**8. Duration and continuity of pollen development**

Pollen development is normally a continuous process that is interrupted only by pollen presentation and dispersal. Pollen meiosis takes only a few hours, but the duration of pollen development after meiosis can vary widely and depends on the plant species. As a rule, annuals develop pollen faster than perennials and woody species: pollen development takes 8 days for the herbaceous perennial *Lycopersicum peruvianum* [211] and approximately 7 days for geophytic *Lilium* species [212], while 18 days are required for the grass *Phalaris tuberosa* [213]. However, in some plants the process can be interrupted at various stages before presentation and dispersal. In some woody plants from temperate environments, the process can be paused once or twice at the microspore or bicellular stages. The ability of pollen development to be interrupted is an adaptation mechanism to protect pollen against extreme environmental conditions during summer or winter. Interruptions are more likely to occur in plants where pollen development takes longer, especially in temperate climates where unexpected harsh weather conditions can occur. Some gymnosperms (e.g. *Juniperus commu‐ nis*) and woody perennial angiosperms (e.g. birch, elder and hazelnut) that disperse their pollen at the end of winter differentiate their flower buds in autumn when environmental conditions are favourable [214, 215]. Under severe winter conditions, flower development is arrested and resumed in early spring. In hazelnuts, this interruption occurs at the bi-cellular stage [215]. The developing pollen grains appear dormant and anther metabolism is repressed. The influx of substances from the mother plant and the activity of anther wall chloroplasts are also reduced, suggesting that developmental arrest may be regulated by the mother plant. In some species, developmental arrest occurs prior to pollen meiosis. In some Mediterranean plants, flower buds develop during late spring but stay dormant during the hot and dry summer and development resumes in autumn [216]. The dioecious bay laurel (*Laurus nobilis*) flower buds of both sexes develop in early autumn, they pause development in winter and flower ripening and pollination occurs during early springtime [163]. It is not known how this developmental arrest of pollen development is controlled at the molecular and physiological level, but it provides a powerful defence mechanism to protect pollen and maintain fertility under sub-optimal climatic conditions.
