The impact of O3 at different stages of plant development was investigated in growth-chamber-cultivated Plantago major L. Six-d-old plants of an O3-sensitive population (‘Valsain’) were exposed to one of the following six treatments; 56 d charcoal/Purafil®-filtered air (CFA): 56 d CFA plus 70 nmol mol−1 O3 for 7 h d−1; CFA with a 14-d episode of O3 administered at days 1, 14, 28 or 42. Harvests were made every 14 d, and at the final harvest (56 d) the influence of O3 on reproductive structures was assessed.

Analysis of the effects of O3 on growth and reproductive performance confirmed the sensitivity of the population to the pollutant. In the absence of the development of typical visible symptoms of foliar damage, the total d. wt of plants maintained in O3 over a 56-d period was 35% lower than that of control plants. However, the impact of the pollutant was found to decrease as plants aged. Plant relative growth rate (R) was only affected in seedlings, suggesting that effects of O3 on seedling growth were largely responsible for the decrease in accumulated biomass; the growth rate of older plants was not affected by O3. The observed shift in O3 resistance with plant age was mediated by both ‘acclimation’ and ontogenetic changes. ‘Acclimation’ was not associated with changes in O3 uptake, and there was some evidence to support the existence of compensatory growth responses. In addition to effects on vegetative growth, plants exhibited an O3-induced decline in reproductive performance; O3 reducing the number of flower spikes and seed capsules produced per plant. Ozone episodes administered at different stages of development indicated that reproductive development was particularly sensitive to O3 during the early stages of flowering.

The findings of this study are discussed in relation to evolutionary adaptation to O3 in natural plant communities.The importance of plant age, prior exposure to the pollutant and the timing of O3 episodes in relation to plant developmental stage are highlighted.