INFLUENCE OF PLANT-AGE ON OZONE RESISTANCE IN PLANTAGO-MAJOR

The impact of O-3 at different stages of plant development was investi gated in growth-chamber-cultivated Plantago major L. Six-d-old plants of an O-3-sensitive population ('Valsain') were exposed to one of the following six treatments; 56 d charcoal/Purafil((R))-filtered air (CFA ): 56 d CFA plus 70 nmol mol(-1) O-3 for 7 h d(-1); CFA with a 14-d ep isode of O-3 administered at days 1, 14, 28 or 42. Harvests were made every 14 d, and at the final harvest (56 d) the influence of O-3 on re productive structures was assessed. Analysis of the effects of O-3 on growth and reproductive performance confirmed the sensitivity of the p opulation to the pollutant. In the absence of the development of typic al visible symptoms of foliar damage, the total d. wt of plants mainta ined in O-3 over a 56-d period was 35% lower than that of control plan ts. However, the impact of the pollutant was found to decrease as plan ts aged. Plant relative growth rate ((R) over bar) was only affected i n seedlings, suggesting that effects of O-3 on seedling growth were la rgely responsible for the decrease in accumulated biomass; the growth rate of older plants was not affected by O-3. The observed shift in O- 3 resistance with plant age was mediated by both 'acclimation' and ont ogenetic changes. 'Acclimation' was not associated with changes in O-3 uptake, and there was some evidence to support the existence of compe nsatory growth responses. In addition to effects on vegetative growth, plants exhibited an O-3-induced decline in reproductive performance; O-3 reducing the number of flower spikes and seed capsules produced pe r plant. Ozone episodes administered at different stages of developmen t indicated that reproductive development was particularly sensitive t o O-3 during the early stages of flowering. The findings of this study are discussed in relation to evolutionary adaptation to O-3 in natura l plant communities. The importance of plant age, prior exposure to th e pollutant and the timing of O-3 episodes in relation to plant develo pmental stage are highlighted.