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It is well established that exposure to ozone (O 3 ) may impair vegetative growth and reproductive development in plants, although the consequences for yield depend on the effectiveness of the compensatory processes induced. This study examined the effects of exposing the terminal inflorescence of Brassica campestris L. to 100 ppb O 3 for 6 h d −1 on four consecutive days during early flowering while the vegetative organs received charcoal-filtered air. The ordered predictability of development in B. campestris is ideal for studies of the impact of abiotic stress factors such as O 3 on reproductive development and seed production. Effects on reproductive development and seed yield characteristics were determined for floral sites exposed at different developmental stages. Flower and pod numbers on the terminal raceme were unaffected by exposure, but effects on pod length varied depending on the developmental stage of floral sites during exposure. Increased ovule abortion and precocious seed germination in the pods of O 3 -treated plants reduced mature seed number pod −1 . Although the individual weight of mature seeds was slightly greater in O 3 -treated plants, seed yield pod −1 and seed yield plant −1 were reduced due to the lower seed number pod −1 . Seed from O 3 -treated plants germinated more rapidly than control seed irrespective of the stage of floral development during exposure. The results show that exposure to realistic O 3 episodes during the early stages of flowering significantly reduced seed yield without impairing the vigour of the seeds produced. The physiological origins of these effects are discussed.

Exposure to ozone (O 3 ) affects both vegetative and reproductive growth in plants, although consequences for seed yield depend on the efficacy of compensatory reproductive processes. This study examined whether the limited compensatory capacity of species with determinate reproductive strategies influences seed production following exposure. The first flowering spike (Spike 1) of Plantago major L. populations which exhibit contrasting sensitivity of vegetative growth to chronic O 3 exposure was exposed, while the leaves and second spike (Spike 2) received charcoal-filtered air. Exposure of Spike 1 during flowering influenced seed number capsule −1 on both spikes, even though Spike 2 was not exposed. Responses varied between populations, position on the spike and plant age at the start of exposure; the greater impact on capsules located on the upper section of the spike suggests they were more responsive. Compensatory increases in seed number capsule −1 in the middle and upper sections of Spikes 1 and 2 generally precluded significant effects on seed number spike −1 ; individual seed weight was largely unaffected. However, the combined seed weight of Spikes 1 and 2 was increased by 19% in the two O 3 -resistant populations following exposure to 120 ppb O 3 on 9 successive days, suggesting that they over-compensated for injury, but was reduced by 21% in the most sensitive population. Compensatory reproductive responses and their possible implications for biomass production, biodiversity and carbon sequestration in natural communities in temperate regions are discussed.