Different types of seed dormancy can be distinguished on the basis of the developmental timing of dormancy onset. Newly dispersed mature seeds that fail to germinate under normal conditions exhibit primary dormancy, typically induced by abscisic acid (ABA) during seed maturation. (ABA regulation of seed dormancy is discussed later in the chapter.) Once primary dormancy has been lost, non-dormant seeds may acquire secondary dormancy if exposed to unfavorable conditions that inhibit germination over a period of time. For example, non-dormant seeds of Avena sativa (oat) can become dormant if exposed to temperatures higher than the maximum for germination. In contrast, seeds of Phacelia dubia (small-flower scorpionweed) can acquire secondary dormancy at temperatures lower than the minimum for germination. The mechanisms of secondary dormancy are poorly understood.

Secondary dormancy can be observed in nature in the seed dormancy cycles of annual dicot weed species. To synchronize themselves with the seasons, a population of seeds buried in the soil, called a soil seed bank, must be able to sense when the environmental conditions are suitable for germination and seedling growth. Seed dormancy and longevity (described later in the chapter), as well as seed resistance to biotic and abiotic stress (see Chapters 23 and 24), are key characteristics that contribute to seed persistence in the soil seed bank. In temperate zones, dicot weed seeds germinate at predetermined times of the year. Summer annuals, for example, typically germinate during a limited time in the spring, sometimes followed by a second round of germination in the summer. Although emergence may be strongly stimulated by soil wetting or soil disturbance, the annual cycle of germination does not depend on the prevailing environmental conditions, but on a seasonal fluctuation of the environment. If the conditions for germination are unfavorable, seeds may develop secondary dormancy. In summer annuals like Polygonum persicaria, secondary dormancy is induced by warm summer temperatures. In winter annuals, such as Veronica hederofolia, the summer temperatures break dormancy, whereas the low winter temperatures induce secondary dormancy. Thus, summer annuals germinate in the spring, while winter annuals germinate in the autumn (Web Figure 18.3.A).

Web Figure 18.3.A Secondary dormancy in summer and winter annuals. Seeds were buried in sandy loam every month and the percent germination was measured. The percent germination was then plotted as a function of the month planted. The percent germination of the summer annual Polygonum persicaria was highest in the spring and lowest in the summer due to secondary dormancy (green line). The winter annual Veronica hederofolia germinated best in the autumn (dark red line), while lower winter soil temperatures induced secondary dormancy. The blue line shows the variations in soil temperature. After Roberts and Lockett (1978) and Karssen (1980/81).