species occurs while the seed is still on the parent plant, apparently because of lack of inhibiting chemi-
cals [20].
The germination of seeds, especially those of many woody trees and shrubs, is complex and erratic.
In nature, every species has one or more mechanisms for preventing germination until the seed has been
dispersed, and failure to germinate seeds in cultivation is often due to our lack of understanding of these
mechanisms [8,21,22].
Dormancy is an all-inclusive term used to describe a “resting” state with reduced metabolic rate [23].
In this condition, a seed will not germinate because of constraints associated with the seed itself (physi-
cal or physiological) or with the external environment. Until a seed dies, it remains metabolically active
even during prolonged storage under dry conditions or extreme temperatures. Dormancy is either primary
or secondary (Table 1). Primary dormancy is an adaptation of the plant species to control the time and
conditions for germination. Secondary or consequential dormancy is a further adaptation that prevents
germination of an imbided or nondormant seed if other environmental conditions are unfavorable [8,24].
Geneve [24] categorized commonly grown small-seeded vegetable and flower species according to their
dormancy type.
VI. TYPES OF DORMANCY
A. Primary
- Exogenous
Viable seeds, especially of many woody species, may not germinate after considerable lengths of time
even when the germination environment seems to be ideal. In most seeds with delayed germination, this
condition is due to a hard seed coat restricting water absorption and gaseous exchange and/or due to
the actual mechanical constraint by the seed coat or covering to the developing embryo [8]. Seeds from
plants of the Cornaceae, Geraniaceae, Fabaceae, Malvaceae, and Convolvulaceae families are
characterized by this condition [23]. Usually, this condition can be overcome by any method that soft-
ens, scarifies, or removes the seed coat or covering, including chemical or mechanical degradation
(scarification).
- Endogenous
In many species, delayed germination results from internal conditions of the embryo and food storage tis-
sues, or a portion of these tissues, which must undergo certain developmental (rudimentary or immature
embryos) or physiological changes before seeds will germinate [8,24].
Whereas seeds of many species possess germination inhibitors that are deactivated before germina-
tion occurs, seeds of other species may require “after-ripening,” defined as a short period of dry storage
after seeds are harvested, usually several weeks in duration (nondeep or transitory physiological dor-
mancy) but possibly a few months or even several years for some species [8,24]. During this process,
physiological and mechanical changes occur: growth-promoting hormones and enzymes are synthesized
or activated, inhibiting hormones or related chemicals are deactivated, water is absorbed, and respiration
increases.
The after-ripening requirement is quite common for seeds of herbaceous garden plants and vegeta-
bles but less common for trees and shrubs [21]. Other species may require a period of moist chilling,
which may be moderate (intermediate physiological dormancy) or longer (deep physiological dormancy)
in duration (Table 1). Tables 2 and 3 indicate the chilling requirements for selected flowering herbaceous
and selected woody trees and shrubs, respectively.
- Double or Combinational
Immature or rudimentary embryos are characteristic of many species of seeds with double dormancy,
such as Ginkgo biloba(Maidenhair tree), Ilexspp. (holly), Taxusspp. (yew), Viburnumspp. (viburnum),
andFraxinus nigra(black ash). Seeds in this category do not germinate until dormancy related to exter-
nal and internal physiological factors is overcome sequentially. In many other cases, the embryo may be
fully developed and appears to be mature but may be dormant because of various physiological and chem-
ical constraints [8,24]. Specific regions of the embryos may be responsible for this dormancy, such as the
seed coat or a dormant radicle, hypocotyl, or epicotyl (Table 1).
62 CHONG ET AL.