Effects of a short ozone exposure given at different stages in the development of Plantago major L.

summary This paper reports a comparison of the effects of the same ozone exposure given at different stages of growth on a population of Plantago major L., which is as sensitive to ozone as Bel‐W3 tobacco. Plants were grown from seed for eight weeks in controlled environment chambers and exposed to 70 μl O 3 1 −1 7 h d −1 for the whole period or for 2‐wk episodes during weeks 1 + 2, 3 + 4, 5 + 6 or 7 + 8. Controls had charcoal filtered air. Effects on leaf, flower, seed and dry matter production, and on relative growth rate of shoot and root are given. At whatever time the ozone was given, it altered relative growth rate, dry matter partitioning and seed output. The experiment supports the view that assessment of the sensitivity of a herbaceous species can be indicated by a short‐term exposure of young plants, even though the nature of the response may change with development. Comparison of the effect of ozone on dry matter partitioning at different stages of growth suggested that at each stage in development, ozone relatively strengthened the currently dominant sink, therefore reducing allocation to subsidiary sinks. As a plant develops, the relative strengths of sources and sinks change so the effects of an ozone episode on allocation will depend upon the timing. It is concluded that the commonly held view that ozone tends to reduce root growth more than shoot may be an over‐simplification. For native plants growing in the field, the timing of episodes in relation to development may be crucial to understanding ozone impact. On the ecosystem level we need knowledge of response and source‐sink relationships at times of the year when episodes occur.