Even when climatic factors do not dictate a need for seed dormancy, the characteristic provides a
safeguard for survival. If all seeds germinated immediately, cataclysmic events such as fires and late
freezes could destroy entire species, at least in local areas. Differing levels of dormancy in a seed popu-
lation permit germination over a period of several years or even longer, depending on seed longevity.
II. TYPES OF DORMANCY
Numerous types of dormancy exist. The many types of dormancy exhibited by plant organs have created
problems in terminology and definition. This problem was summarized for seeds by Simpson [1]: “A pre-
cise definition of dormancy cannot be used in the general sense to apply to all seeds, but can only be given
for each individual seed considered in the context of a precisely defined set of environmental conditions.”
Nevertheless, Lang et al. [2,3] and Lang [4] have attempted to classify the many types of dormancy into
three main categories, based on the controlling factor(s): ecodormancy, when growth is prevented by en-
vironmental conditions, such as low or high temperature; paradormancy, when growth is prevented by
conditions outside the meristem but within the plant; and endodormancy, when growth is prevented by
conditions within the meristem itself. Examples of these types of dormancy are the failure of buds of trees
to expand in the late winter, when low temperatures prevent growth (ecodormancy); their failure to grow
in early winter, even when held in a warm greenhouse, because they have not been exposed to sufficient
“chilling hours” (see later) to permit growth (endodormancy); and the failure of lateral buds to develop in
an herbaceous or woody plant when the terminal bud is growing rapidly (paradormancy). In the buds of
perennials, dormancy progresses gradually from paradormancy, also called apical dominance, through
endodormancy to ecodormancy as the seasons progress from summer to fall to winter and spring.
These definitions are more applicable to whole plants or shoots than they are to seeds, and seed sci-
entists have been less receptive to their use [5]. Is a dry bean seed, which exhibits no dormancy, ecodor-
mant just because it will not grow without water? Does paradormancy exist in a seed? Does a single type
of dormancy prevent growth, or are control mechanisms more complex? As we will see, dormancy is in-
deed a complex phenomenon in many systems.
I have spoken of dormancy in seeds and whole plants, but dormancy can occur in other structures as
well. Bulbs, tubers, and corms—all organs that permit plants to survive unfavorable environmental con-
ditions—also exhibit dormancy. This dormancy can be likened to bud dormancy, for all three structures
contain buds, and bud development is the primary indication of the ending of their dormant period. In some
respects the structures represent intermediates between whole plants and seeds in that they are more com-
pact than the former but less compact than the seed, which has in addition a seed coat surrounding the em-
bryo and closely associated parts. Most of the remainder of this chapter deals with seed and bud dormancy.
Given the many aspects of dormancy, I will not address apical dominance in detail. Several reviews [6,7]
provide information on this topic. Khan [8,9], Bewley and Black [10,11], and Bradbeer [12] provide thor-
ough coverage of seed dormancy; Saure [13], Powell [14,15], and Martin [16] have reviewed many aspects
of bud dormancy; and Dennis [17] and Lang [18] offer additional information on dormancy in general.
III. SEED DORMANCY
A. Induction of Dormancy
Some seeds do not become dormant until fully mature. The percentage germination of barley seeds in-
creases with maturation up to a certain point, then declines (Table 1). Germination is further reduced
when mature seeds are held at room temperature for 1 week, but it is stimulated by a brief exposure of
moist seeds to low temperature [19]. Breeders sometimes take advantage of this by harvesting fruits be-
fore they reach maturity, when seeds or embryos can germinate without special treatment. Considerable
research has focused on the physiological basis for the inability of immature seeds to germinate. Kermode
[20] provides an analysis of the problem.
B. Types of Seed Dormancy
Early investigators recognized that many factors could be responsible for the failure of seeds to germi-
nate. One obvious cause of such failure is a nonviable embryo. Death of the embryo can occur during seed
162 DENNIS