Direct effects of ozone on reproductive development in Plantago major L. populations differing in sensitivity

Exposure to ozone (O3) 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 O3 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 O3-resistant populations following exposure to 120 ppb O3 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.