Summary 421

I. INTRODUCTION 421

II. EFFECTS OF OZONE ON REPRODUCTION 423

1. Pollen germination and pollen tube growth 424

2. Floral initiation and development 428

3. Effects on seed and fruit yield and yield components 433

4. Effects of ozone on seed and fruit quality, germination and seedling growth 437

III. INFLUENCE OF REPRODUCTIVE HABIT AND IMPLICATIONS FOR FIELD-GROWN PLANTS 438

IV. CONCLUSIONS AND FUTURE RESEARCH 441

Acknowledgements 442

References 442

Sexual reproductive development is a crucial stage in the life cycle of higher plants as any impairment of the processes involved might have significant implications for the productivity of crop plants and the survival of native species. There is considerable evidence that exposure to ozone, even at current ambient levels in many industrialized countries, reduces grain and fruit yields and adversely affects yield quality. It is also well established that sensitivity to ozone may differ not only between species, but also between cultivars and populations of individual species, and that the impact of exposure is highly dependent on ozone concentration and the duration and timing of exposure. However, few studies have attempted to distinguish between the direct effects of air pollutants on reproductive development, and indirect effects mediated by injury to the vegetative organs and associated changes in the supply of assimilates and other essential resources to support reproductive growth, or the levels of endogenous growth regulators. This review considers the impact of ozone on the reproductive biology of agricultural and native species, and examines its direct effects on specific reproductive processes. The extent to which compensatory responses redress the adverse effects of exposure is also explored, with particular reference to recent studies of Brassica napus (oilseed rape), Brassica campestris (Wisconsin Fast Plants), Plantago major (greater plantain) and Triticum aestivum (wheat).