistance to the critical levels concept is discussed, particularly in relation to exposure indices.