Dormancy and the Control of Germination

Abstract

Whether or not a viable seed germinates and the time at which it does so depend on a number of factors, including those present in the seed's environment. First, the chemical environment must be right. Water must be available, oxygen may have to be present since the seed must respire, and noxious or inhibitory chemicals should be absent. The physical environment, too, must be favorable. The temperature must be suitable and so also, in many cases, must the light quality and quantity. But in many instances all these conditions may be satisfied and nevertheless the seed fails to germinate. The reason for this, as we have indicated in Chapter 1, is that there exists within the seed (or dispersal unit) itself some block(s) that must be removed or overcome before the germination process can proceed: such a seed is said to be dormant. To be released from dormancy, a seed must experience certain environmental factors or must undergo certain metabolic changes. Hence, the control of germination exists at two levels. One---dormancy---is due entirely to the state of the seed itself, and the second involves the operation of environmental factors on both dormancy and germination. We call these the internal and external controls, respectively. The relationship between dormancy and germination and the points at which control exists is shown in Figure 5.1.