Dormancy is one of the most common adaptive strategies of plants to adverse environmental conditions.  Dormancy is defined as a period of temporary physiological and growth inactivity.  It is manifested in almost all groups of plants from the primitive algae to highly evolved seed plants.  It may occur in plants of widely differing habitats.  For example, dormancy enables many desert plants to survive the hottest and driest period of the year, whereas in temperate and polar zones, dormancy is an adaptation to the adverse conditions of winter.

The main danger to plants from low winter temperatures is the formation of ice crystals within the protoplasts of cells.  The formation of ice results in dehydration of the protoplasm, and the crystals may disrupt the organelles and rupture the cell walls.  Plants or plant parts that are in the dormant state are characterized by low rates of metabolism and very low cellular water content.

All parts of plants including stems, buds, underground stems and buds, and seeds may achieve the dormant state.  Dormancy is not simply the cessation of physiological and growth activity; it is a programmed series of metabolic events.  In the terminal buds of many deciduous tree species dormancy is initiated by the short days of autumn.  The environmental conditions necessary for breaking dormancy are often the reverse of those that initiated the state.

The dormancy process was studied in detail in two European tree species, European white birch (Betula pubescens) and sycamore maple (Acer pseudoplatanua).  In an experimentally induced dormant birch, the leaves were removed and the buds were exposed to long days.  The result was the breaking of dormancy by the buds.  In another experimentally induced dormant plant the buds were exposed to long days and the leaves to short days.  The result was that the buds did not break dormancy.  This suggests that the dormant state is controlled by the leaves.

The investigators hypothesized that a dormancy-inducing substance was synthesized in the leaves and transported to the buds.  This was later identified as a growth inhibitor and called abscisic acid (ABA).  This is an unfortunate name since this substance is not involved in the formation of the abscission layer and the shedding of leaves.  However, ABA or other growth inhibitors play an important role in the initiation and maintenance of winter bud dormancy in many native trees and shrubs.

In order to break dormancy in spring, native species of trees and shrubs require exposure to an extended cold period.  The length of the period and the degree of cooling have not been determined for all species.  However, among cultivated plants, peach trees require 400 or more hours of exposure to temperatures below 7^oC, and blueberry plants require 800 hours of exposure at this level in order to break dormancy.  After the period of cold exposure, plants require increasing temperature and increasing photoperiod to resume growth.  Most deciduous tree species require a minimum of 300 hours at temperatures near 25^oC before new growth begins.

The dormancy requirement of plant species has a profound influence on geographic distribution.  If such a plant is not exposed to the required minimum cold period it may not break dormancy in spring; or if it does break dormancy growth may be weak and the plant devoid of flowers.  Thus a species may be limited in its southward distribution by the length and severity of winter temperatures.  Undoubtedly one reason why apple trees do not flourish in Florida is that it is not cold long enough to fulfill the dormancy requirements of apple trees.

SEED DORMANCY

The seeds of many of plant species will not germinate when mature even though all the factors necessary for growth are favorable.  They require an after-ripening period that may vary in length for different species and may include exposure to some special conditions.  This failure to germinate is generally referred to as dormancy, and a period of low temperature is usually required for its termination.  Although the circumstances in which it is initiated may be different, the state of dormancy of the seed is similar to that of the bud.

The failure of seeds to germinate may be the result of several factors:  (1) the physical conditions of oxygen, temperature, and water, may be unfavorable; (2) the seed coat may impose restrictions; (3) a metabolic block within the seed tissue which requires chilling and/or a particular light exposure to overcome dormancy; or (4) a combination of the above.

Seeds that fail to germinate only because environmental conditions are unfavorable for growth have been described as quiescent rather than dormant.  The seeds of most crop plants are in this category because one of the criteria in their development has been ease of germination.  As the term is used here, "dormancy" refers to the condition in which failure of germination is the result of physical or chemical properties of the seed itself.

Dormancy imposed by the seed coat may take one of several forms:  (1) it may be the result of mechanical resistance to embryo enlargement; (2) the seed coat may be impervious to water and/or oxygen; or (3) the seed coat may contain growth inhibitors that must be leached away before germination can occur.  The latter is a characteristic of some desert annuals in which the seed requires at least 2 cm or more of rain to initiate germination.

The cause of dormancy in those seeds that have an internal metabolic block is not clearly understood.  The dormant seeds of some of these species are characterized by the presence of the growth inhibitor ABA.  In still other species the dormant seeds are characterized by a different growth inhibitor.  Some investigations have suggested that the formation of a growth promoting substance is necessary for the resumption of growth.  Investigators have identified a group of plant hormones known as the gibberellins as the most likely growth promoting substances.  It is probable that inhibitors are responsible for initiating and maintaining dormancy and that growth promoters are necessary for ending the dormant state.

One of the most common requirements of dormant seeds is a period of chilling.  Physiological changes that may take place during this period include a reduction in growth inhibitor substances, an increase in growth promoting substances, or both.  The release from dormancy may depend on a specific balance between growth inhibitors and growth promoters.  In any event, the physiological changes necessary for breaking dormancy will not take place in the absence of chilling.

Seed dormancy is widespread among species of the temperate zone.  Although dormancy is often induced at a time when climatic conditions are favorable for growth, the dormant state normally coincides with a period during which environmental factors do not favor growth.  During the dormant period, the important function of seed dispersal also takes place.  Seed dormancy and seed dispersal are phenomena that can be investigated during the winter season (see "Seed Dispersal by Winter Weeds", "Seed Dormancy and Germination", and "Seed Dormancy: A Stratification Experiment").

Stratification is the artificial imposition of a period of low temperature in order to stimulate germination in seeds.  Experiments with stratification and germination may yield data on two aspects of dormancy:

In the Encyclopedia of Plant Physiology, Stokes lists 86 species with seeds that require moist stratification to overcome dormancy.  The table below lists a few widespread species along with their common name and the necessary moist pretreatment.

REFERENCES

Barton, L. V.  1965.  Dormancy in seeds imposed by seed coat. Encyclopedia of Plant Physiology 2:727.

Barton, L. V.  1965.  Seed dormancy:  General survey of dormancy types in seeds and dormancy imposed by external agents. Encyclopedia of Plant Physiology 2:699.

Dale, H. M.  1970.  Germination patterns in Daucus carota: Variations in the 1967 collection.  Can. Jour. Bot. 48:413-418.

Dale, H. M. & J. Harrison.  1966.  Wild carrot seeds:  Germination and dormancy.  Weed Science 14:201-204.

Duke, S.  1970.  Seed hydration and secondary dormancy in Rumex crispus seeds.  Plant Physiol. 59:(6) 34.

Evenari, M.  1965.  Light and seed dormancy. Encyclopedia of Plant Physiology 2:804.

Haydecker, W., (Ed.) 1972.  Seed Ecology.  Pennsylvania State University Press, University Park, Pa.

Sterns, F. & J. Olson.  1958.  Interactions of photoperiod and temperature affecting seed germination in Tsuga canadensis. Amer. J. Bot. 45:53-58.

Stokes, P.  1965.  Temperature and seed dormancy. Encyclopedia of Plant Physiology 2:746.

Villiers, T. A.  1972.  Seed dormancy. In Seed Biology, (T. T. Kozlowski, Ed.) Volume II, Germination Control, Metabolism and Pathology.  Academic Press, New York.