Abstract:Introduction:

A dormant seed will fail to germinate under normally favourable conditions of moisture, temperature and oxygen supply. Temperate climate plants especially produce dormant seeds, but they are also found in tropical and subtropical species. Methods for breaking seed dormancy have been devised which override the physiological mechanisms involved in dormancy preservation. In this essay I intend to outline the most popular mechanisms of dormancy breaking and discuss the physiological mechanisms and ecological significance of stratification, photosensitivity and scarification.

Seeds are the agency of propagation for plants. Dormancy offers plants the opportunity to regulate this stage of their growth cycle. Through dormancy plants can pause and later benefit from both seasonal and fortuitous changes in the environment. Dormant seeds can remain viable for remarkably long periods of time. Lotus seeds, Nelumbo nucifera, dug from archeological sites over 1000 years old have been found to be viable (Weir et al., 1982). In certain species, the dormancy characteristics of seeds within the same harvest are different. The timing of germination would be spread out over months or years, increasing the likelihood of some seed survival. Other seeds display secondary dormancy. These seeds when shed will germinate readily if conditions are favourable. However if conditions are not favourable, secondary dormancy is induced. Studies of dormancy by plant physiologists have provided valuable knowledge on the mechanisms of seed dormancy initiation of germination. "Dormancy mar be due to an immature embryo, impermeability of the seed coat to water or to gases, prevention of embryo development due to mechanical causes, special requirements for temperature or light, or the presence of substances inhibiting germination" (Mayer & Poljakoff-Mayber, 1982). An understanding of germination initiation is of economic importance in agriculture, horticulture and forestry. Without this, important seed which is dormant would be reduced to germinating naturally in a very narrow range of micro-environmental conditions. To break dormancy, special treatments may be necessary before sowing or during imbibment of batches or seed banks.

The mechanisms for breaking dormancy correspond to the various dormancy inducing and maintaining mechanisms in action for each species. In a few species dormancy is related to the undeveloped state of the embryo at the dehiscence. A considerable period of time must pass during which the embryo will continue to develop until it reaches `maturity’. In other seeds there are no obvious changes anatomically or morphologically but still a period of after- ripening is required. The subtle differences which occur within the seed must be the consequence of chemical or physical changes within the seed or seed coat.

A hard seed coat is capable of causing dormancy in three was; it may be impermeable to water, impermeable to gases or it may mechanically restrain the embryo (Mayer & Poljakoff-Mayber, 1982). `Impaction’, vigorous shaking of the seeds can be effective in making the seeds permeable to water. Treatment of the seeds with microwave energy (2450 MHZ) Tran 1979; Mayer & Poljakoff-Mayber, 1982) has also been successful for hard coated seeds. Dry heat and boiling water treatments are commonly used for seeds of Acacia melanoxylon. Shaking with abrasive material causes mechanical breakage of the seed coat and is a frequently used method as are scratching and pricking. Chemical treatment is usually with a solvent such as alcohol or an acid. Treatment with alcohol is suggested for members of the family Caesalpiniaceae (Mayer & Poljakoff-Mayber, 1982). Often combined types of dormancy restrain germination. Seeds of Rosa and Crataegus sp. have extremely hard and durable coats. Great mechanical pressure is needed to destroy the stony endocarps. Additionally, endogenously imposed dormancy mechanisms complicate the inducement of germination in Rosa species. Long imbibment periods without chilling will initiate secondary dormancy (Bradbeer, 1988).

In order to germinate many seeds require exposure to specific temperatures (high or low), at which germination is not normally favourable. In temperate regions chilling or ‘stratification’ is an extremely important control factor in dormancy breaking. This involves exposure of imbibed seeds to temperatures normally between 10 and 100 C, for extended periods. The term stratification was coined from the procedure used in temperate forest nurseries. The seed material is placed in alternating layers of sand or soil. At the beginning of spring the seeds are dug up for sowing in seed beds (Bradbeer, 1988). Routinely though, standard chilling methods employ refrigerated incubation.

Treatments with high temperatures are not particularly successful even with tropical species. Although brief exposure to slightly elevated temperature has promoted subsequent germination at lower temperatures after exposure to light n Poa pratensis and Lepidium virginicum (Mayer & Poljakoff-Mayber, 1982). Tropical species more often respond to combinations of temperature and light.

Bradbeer recognizes specific groups of light related dormant seeds:

(1) Positively photoblastic seed in which germination was either induced or promoted by light

(2) Negatively photoblastic seed in which germination was either wholly or partially inhibited by light

(3) Apparently non-photoblastic seed in which no difference between dark and light germination has been reported.

For positively photoblastic seeds the level of illumination required may be low and germination relies more upon the quantity of light received (Villiers, 1972). )Only imbibed seeds will respond to light and there is usually a complex interaction between light and other external factors as well as the age of the geed (Mayer & Poljakoff-Mayber, 1982).

The control of dormancy in some species is dependent on the combined actions of inhibitory and promotive substances. Frequently many of these dormancies can be overcome by the use of chemical promoters. Commercial products such as Dalapon, Thiourea, Knop’s potassium nitrate, and hormones such as gibberellins and cytokines have been used. These substances can wholly or partly replace light or temperature in breaking of dormancy. More simply, natural substances enveloping the seed can produce an environment of high osmotic pressure. Seeds of plants from saline environments often are held dormant by this mechanism. Tomato seedlings are encapsulated in a gelatinous substance which when removed allows germination of the seed (David Midmore, pers com). This gel may also contain a germination inhibitor such as caffeic and or ferulic acids (Mayer & Poijakoff-Mayber, 1982). Leaching can be used to attempt to remove any of these water-soluble inhibitors. Birch seeds need prolonged rinsing, 16 hours whereas Xanthium pennsylvanicum require only a small amount of water (Wareing & Foda 1975; Badbeer, 1988).

Stratification

The exact molecular mechanisms of breaking dormancy through chilling are shadowy. Frankland and Wareing (1966), working on hazel seeds, found that initially dorrmant seeds contained no detectable gibberellic acid (GA) substances. ) however, after 12 weeks of chilling, the hazel seeds contained the biological equivalent of 0.2 picomoles of GA3 (Bradbeer, 1988.). Through further work it was established that GA is synthesized in the embryonic axis and is translocated to the cotyledons not during stratification but in the first days after the temperature is increased (Ross and Bradbeer, 1971; Bradbeer, 1988). This would imply that chilling brings about changes which capacitate the biosynthesis of GA when the temperature is raised after stratification. Bradbeer (1988) continued by suggesting it was the presence of growth inhibitors which retarded synthesis of GA in the axis. Moreover that chilling influenced the levels of growth inhibiting substances. Studies by Mayer et al. (1982) upheld this. In the seed of Fraxinus americana, Juglans regis and Corylus avellana, it was reported the amount of growth inhibiting substance ABA,(abscisic acid), dropped during stratification. Walker-Simmons et al. (1989), whilst working on wheat embryos from dormant grain, showed that ABA deficient mutants in maize, potato and tomato precociously germinate. It has been shown that gibberellins interact with ABA in the synthesis of hydrolytic enzymes in the aleurone layers of germinating cereal seeds (Chrispeels and Varner, 1966; T.A. Villier , 1972). It is generally accepted that dormancy is regulated by a balance between and sensitivity to the activities of growth-promoting and growth-inhibiting substances. During stratification this balance and sensitivity is changed considerably.

In temperate climates where dormant seeds are most abundant, many seeds are dispersed in autumn and covered by moist leaf litter or soil over the winter months. It is strategic to delay germination until spring when conditions are more promising for germination and seedling growth. Stratification mimics these conditions. For the seeds of tropical plants stratification is not as effective. An alteration between high temperatures and exposure to light, with low temperatures and high humidity is often needed to induce germination. Dormant seeds of Oldenlandia corymbosa will not germinate in the light at high temperatures without prior exposure to lower temperatures 25Â?° C, at high relative humidity (Attims & Come, 1978; Mayer & Poljakoff-Mayber, 1982). In and regions such as the desert of Western Australia summer grasses remained dormant during winter rains and only germinated in the following rains of summer (Mott, 1972; Mayer & Poljakoff-Mayber, 1982).

Light in Dormancy Control

"The effect of light on dormancy is dependent on the intensity and duration of irradiation, on the wavelength, on moisture content of the seed and on the time of the exposure to irradiation, including the whole of the previous history of the seed during development on the parent plant and subsequently" Bradbeer, 1988. This statement hints at the enigmatic role of light as a mechanism for breaking dormancy in seeds. The effective wavelength of light for promotion of seed germination has been shown to be the red region of the spectrum, 660 nm. Far-red irradiation, 730 nm, inhibits the germination of light promoted seeds (Borthwick et al., 1952; T.A. Villiers, 1972). The regulatory pigment involved is Phytochrome, a chromoprotein in which the chromophore is a tetrapyrrole (Bradbeer, 1988). It has two photoconvertible forms PR and PFR. The pigment is converted from one energy form, PR, germination inhibitor, to the other, PFR, germination promoter, by exposure to red irradiation. Exposure to far red irradiation converts the pigment back to PR form. Villiers (1972) suggests that it is the higher energy of the active PFR form that drives the initiation reactions which promote germination. The lower energy levels of PR form are insufficient to bring about the cellular response required to initiate germination. Seeds may be light requiring when newly formed on the plant but over time and with air drying or chilling become less or no longer light sensitive. Unchilledseeds of Betula pubescens require day long illumination for dormancy release but chilled seeds will germinate in the darkness (Black & Wareing, 1959; Villiers, 1972). Moreover light sensitivity can be induced in negatively photoblastic seeds by maintaining seeds in unfavourable conditions. Light insensitive tomato seeds exposed to far-red irradiation for 18 hours from the beginning of imbibition then required red irradiation to promote germinate. It is suggested that prolonged irradiation causes far red absorbing pigment to be synthesised (Mancinelli et al., 1966; Villiers 19-72).

The conditions under which a particular batch of seeds has matured on the plant might be important in manipulating light sensitivity (Koller, 1962; Villiers 72).

The use of Phosphon D, an inhibitor of gibberelin synthesis, can reverse the effect of light on dormancy of Verbascum seeds (McDonough, 1965; T.A. Villiers, 1972). Such a finding suggests that gibberelin synthesis may. play a pivotal role in light-stimulated dormancy breaking of seed. The ecological value of light regulated germination is not totally clear.

Superficially, seeds requiring light for dormancy breaking are likely to be

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