Handbook of Plant and Crop Physiology

development (abortion) or after shedding of the mature seed. Some seeds (silver maple, citrus) are very
short-lived; if germination does not occur within a few weeks, the seed does not survive. Other seeds, in-
cluding many nuts, as well as avocado and cacao, lose viability rapidly when dried; if stored, a high rel-
ative humidity should be maintained. Information on methods of evaluating and prolonging seed viabil-
ity are available elsewhere (e.g., Bewley and Black [10]) and will not be discussed here.
By definition, a seed that is dormant has the potential to germinate (is viable) but requires exposure
to certain treatments or environmental conditions before germination can occur (Table 2). Some fruits
contain inhibitors that prevent seed germination. Seeds of tomato and cucumber, for example, will not
germinate within the fruit; the pulp must be removed and the seeds washed before germination can occur.
In other species (e.g., peach, cherry), the presence of a hard pit (endocarp or inner ovary wall) may limit
germination. Although such seeds can germinate following the breaking of endodormancy by chilling
(see later), germination is improved by endocarp removal. Neither of these conditions represents true seed
dormancy, as control is external to the seed, but they are often discussed in relation to seed dormancy.
Some of the conditions that break seed dormancy are given in Table 3.
Like the endocarp, the seed coat itself can prevent germination in some species, especially legumes
such as alfalfa, locust, and redbud. The structure of the seed coat (testa) prevents the entry of water and its
absorption by the embryo (imbibition); thus the embryo cannot germinate. The seed coat must be weak-
ened, either naturally by abrasion or by exposure to fire or to HCl during passage through the gut of an an-
imal, or artificially by scarificationbefore imbibition can occur. Ground fires damage hard seed coats,
thereby permitting germination of seeds that might otherwise remain dormant [31]. Scarification can be
either mechanical, by rotating seeds with gravel or filing the seed, or chemical, by brief exposure to con-
centrated H 2 SO 4. “Heat shock” by immersing seeds briefly in boiling water can be more effective than me-
chanical scarification in some species. For example, Bell et al. [31] reported that germination of seeds of

DORMANCY: MANIFESTATIONS AND CAUSES 163

TABLE 1 Effect of Stage of Development and Cold
Treatment on Germination of ‘Cape Coast’ Barley

Stage Germination (%)

Milk stage 5
Yellow-ripe 60
Mature 36
Maturestored 1 week 1
Maturestratified for 2 days 64
Source: Ref. 19.

TABLE 2 Types of Seed Dormancy, Conditions That Break Dormancy, and Specific Examples

Cause of dormancy Conditions that break dormancy Species

A. Control outside the seed

1. Inhibitors in the fruit


2. Hard endocarp
   B. Control by seed coat


3. Coat impermeable to H 2 O


4. Coat impermeable to O 2
   C. Morphologically immature
   embryo
   D. Physiologically immature
   embryo


5. “Shallow” dormancy


6. “Deep” dormancy
   a. Epicotyl dormancy
   b. Double dormancy
   CD. Hard seed coat plus deep
   dormancya

Seed removal, washing

Acid treatment, endocarp removal

Acid or mechanical scarification, fire

Seed coat removal?

Warm-moist storage

Cool-moist storage

Light, alternating temperature, dry storage

Cool, moist storage

Cool, moist storage

Cool, moist storage

Scarification, followed by cool, moist

stratification

Tomato, cucumber

Stone fruits

Some legumes

?

Ginkgo, coconut

Cowparsnip

Lettuce, celery, oats

Apple, peach

Tree peony

Trillium

Redbud

aSome authors use the term double dormancyfor this phenomenon.