Hard-seededness, seed dormancy and pretreatment In many naturally reproducing plant populations a delay between seed shed and subsequent germination can be a significant ecological advantage. Postponing emergence clearly allows extra time for dispersal (whether by wind, water, birds or animals) to potentially greater distances (Figure 8). In any region with seasonal variations, it has the additional advantage that the germination of at least some seeds will coincide with the most favourable season for seedling growth and establishment. Natural selection has therefore favoured many species with seeds which do not germinate immediately after dispersal – even when the environmental conditions are ideal (e.g. ample water, a suitable temperature for growth and good aeration). When seeds are inactive because they are ‘immature’ or possess a layer of tissues that prevent water uptake or gaseous exchange, they are said to be ‘quiescent’. But when seeds can be shown to be fully hydrated and alive yet still remain outwardly inactive – they are said to be ‘dormant’.
Figure 8 A blackbird eating a rowan berry. It is likely that passage through a bird gut removes germination inhibitors from the fruit skin and pulp, however, intact seeds still retain their innate 'deep-dormancy’.
A dormant seed is one that is known to be alive, but remains outwardly inactive and does not germinate when we expect it to. The seeds of some species remain dormant under all conditions, and the seeds of others are only dormant under some conditions. There are a number of natural processes, seasonal stimuli or combinations of the two which overcome dormancy and stimulate germination. And there are a number of artificial substitutes which humans can use to mimic such ‘dormancy breakage’. These are called pre-(sowing) treatments or pretreatments for short. Nearly all tree seeds exhibit dormancy of one sort or another and therefore fast, uniform and complete germination is only possible after live, dormant seeds have been exposed to a natural or artificial dormancy breakage pretreatment. This section describes a few of the features of hard–seededness, seed dormancy and seed pretreatment and the properties of seeds with different types of dormancy are illustrated, see Figures 9a, b and c.
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