In this article we will discuss about Seed Dormancy. After reading this article you will learn about: 1. Barrier Effects of Seed Dormancy 2. Inhibitor-Promoter Concept of Seed Dormancy 3. Hormonal Regulations.
Contents
Barrier Effects of Seed Dormancy:
The degree of dormancy of seeds has been associated with hard seed coat. The barrier effect of seed coats can be due to physical and chemical characteristics of the seed coats as well as due to permeability changes to water, gases and solutes.
There are examples of seeds which have remained dormant for a few days to thousands of years without the loss of viability, i.e., ability of germination.
(i) Water Impermeability:
Presence of hard and impervious seed coat is no doubt an important aspect of dormancy because less water is available for germination under this condition. Removal of the seed coat or weakening of the seed coat by various means leads to dormancy release. One mechanical method is scarification in which the seed coat is cracked to permit entry of water.
Under natural conditions in the soil, fungi and bacteria infecting the seeds hydrolyze the seed coat components and thereby soften them so that water can penetrate into the embryo. The testas can be made permeable by treatment for short periods with concentrated sulphuric acid.
(ii) Gas Impermeability:
Impermeability of seed coats to gases, such as oxygen and carbon dioxide is another characteristic of certain seeds showing dormancy.
Hard seed coat is likely to impose restriction of oxygen supply to the embryo, thereby impairing aerobic respiration. Respiration also involves the release of CO2 from the embryo which cannot cross the permeability barrier imposed by the coat and the CO2thus accumulated would further inhibit germination.
Either removal or puncturing of the covering structures which facilitates diffusion of gases is most effective in removing dormancy. It is postulated that dormancy is related to the presence of phenol oxidase in the covering tissues and oxygen which is preferentially consumed by the enzyme there becomes less available to the embryo.
(iii) Mechanical Resistance:
Seed coats may also act as physical barriers, thereby restricting the expansion of the embryo. In such cases, a balance between the expansive force of the embryo and strength of the coats determine dormancy release. The balance may be modified by various seed treatments like light, hormones and oxygen. Mechanical scarification which weakens the seed coat will permit embryo growth.
(iv) Rudimentary Embryos:
Many species of plants have seeds in which the embryo does not develop as rapidly as the surrounding tissue so that when the seeds are shed, the embryo is still imperfectly developed.
In some other species, embryos have grown little beyond the egg stage. Germination of such seeds is usually delayed until the development of the immature embryo is complete. Seeds showing this type of dormancy include Ficaria verna, Caltha palustris and Anemone nemorosa.
The embryos of Fraxinus excelsior are morphologically developed but undergo considerable growth after shedding. A suitable after-ripening period during which the embryo completes its development is necessary for breaking dormancy of such seeds.
(v) Cellular Membranes:
Some other factors have been recognized with seed coats and cellular membranes with respect to differential permeability of solutes along with the differential exchanges of anions and cations. Fusicoccin, a diterpene glucoside, produced by Fusicoccum amygdale has been shown to release dormancy in a number of seeds.
It has been interpreted that fusicoccum activates at the cell-membrane level an energy-dependent proton extrusion mechanism which is subject to hormonal regulation.
According to Marre, the effects of well-known plant hormones like auxin, gibberellin and cytokinin on the dormancy release process is mediated by the same mechanisms involved in fusicoccin action. Marreet al., proposed a model to explain the action of fusicoccin and plant hormones in removing some kind of factors impeding cell elongation.
Inhibitor-Promoter Concept of Seed Dormancy:
Of the growth hormones, only GA is known to consistently enhance seed germination and is positively implicated in many seed processes.
Until the middle 60s, only GA and inhibitors were held responsible for the control of seed germination and dormancy. It was later discovered that cytokinins oppose the action of germination inhibitors like coumarin and ABA and they have the ability to allow gibberellins to function.
Such cytokinin-inhibitor antagonism plays a prominent role in germination process particularly under those conditions in which germination is blocked by the accumulation of inhibitors whereby GA alone is not capable to reverse the effects of the inhibitor and simultaneous application of both GA and cytokinin can break dormancy and promote germination.
Thus, dormancy release or germination is the result of a cumulative action of several hormones and not always the consequence of increase or decrease of promoters and inhibitors. This finding led Khan (1975) to propose the hypothesis that GA, inhibitors and cytokinins may have primary, preventive and permissive roles in seed germination.
Based on this hypothesis, a model for the control of dormancy and germination has been proposed in several possible ways by which the hormones exert their effects. Eight hormonal situations have been envisaged according to the presence or absence of gibberellin, inhibitor and cytokinin (Fig. 14.10).
Gibberellins have primary roles in bringing about germination while the roles of inhibitors and cytokinins are secondary-preventive and permissive, respectively.
(i) Light Requirement:
In numerous seeds, light is required in the release of dormancy. These seeds are termed photoblastic as opposed to the non-photoblastic seeds which do not require light for dormancy release. Early studies conducted by Flint and McAllister established that shorter wavelengths (630-680 nm) promote germination in light-sensitive seeds whereas longer wavelengths (730-750 nm) prove to be inhibitory.
Investigating the influence of light on lettuce seeds, Borthwick et al. (1952), first described the involvement of the photo-reversible pigment phytochrome in the regulation of germination. Red (R) promotes and far-red (FR) retards the germination of light-requiring lettuce seeds. Basically the phytochrome function in seeds is analogous to its function in other aspects of light-mediated phenomena.
In lettuce seeds, Pfr appears to be promotive and Pr to be suppressive, while in light-inhibited seeds the reverse may be true. The role of phytochrome in photoblastic seeds may be compared to that in floral induction in long-day plants, whereas light-inhibited seeds can be compared to short-day plants.
In order to explain the action of red light, Smith (1970), proposed one model which recognizes that phytochrome action is similar to a specific carrier for an important metabolite (Fig. 14. 11).
(ii) Temperature-Sensitive Seeds:
A range of temperatures exists for all viable seeds over which they will germinate. There is a minimum temperature below which no germination occurs. Similarly, seeds will not germinate at temperatures above the maximum.
The temperature range for germination can be widened through treatment during early imbibitional phase. Seed lots hardly capable of germination at 5°C are enabled to do so by priming in an osmotic solution at higher temperature before being transferred to 5°C.
With many seeds, alternating or fluctuating temperatures are beneficial in increasing the rate of germination. It has been noted that alternation of a sub-optimal with a lethally high temperature can promote germination. For the study of temperature stress, lettuce (Lactuca sativa) is a popular experimental material. Regarding its response to temperature stress, lettuce seeds present an interesting situation.
Although short periods at high temperature can accelerate germination, prolonged high- temperature periods induce light requirement which would not be noticed at lower temperatures. A Seed lot capable of germination at 25°C in the dark can be made incapable of germination by exposure to 30°C in the dark for seven days.
It now either requires light at 25°C or a lower temperature (20°C) in the dark in order to be able to germinate, which indicates that both light and cold sensitivity can be induced by high temperature. Exposure of lettuce seeds to 30°C for prolonged period, however, imposes thermo-dormancy and make them completely non-germinable.
In many seeds, dormancy is broken by exposing the moist seeds to low temperature, between 0°C and 10°C for several days. This is termed stratification and can be compared to vernalization with regard to floral induction. Working on the stratification behaviour of wild oat (Avena fatua), birch (Betula pubescens) and ash (Fraxinus sp.).
Crocker and Barton (1953) noted that while kept at stratifying temperatures, the seed may remain quiescent, and germination can only take place when the seed is exposed to higher temperatures. Stratification is not possible at temperatures below freezing point.
Thus, under the influence of cold stratification, the enzyme molecules are activated which under favourable conditions are responsible for the establishment of hormonal balance required for germination. On the contrary, an increase in temperature during stratification results in inactivation of the enzymes and thus brings about a reversion to dormancy.
(iii) Shifts in Metabolic Pathways:
Roberts (1973), proposed a hypothesis in which the pentose phosphate pathway plays a central role as the major oxidative process in regulating seed dormancy release and germination. It was postulated that the loss of dormancy was due to some oxidative reaction and the conventional respiration may not be the only oxidative pathway regulating seed germination.
Based on his studies with respiratory inhibitors on rice (Oryza sativa) and other seeds, Roberts (1969) found that all the common inhibitors of cytochrome oxidase (potassium cyanide, sodium azide, carbon monoxide, hydrogen sulphide and hydroxylamine) and other respiratory inhibitors, viz., sodium fluoride, iodoacetate, malonate and fluoroacetate produce significant increases in the germination of dormant seeds.
These inhibitors can inhibit the conventional respiration but do not inhibit the oxidation of glucose-6-phosphate or the activity of the dehydrogenases of the PP pathway. It was further noted that hydrogen acceptors other than oxygen like nitrate, nitrite and methylene blue which break dormancy also stimulate PP pathway.
Thus, the operation of the PP pathway is essential to the release of dormancy.
The rate-limiting step of the pathway is the re-oxidation of NADPH which is the coenzyme of two dehydrogenation steps of this pathway (glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase). The re-oxidation of this coenzyme will not affect conventional respiration (e.g., glycolysis), since the major coenzyme here is NAD.
Thus, any reaction which is coupled with the re-oxidation of NADPH would be expected to remove dormancy. The reason why both nitrate and nitrite stimulate the activity of the PP pathway is due to the fact that nitrate is always reduced to nitrite and nitrite is reduced still further by reduction specifically with NADPH.
Thus, the latter reaction would ensure the re-oxidation of NADPH making NADP available for oxidation by the PP pathway dehydrogenases.
Many chemicals containing sulphydryl groups such as thiourea and mercaptans have been reported to break dormancy. Here these compounds are reported to act as reducing agents preventing the formation of disuphide linkages and the resulting thiol/disulphide balance is reported to influence enzyme activities including those of the PP pathway.
Hendricks and Taylorson (1975) postulated a scheme of oxidative pathway (Fig. 14.12) in the release of dormancy by ensuring the re-oxidation of NADPH. This scheme suggests that the dormancy-breaking activity of nitrite, azide and hydroxylamine is due to their inhibitory effects on catalase.
They suggested that the source of H2O2 might be due to the glyoxysomal enzymes during the conversion of fat to carbohydrate. Thus, through the inhibition of catalase, H2O2 is spared for peroxidase action involved in the NADPH oxidation system. They also suggest that red light stimulating the release of dormancy in light-sensitive seeds activates NAD kinase, thus providing NADP for use in the PP pathway.
Several studies have indicated that the glyoxyiate cycle is possibly involved in the dormancy release mechanism. In the seeds of Corylus avellana which require GA, for releasing dormancy, an enormous increase in isocitrate lyase, a key enzyme of the glyoxyiate cycle, is noted. Again, increased sucrose production from fats in hazel cotyledons might be a result of glyoxyiate activation.
Thus in these seeds, a shift in metabolism from a catabolic to an anabolic one is likely to be associated with dormancy release.
The proposal that dormancy breaking is associated with the stimulation of the PP pathway is further supported experimentally by the application of glucose-6-14C and glucose -1 –14C to separate samples of the same tissue followed by collection and estimation of evolved 14CO2 from each, and the results are expressed as C6/C1 ratio.
This experiment depends upon the fact that during glycolysis, a glucose molecule is split into two 3-C units and both units are then decarboxylated in an identical fashion in the citric acid cycle. Thus, if both forms of radioactive glucose are respired solely via glycolysis, then the C6/C1 ratio will be unity.
On the other hand, if the pentose phosphate pathway is operative, the carbon in position 1 of glucose is removed as CO1 by the activity of 6-phosphogluconate dehydrogenase during the conversion of 6-phosphogluconate to ribulose-5-phosphate, whereas C6 is not removed.
Hence the operation of the PP pathway in a tissue reduces the C6/C1 ratio due to a greater release of 14CO2 from glucose – 1 – 14C. Roberts (1969), compared the C6/C1 ratios of dormant and non-dormant barley grains following imbibition. A lower ratio (about 0.1 4) indicating PP pathway is found in non-dormant grains prior to germination, whereas the ratio in dormant seeds is 0.44, i.e., about three times the other value.
Hormonal Regulations During Seed Dormancy:
(i) Gibberellins:
The germination of both dormant and non-dormant seeds has been shown to be stimulated by applied GAs. The stimulatory effect of GAs has been widely reported in seeds where dormancy or quiescence is imposed by different mechanisms like incomplete embryo development, mechanically resistant seed coats and presence of germination inhibitors.
Based on these observations, Amen postulated that GAs play a universal role in seed germination. Endogenous GA levels have been found to undergo change in relation to dormancy breaking conditions.
There are reports which show that low temperature treatment of dormant seeds lead to increased level of GA-like substances and germination. Dormancy induced in hazel (Corylus avellana) seeds by dry storage can be overcome either by GA or by cold treatment or stratification.
From the observation that GA content increases when the hazel seeds are transferred from the stratification temperature (5°C) to the germination temperature (30°C), Williams et al., postulated that the dormancy-breaking effect of chilling is to activate the GA-producing mechanism, whereas the subsequent synthesis takes place at higher germination temperature.
It is generally observed that the cereal seeds are not able to germinate after harvest and storage for 1 -3 months at room temperature is necessary to allow maximum germination.
Thus, dormancy in these seeds can be removed by dry storage after-ripening period during which the embryos acquire the capacity to produce GA. A study on GA synthesis by germinating barley by Radley suggests that the synthesis of GA by the embryo scutellum is regulated by the levels of sugars present in the endosperm.
He proposed that the inhibition of GA synthesis by the accumulation of sugars could serve an important regulatory role in cereal seed germination. Premature germination may be prevented by sugars present in the ripened grains whereas depletion of endosperm sugars during dry after-ripening would permit the production of GA and subsequent germination.
In the germination of photo dormant seeds, light influences GA metabolism. Both positively photoblastic seeds (e.g., Lactuca sativa) and negatively photoblastic seeds (e.g., Phacelia tenacetifolia) germinate under non-inductive conditions when treated with GA. It was shown that red light illumination of lettuce seeds could increase GA-like substances.
(ii) Cytokinins:
It was discovered that the application of exogenous cytokinins can counteract the blockage of GA-induced germination or enzymatic processes by naturally-occurring germination inhibitors. This would suggest that the hormonal control of seed dormancy and germination involves a balance between stimulatory and inhibitory compounds within the seed.
Based on this concept, Khan proposed a model in which the gibberellins assume the primary role in germination, whereas cytokinins and inhibitors are essentially permissive and preventive, respectively. Burrows (1975) has suggested three major sites at which cytokinins are involved in seed germination.
These are:
(a) Control at the gene level—a cytokinin-receptor protein has been isolated from pea bud chromatin and there is a direct interaction between cytokinins, its receptor protein and chromatin. Binding of cytokinins to ribosomes has also been confirmed,
(b) Control at the translational level—presence of cytokinin in different amino acid specific tRNA suggests that there is a specific relationship between cytokinins and tRNA and they have action on protein synthesis. Cytokinin- binding sites on higher plant ribosomes may constitute the control mechanism for determining which tRNA species are permitted access to the codon.
(c) Regulation of membrane permeability—cytokinins affect the permeability of cell membranes in a wide variety of plant tissue including seed. This is supported by the fact that cytokinins can influence many phytochrome-controlled processes and that phytochrome is involved in altering the selective permeability of cell membrane,
(iv) Regulation of GA levels—there is considerable evidence that red light stimulates the release of GA from etiolated chloroplasts and it is suggested that the primary effect is increased permeability of etioplast membrane.
Since cytokinin effect is similar to red-light effect, it seems possible that they can control membrane permeability and release of GAs. This hypothesis can explain as to why the low levels of GAs which are normally without effect on seed germination become effective when combined with cytokinins. The cytokinin-inhibitor antagonism model proposed by Khan (1975) is quite consistent with this hypothesis.
(iii) Abscisic Acid:
Several reports indicate that endogenous ABA levels drop in seeds which are subjected to a dormancy-breaking treatment like cold stratification and germination can also be inhibited by exogenous ABA.
The addition of ABA specifically inhibits the synthesis of certain enzymes which play a key role in germination. One of the earliest reports of the effect of ABA is its inhibition of GA-induced hydrolase synthesis in barley aleurone layers. Inhibition of hydrolase synthesis by ABA appears to be at the level of translation rather than transcription.
Ho and Varner (1976), have postulated that ABA might de-repress a regulator gene or interact with a regulator RNA or protein to inhibit the translation of a-amylase RNA. It appears that ABA inhibits the translation of specific mRNA species.
(iv) Ethylene:
Seeds have been shown to produce ethylene during germination. In castor bean, three peaks of ethylene production have been detected and those ethylene maxima coincide with the rapid growth phases of the seedlings. Non-germinating seeds, on the other hand, produce low levels of ethylene. Ethylene production pattern indicates the emergence of hypocotyl-radicle and subsequent growth of these organs.
It was noticed that any substance which broke seed dormancy also stimulated ethylene production, generally reaching the peak value within 24 hours of germination.
Germination rate of seeds is enhanced by exogenous ethylene treatment. In cocklebur promoters like gibberellin and cytokinin may influence ethylene production by seeds, as a means to stimulate germination.
(v) Bud Dormancy:
The seeds are not the only organs which show the failure to germinate even under environmental conditions favourable for growth. The meristematic regions such as buds of woody or herbaceous plants also show dormancy and fail to grow. The winter temperatures prove to be lethal to the temperate herbaceous and woody perennials.
Most woody plants have developed for their buds a dormancy mechanism which helps them to survive winter cold. In the annual growth cycle of the temperate woody plants, the bud break usually takes place in spring.
Dormancy of buds may be defined as cessation of growth showing a rest condition. These buds in early spring, however, do not show any physiological dormancy and only the warm temperature of spring is needed to activate the buds into growth. In the early stages of growth, the buds do not have photosynthetic activity and thus depend on stored reserves.
Ultimately, the young leaves are formed in buds which attain photosynthetic capacity.
It is generally observed that the terminal growth of many woody plants of temperate zone ceases with the onset of cold weather. At some point after the cessation of shoot growth, physiological dormancy, also called rest, develops in the shoot.
This type of dormancy induced by rest condition is quite different from the dormancy under apical dominance displayed by-axillary buds which are prevented from growing by the presence of shoot tip.
If the rest condition of the shoot is largely advanced, removal of shoot tips will fail to release apical dominance and axillary bud burst does not occur. The intensity of bud rest has been shown to vary seasonally.
Hatch and Walker (1968) have shown that the rest intensity in peach increases from time when it can be first detached in September to a maximum in November, it then declines to a less pronounced rest by late December.
The rest period of several buds can be eliminated by chilling temperatures with an optimum about 5-7o C. Temperatures near 0°C seem to be too cold for physiological chilling purpose, whereas temperatures above 10°C appear to be too warm for the chilling process.
The amount of chilling required to fulfil the chilling requirement varies widely in different species which need either low or high chilling treatment.
But once the chilling requirement is satisfied, growth starts again by breaking of dormancy. Since the chilling requirement is generally met in winter with the return of warmer temperature of spring, bud burst and shoot elongation can start once again and the annual shoot growth cycle begins.
Some plants with very high chilling requirements are characterized by late growth and blooming. These plants receive part of their chilling in autumn and early winter. They have been shown to receive an additional period of chilling till late winter and early spring, and are able to grow later in spring resulting in late blooming.
Such delayed growth in spring is possibly due to the variation in the ability of some cultivars to resume growth at lower temperatures than other species. For example Populustremeloides and Betula papyrifera are trees capable of initiating growth when daily minimum low temperatures are below 0°C, while Acerrubrum and Fraxinus nigra have much higher minimum temperature requirements.
In studies on the site of rest within the bud it has been shown that bud scales have some influence on bud dormancy but it may not be the only factor Swartz et al, have demonstrated that the removal of the bud scales may permit bud growth when rest is not strongly developed, but scale removal has little influence when the buds are in profound rest.
Thus, it can be indicated that the rest influence possibly resides in the meristematic part of the bud.
Hormonal relationships of bud dormancy in woody plants have been established whereby hormones have been shown to play a major role in regulating dormancy.
Of the different naturally-occurring plant hormones, ABA, gibberellins and cytokinins are positively implicated in the control of dormancy whereas IAA or ethylene has not been shown to participate in a direct manner. Hemberg (1949), first enunciated the hypothesis that specific inhibitory substances play an essential role in bud dormancy.
Subsequently, it was found that endogenous gibberellins and cytokinins overcome dormancy in a range of woody species and in organs such as potato tubers. It was further postulated that bud dormancy may be controlled by a balance between endogenous inhibitors such as ABA, and the growth-promoting hormones, especially the GAs.
The formation of resting buds and the onset of dormancy is promoted by short-days in many woody species, it seems reasonable to suggest that the cessation of growth and formation of resting buds are due to an inhibitory factor produced by the leaves under SD condition which is transmitted to the apex Philips and Wareing (1958), followed seasonal changes in the growth substance content of buds and leaves of Acer pseudoplatanus and noted changes in growth-promoting and growth- inhibiting activity correlated with the onset of bud dormancy in trees growing under natural conditions with shoot apices of Populus tremula cuttings, Eliasson (1969), observed a marked SD-induced reduction in GA-like activity.
A balance between promoter and inhibitor levels is an important factor in the control of bud dormancy. Exogenous applications of gibberellins and cytokinins are well known to be effective in overcoming dormancy of tree buds but the effect has not been confirmed in all plants studied so far.
On the other hand, the formation of resting buds by the application of ABA has been achieved in several woody species like Betula pubescens, Acer pseudoplatanus and Ribes nigrum.
Experiments in which ABA and GA were applied together to tree buds indicate that the growth-inhibitory action of ABA can be largely overcome by increasing concentrations of GA. Thus, experiments with externally applied ABA, GA, and kinetin give some support to the hypothesis that bud dormancy may be regulated by an interaction between these endogenous hormones.
More recently, ABA level has been measured in buds of several species from the onset of rest in autumn through the breaking of rest in late winter and early spring. In some cases, positive correlations between ABA contents and intensity of dormancy have been established, but in others the relationship is less certain.
Based on the present state of knowledge. Powell has concluded that ABA in buds of many woody plants declines during the cold winter months, although it is not certain whether cold temperatures are responsible for this decline.
It is also well established that growth-promoting hormones tend to fall to low levels in shoots late in the growing season. He has argued then, that dormancy may be due to insufficient growth hormones and not due to an inhibitor.
But the failure of growth-promoting substances to promote growth when rest type of dormancy has already started suggests that dormancy is due to the gradual increase of some kind of an inhibitory influence, which is either ABA or some other agent.