Seed Dormancy and Germination, We often visualize plant morphogenesis and growth as a continuous process from germination through flowering to death. The life cycles of almost all plants, however, are characterized by periods of temporarily arrested growth. They become quiescent that is they continue to live, but with very low, almost non measurable metabolic activity. Plants or plant parts seem to enter a state of suspended animation An understanding of this phenomenon will be highly significant to us in agriculture and in other ways, including space travel.
Plant scientists often use the term dormancy to describe the arrest in growth and development of seeds (embryos), buds, and other plant parts under conditions seemingly suited for growth Growth may also be suspended by adverse conditions in the environment Seeds, for example, will not germinate under dry conditions, but they will readily germinate if they imbibe water, A suspension of growth may also occur because of the concentration of some growth inhibitor, or it may be caused mechanically by the mere presence of a strong, durable enclosing structure that does not allow for expansion. The presence of membranes or seed coats impermeable to water or oxygen may also keep growth in an arrested state. Finally, many seeds and buds require special conditions of light and temperature. The onset of dormancy and the deciduous habit of North Temperate zone plants serve as excellent examples of photoperiodic and temperature-regulated growth.
Some distinction is made between a suspension of growth due to a lack of a necessary external environmental factor (e.g., water) and a suspension of growth due to internal limitations. Arrested growth because of the lack of some necessary external environmental factor is referred to by many as quiescence. However as we have mentioned, many seeds and buds are unable to grow even if provided with water because of internal limitations, a situation referred to by many as dormancy (a rest stage). Since the general result, growth suspension, is the same, we include both situations under the general term dormancy.
In temperate zones, seasonal changes in temperature range hem near 38°C in midsummer to well below freezing in midwinter. Obviously, most plants could not survive the cold temperatures of winter in the vegetative or flowering state. Thus in many plants, seed and bud dormancy begin at the onset of winter cold, thereby allowing the plant to pass through the winter with little or no damage. In the grain areas of the United States and Canada, for example, wild oat infestation is a serious problem because of the ability of the grains to survive the winter in a dormant state and then germinate the following spring. In contrast, the seeds of many other noxious weeds have only a brief dormant period, germinate in the fall, and are killed off during the severe winters that are Common to the northern , Midwest areas.
The importance of dormancy among plants growing in arid regions is immediately apparent. For those plants, it is certainly significant if germination and growth can be managed during the relatively brief periods of rainfall in these areas. Seeds that can remain viable but dormant until sufficient water is available have a very good chance of survival. We find an even more bizarre example of the importance of dormancy in the adaptation of a plant to adv region in the desert shrub guayule In this plant, the chaff covering the seed contains a germination inhibitor that causes the seed to remain in a dormant state. However: with a rather strong rainfall, sufficient dilution of the inhibitor occurs to allow germination.
While talking about the beneficial role of dormancy in plants, we should also mention how seed coats impermeable to water help in the persistence of a species. Some species of Convolvuius, which grow in arid regions, have this type of seed coat in order for these seeds to imbibe water and germinate, the seed coats must be mechanically broken. However, permeability to water gradually occurs over a long period of time. The advantage here is that all of the seeds will not germinate at one time, only a certain number will germinate each year. Thus it is virtually impossible for the entire species to be wiped out during the vulnerable seedling stage due to some adverse environmental condition;
Dormancy in plants is both a convenience and an inconvenience to humans. The temporary dormant period experienced by many cereal grains allows for their harvest, dry storage, and ultimate use as food. Otherwise, these gears would germinate and be useless to us. The ability, however, of certain Weed seeds to lie dormant for many years in the soil has proven to be a great inconvenience. During plowing, the dormancy of many of these seeds will be broken, thereby allowing them to compete with any economic crop sown in that area. The eradication or even control of many of these weeds is almost impossible because they can never all be caught in the vulnerable seedling or vegetative state. Although some are triggered to germinate by the soil disturbances caused by plowing, there are always some that remain dormant in the soil. Therefore, each year farmers are presented with the same problem, the germination of some but not all of these weed seeds. They Can only destroy those that germinate and they have almost no control over those lying dormant in the soil.
Seed Dormancy and Germination
We may define the process of germination as the sequence of Steps that begins with the uptake of water and leads to the rupture of the seed coat by the radicle (embryonic root) or the shoot. Cell division, cellular enlargement, and overall increase in metabolic activity (food digestion and assimilation, for example) accompany the observable morphological steps. Even though these events commence long before the rupture of the seed coat, we usually determine the occurrence of germination visibly by noting the protrusion of the radicle. Let us consider the various factors causing dormancy and the different methods for breaking dormancy.
The absence of some external factor considered necessary for the process to occur inhibits the germination of seeds. Thus in the absence of water, the proper temperature, or the proper mixture of gases, germination is inhibited. However, many seeds may be placed in an environment considered optimum for germination and still not germinate because of some factor associated with the seeds. This factor may be a hard seed coat that is impermeable to water or gases or is physically resistant to embryo expansion an immature embryo, a need for after ripening a specific light requirement, a specific temperature requirement, or the presence of a substance that inhibits germination.
Hard Seed Coat
One of the most common factors associated with seed dormancy is the presence of a hard seed coat. The hard seed coat mar be responsible for dormancy by preventing the absorption of water, by preventing gas-eons exchange, primarily oxygen absorption, and by ‘mechanically restricting the growth of the embryo.
Inhibition of water absorption. Many plants produce seeds with hard seed coats impermeable to water. In this respect, the Leguminous family has by‘far the largest number of species (14). In addition to having hard seed coats, seeds of many members of the Leguminous have an external waxy covering (25). Some of these seeds may be totally impermeable to water. The hardness factor in seed coats is primarily an inherited trait; however, in at least one case the hardness of a seed coat is determined by environmental conditions. Cracker (6) observed that seeds oi white sweet clover are hard when they ripen during hot, dry weather, but they are soft when they ripen during rainy weather.
Hyde (21), in a study of some legume seeds, described an interesting mechanism for the control of water entering the seed. In the seeds of some legumes (e g, Lupinus arboreus), water enters only through the hilum. Hyde found that the absorption of water by these seeds is controlled by gyroscopic tissue, which makes up the hilar fissure When the relative humidity is high this tissue swells, closes the hilar fissure, and prevents water absorption; and when the relative humidity is low, the fissure opens and allows the seed to dry out.
We can ascertain that drying out of the seed is the inevitable result under these circumstances by measuring the moisture content of scarified and un-scarified seeds of white clover after they have been subjected to different relative humidifies. As we shall gee later, scarified seeds are those in which the seed coats have been rendered permeable to water and gases. The white clover seeds contain the same type of mechanism for controlling water absorption as do the seeds of Lupinus arboreus. illustrates that the moisture content of the un-scarilied white clover seeds never rises when the seeds are transferred from a low to a high relative humidity and always falls when they are transferred from a high to a low relative humidity. The moisture content of the scarified seeds, in contrast, rises and falls relative to the humidity treatments, as we would expect of a seed that is permeable to water.
Inhibition of gas absorption. Many seeds that are permeable to water are impermeable to gases (25). We find the classical example of this type of impermeability in the cocklebur (Xanthium) in the burr of the cocklebur plant, there are two seeds, one borne higher‘ up in the burr, called the upper seed, and one borne lower in the burr, called the lower seed. Cracker (6) found that the seed coats of both seeds are, permeable to water. The lower seed will germinate under normal conditions of moisture and temperature but the upper seed will not germinate under these conditions unless the seed coat is punctured or removed. However, if the upper seed is placed under high oxygen conditions. it germinates readily. Crocker concluded that the seed coat of the upper seed limits the supply of oxygen to the embryo so that the minimum needed for germination cannot be reached. Subjection of the seed to high concentrations of oxygen overcomes this block in germination.
Later work by Shull (36, 37.) and Thornton (39) demonstrated the accuracy of Crocker’s observations. These two workers showed that the naked embryos of both the upper and the lower seeds have a much lower oxygen requirement than does the intact seed and as temperature increases the oxygen requirement decreases For the naked embryo of the upper seed, 1. 5 percent oxygen is needed at 21°C and 0 9 percent oxygen is needed at 30°C for 100 percent germination. When the upper seed is left intact the oxygen requirement for genuineness increases considerably. Pure oxygen is needed at 21°C and 80 percent oxygen is needed at 30°C to give 100 percent germination. Figure 23-2 presents some of Thornton’s data. We do not as yet know whether
the limiting of oxygen supply by the seed coat retards metabolic activity to the point of blocking germination or whether the high oxygen concentration has some other function that promotes germination. Wearing and Foda (53) claimed that high oxygen tensions cause the oxidation of an inhibitor present in the upper seed, thus allowing germination.
Mechanical restriction of embryo growth. Seed coats may be permeable to both oxygen and water, yet still effect a dormant state in a seed. For example, the seeds of pigweed (Amaranthus retroflexus) have a seed coating that is permeable to oxygen and water but that is strong enough to resist embryo expansion (26). These seeds may sometimes lie dormant but viable for many years. Dormancy and modes of storage have maintained seeds viable for as long as 10,000 years. Dried seeds have been shown to exist in soil for thirty-one years, in dry laboratory storage for one hundred years, and in peat bogs and frozen earth consider ably longer.
Where germination is inhibited by mechanical resistance of the seed coat or impermeability of the coat to water or oxygen, dormancy may be broken by scarification. The term scarification refers to any method that renders the seed coat permeable to water and oxygen or breaks the seed coat So that embryo expansion is not physically retarded. The process can be accomplished in the -laboratory by forms of abrasion, cutting, or chemical treatment. Scarification is roughly divided into mechanical scarification and chemical stratification. Mechanical scarification of hard coated seeds is effected by any treatment of the seeds that will crack or scratch the seed coats, such as shaking the seeds With some abrasive material (e. g., sand) or scratching or nicking the coat with a knife. The cracks or scratches resulting from such treatment promote germination by decreasing the resistance of the seed coat to water or oxygen absorption and to embryo expansion.
Chemical scarification rs also an effective way of breaking dormancy resulting from the seed coat. Dipping seeds into strong acids, such as sulfuric acid, or into organic solvents, such as acetone or alcohol and then rinsing the seeds with water can break this type of dormancy. Even boiling water may be a successful treatment. As m mechanical scarification, chemical scarification breaks dormancy by weakening the seed coat or by dissolving waxy material that renders the coat impervious -to water.
In nature, the process is accomplished by the acid and enzymatic conditions of the digestive tracts of birds and other animals, by abrupt changes in temperature, and by the action of fungi and other microorganisms. In some areas of the world, seeds of certain species require the for scarification. These seeds have a competitive edge immediately after the denudation of a dense vegetative area.
Failure of a seed to germinate may be a consequence of partial development of the embryo Germination will occur only when the embryo development is complete, and embryo development may occur during or before the germination process (25). Dormancy due to immature embryos may be found in Orchidaceae and Ovobancheae, as well as some Fraxinus and Ranunculus species. Dormancy due to immature embryos can only be broken by allowing the embryo to complete development within the seed in an environment favorable to germination.
Afterripening and Stratification
A large number of plants produce seeds that do not germinate immediately but do so after a period of time under normal conditions for germination. A prerequisite to germination for this type of seed, then, is a period of afterripening (development of the embryo). In nature, afterripening occurs during the period between the fall of the seed to the ground in the autumn and its germination the following spring. During this time, the seeds are covered over i with debris and winter snows.
Afterripening occurs for some species during dry storage. For others, moisture and low temperatures are necessary a process called stratification. Natural stratification occurs when seeds shed in the tall are covered with cold soil, debris, and snow. We have learned to copy and improve on nature in this respect by devising a method of artificial stratification. In artificial stratification, layers of seeds are alternated with layers of moistened Sphagnum, sand, or some other appropriate material, and stored at low temperatures. The effect of artificial stratification on the germination of Pinus rigida may be seen in Figure 23-3.
Because many workers refer to the period of afterripening as a dormant, or test, period, the implication persists that nothing is occurring within the embryo during this time. However, many studies have demonstrated that considerable physiological activity may be observed during the so called after-ripening, or dormant, period (29, 31). Figure 23-4 shows the effect of afterripening time and temperature on growth of the embryonic axis of cherry seeds. During afterripening, extensive, transfer of compounds from storage cells to the embryo, sugar accumulation, and digestion of various storage lipids take place.
Stratification may effect the disappearance of inhibitors and the buildup of semi nation promoters such as the belligerents and cytokines. Certainly many changes in the levels and inter conversions of food materials take place
Light Requirements for Germination
With respect to germination, seeds vary considerably in their response to light. Some seeds have an absolute light requirement for germination. In other seeds, exposure to light is inhibitory to germination, And in still others, germination is associated with a photoperiodic response that is, an alternation of light and dark periods. The determination of a seed’s light requirement is made even more complex by the fact that temperature may interact with light in the germination of a seed.
As inmost studies in which light is implicated as a catalytic agent, experimenters are searching for the most effective wavelengths. We hate already seen that light (red and far-red) operating through phytochrome regulates germination Grand Rapids lettuce seeds. We did not mention, however. what effect the time of water imbibition prior to light treatment and temperature changes had on Grand Rapid lettuce seed germination.
Borthwick and colleagues (4, 5) found that the response of lettuce seeds to light can be modified by the amount of time the seeds are allowed to imbibe water before exposure. Red-light promotion of germination increases with time of exposure up to 10 hours, at which point a plateau reached. However, if the seeds are allowed to imbibe water for more than 20 hours, the germination response falls off in contrast the inhibitory effect of far-red irradiation has a tendency to decrease as previous inhibition time increases up to 10 hours. In keeping with this contrast, the sensitivity of lettuce seeds to fan-ed irradiation increases when the seeds imbibe water for more than 20 hours.
Effects of Temperature on Germination
Photocontrol of seed germination is, in many cases, interrelated with temperature, as evidenced by data in Table 23-1, which shows a decrease in sensitivity to light with an increase in temperature above 25°C (42).
Another, more complex, example of temperature-light interaction may be found in germination of pepper grass seeds (Lepidiam virginicum). Maximum germination is achieved if the seeds are kept at a cool temperature before being irradiated with red light and at a relatively high temperature for a period of time after being irradiated (46). Table 23-2 shows these relationships.
In discussing the various aspects of seed germination, we have seen the importance of temperature in the prolonging or breaking of dormancy. Many seeds need a period of prechilling under moist conditions before adequate germination can take place. In natural and artificial stratification, this requirement is satisfied. After the cold requirement is satisfied, the actual germination, in most situations, takes place efficiently at about 20°C.
In some seeds, the cold requirement is modified by the age of the seed. For example, seeds of Brassica juncea show a definite cold requirement immediately after harvest, and this cold requirement decreases with age (45). Immediately after harvest, 97 percent of the seeds germinate at temperatures of 10° or 15°C, 63 percent at 20°C, and only 8 percent at 25°C. However, after 3 weeks, 95 percent of the seeds germinate at 25°C. The sensitivity of seeds to high temperatures varies greatly. In some, it may persist for a long time in others-for example, Brassica juncea sensitivity is lost in 3 weeks.
In many seeds, an alternation of temperatures gives maximum germination. In some cases, such as with Poa pratensis, alternation of low and high temperatures repeated several times gives the best results. For example, a single alternation from 15° to 25°C in connection with light treatment of pepper grass seeds may significantly increase germination (see Table 23-3).
Low temperatures promote germinastion of the light sensitive Grand Rapids lettuce seeds, and Pugh temperatures.
Low temperatures can Substitute for red-light promotion of light-sensitive lettuce seeds (22). Low-temperature promotion of lettuce seed germination is much less efficient with respect to time than is red-light promotion. We should note from Table 23-3t , that far-red irradiation cannot reverse low temperature stimulation, thus Suggesting that low-temperature promotion of germination is not Controlled by phytochrome (3, 22).