PHYSIOLOGY OF DORMANCY
During the developmental cycle of the plant, at some phase or the other certain structures like buds, tubers, seeds, etc., go through a period of temporary suspension of growth activity. Such a state is called dormancy. It may be imposed by certain environmental factors or internal factors or genetic factors included. Generally plants or plant structures, in order to overcome or survive against hostile environmental conditions undergo a period of dormancy with suitable modifications. In lower plants production of endospores, zygospores, auxospores, akinetes, etc, are some of the methods involved in tiding over unfavorable conditions. Even vascular plants with their complex structural organization and reproductive methods produce dormant structures to overcome unfavorable conditions. Some of the dormant structures that develop in plants greatly facilitate in the dispersal mechanism. Among the many structures which exhibit dormancy, seeds and buds are important.
Perennial plants like shrubs, trees have to go through different seasons in a year. The onset of winter is always an unfavorable season for the growth and even survival of plants become difficult because of extreme variations in the temperature, especially cold conditions. In order to survive against such hostile conditions the growth regions like apical buds, axillary buds and underground structures like rhizomes, tubers, etc, undergo a period of dormancy “suspended growth”. It is during this season that leaves wither. In fact, it is not the change in the temperature that acts as the signal but it is the change in the photoperiodic conditions which triggers the falling of leaves.
Aesculus hippocastanum-floral dormancy
Carlina sunshine-floral dormancy
Dormant leaves of Adenium socotranum
Gymnocalycium badldianum buds
Protea dormant floral structures
It is again during the same season, meristematic cells found in the buds undergo temporary suspension of their activities. In addition to it, the meristematic regions will be armed and protected with a number of leaves called bud scales which are thick, waxy and covered with a dense coat of hairs. Such structures provide thermal insulation to the meristematic zones and prevent them from cold injury or frost bite.
Site of Perception:
Though leaves perceive changes in photoperiodic effects of the day, it is the buds that act as the sites of perception for inducing dormancy. The induction of dormancy in buds starts only after the falling of leaves. Nonetheless, in many plants even old leaves act as the sites of perception in inducing bud dormancy. Such buds can be induced to break the dormancy by subjecting the same to long photoperiodic treatment or interrupting the long dark periods by red light.
Mechanism of Induction of Dormancy in Buds:
The onset of short day or long dark photoperiods in winter stimulates the synthesis of various growth inhibiting compounds of which Abscissin dominates. Quantitative estimation by solvent extraction methods reveal that dormant buds contain greater amounts of ABA than actively growing buds. Abscissin is a well known growth inhibiting hormone. By inhibiting the synthesis of proteins, RNA and other metabolic processes (See ABA chapter), ABA imposes dormancy on meristematic tissues of the plant body.
The involvement of photoperiodic effect on bud dormancy indicates that the phytochromes have a role in imposing dormancy. Phytochromes are known to be present in plastid membranes and other surface membranes. It is also known that ABA is synthesized and often stored in plastids. Phytochromes by perceiving the changes in photoperiodic conditions probably induce the synthesis of ABA and also facilitate the release of ABA. Then ABA is translocated to the buds where it brings about temporary suspension of metabolic activities and totally inhibits mitotic activity. The presence of Abscissin II, which is also called ‘Dormin’ has been detected from various plant structures involved in dormancy ex., dormant potato tubers, birch leaves, winter buds in clerodendron, pinus, etc.
Breaking Bud Dormancy:
The dormant buds can be induced to sprout again by treating with cytokinins and gibberellins. But in natural course, the onset of spring and long photoperiods, the dormant buds become active and develop into branches.
Cytokinins are known to be synthesized in root tips but under cold conditions because of the snow fall, the root meristems are very inactive and they don’t synthesize sufficient quantities of cytokinin required for the buds to be active. That is probably one of the reasons why buds remain dormant. As soon as cytokinins are provided to dormant buds, mitotic activity is initiated and buds start sprouting. Besides, cytokinins also counteract ABAs inhibitory effect of the metabolic activity level and promote growth activity.
Another class of phytohormones, which over comes the bud dormancy is Gibberellins. Now it is certain that Gibberellin synthesis takes place in plastids. Moreover, the synthesis of GA and ABA starts from the same precursor called mevolonate. Under short day conditions, the pathway from mevolonate is directed towards ABA synthesis and GA synthesis is inhibited, but during long day photoperiods it is directed towards GA synthesis and ABA synthesis is blocked. That is the reason why gibberellins under long day conditions or not light treatment, break bud dormancy and nullify the effect of ABA present in such dormant buds.
The photoperiodic effect either in breaking the dormancy or induction of dormancy is explained on the basis of phytochrome involvement. When long day conditions prevail, more amount of PfR form of phytochrome accumulates in the cells which initiates not only the synthesis of more GA and but also it facilitates the release of it from the plastids into the cytoplasm. Once GA is released, it brings about the activation of dormant cells, and thus GA breaks bud dormancy. It is really very fascinating to understand the three way interaction between phytochrome, photoperiod and GA/ABA synthesis in imposing bud dormancy or breaking it.
Angiosperms produce seeds within the ovary and ovary itself develops into a fruit. In Gymnosperms only seeds develop, because the ovule is naked and it is not enclosed by any ovary wall as in the case of angiosperms. During the development of a seed or the fruit, some remarkable changes occur in the ovule as well as in the ovary wall. The development of ovary into fruit and seed go hand in hand.
Sometimes, one develops faster than the other, but ultimately the slower one catches up with the other at the time where fruit is ready to be shed.
Gene expression during dormancy and breaking dormancy
Left) Seeds of Arabidopsis thaliana Cvi that have lost dormancy (G) are characterised by low expression of two dormancy genes (ATS2, ATS4) and high expression of two germination genes (AtRPL36B, AtRPL27B). The two dormancy genes (ATS2, ATS4) display high expression in dormant seeds (D) that do not complete germination. (Contact: Dr Peter Toorop
Role of GA and ABA in antagonistic manner during dormancy and breaking dormancy
Model for the regulation of dormancy and germination by ABA and GA in response to the environment. According to this model ambient environmental factors (e.g. temperature) affect the ABA/GA balance and the sensitivity to these hormones. ABA synthesis and signalling (GA catabolism) dominates the dormant state, whereas, GA synthesis and signalling ( ABA catabolism) dominates the transition to germination. The complex interplay between hormone synthesis, degradation and sensitivities in response to ambient environmental conditions can result in dormancy cycling. Change in the depth of dormancy alters the requirements for germination (sensitivity to the germination environment); when these overlap with changing ambient conditions, germination will proceed to completion. Model based on work with A. thaliana ecotype Cvi, modified from Cadman et al. (2006). Key target genes are in parenthesis.
Analysis of mRNAs and proteins during different stages of seed dormancy.
Almost every angiosperm produces seeds (with a few exceptions) and the same are dispersed when the fruit dehises or decays. A large number of seeds germinate on shedding if conditions are favourable, but there are some which germinate even before the fruit is shed. They are called viviparous seeds. But some do not germinate till they pass through certain period of time. Even favourable conditions they do not germinate. Such seeds are called dormant seeds. The time at which they shed and the time at which they germinate is called dormant period.
Duration of seed dormancy:
The duration of dormancy in seeds varies from species to species and it is species specific. It has been noted that the duration of dormancy varies from few months to many years. The controlling factors that impose dormancy are many ex., the strength or the susceptibility of seed coat, ability of the seed to absorb and utilize water or oxygen, presence of inhibitors and environmental factors like temperature, water, day length, etc.
Viability of Seeds:
The period for which the embryo remains healthy within the seed coat and capable of germination under permissible conditions is referred to as seed viability. In spite of favourable conditions, seeds fail to germinate beyond the viable period. In fact, the period of viability refers to the longevity of the embryo. The half life of seeds vary from species to species, where some have few days or months and some remain viable for a period as long as 100 to 1000 years. The conditions at which the seeds are preserved are also important. Horace Wester (1971) reported that the lotus seeds obtained at an excavation point were as old as 900-1200 years, still they were viable. Majority of crop plants that are invaluable for human beings are viable for 1 to 2 years.
Seed dormancy and its advantages and disadvantages:
Dormancy has its own advantages as well as disadvantages for seeds. Seeds with a longer period of dormancy and viability are capable of surviving the worst hazards of environmental conditions. With time, they can be dispersed to longer distances; still they are capable of germination under favourable conditions and propagate their population in distant areas. Such seeds have the capacity to survive all the hazards of environmental factors and still survive. Unfortunately, some the plant seeds which have such survival capacity are weeds and they create nuisance to human beings and there is no way to destroy them. However, seeds with longer period of dormancy will greatly help in storing the food grains or seeds.
Causes for seed dormancy:
Many seeds are incapable of germinating immediately because of the hard seed coat which is thought to break open by the developing embryo. It often does not imbibe water; even O2 does not difuse in which are the most important factors favourable for germination. Thus the seed coat, though it acts as a protective structure often imposes dormancy in many seeds in the following ways. It is a necessary evil.
(1) Prevents water uptake: Some plants produce seeds with hard seed coat which is also waxy. Thus the seeds are rendered impermeable to water. For example, in some Fabaceae members like is Pinus arborous, water first enters through hilum which is made of hygroscopic tissue. On coming contact with water, the tissue swells and closes the micropyle. Thus it prevents the entry of water into the seeds.
(2) Prevents oxygen: Some seeds though they are capable of imbibing water, they are incapable of taking in atmospheric air. In xanthium every fruit contains two seeds placed one above the other. Curiously, the lower one germinates under normal conditions, but the upper one does not until and unless, it is subjected to high oxygen pressure. This behavior has been attributed to the presence of an inhibitor which will be oxidized only under higher concentration of oxygen. Otherwise it inhibits the growth of the embryo.
(3) Prevents the growth of the embryo. Amaranthus and some other seeds belonging to this category are capable of absorbing water and oxygen, with this the embryo is capable of growing, but unfortunately it cannot break open the hard seed coat, thus the germination of seeds is prevented. In this condition, the seeds may remain for months or years until the said seed coat gets cracked or loosened.
In all the above mentioned cases the embryos are normal and do not posses any growth inhibiting factors. But its growth is prevented by the presence of a hard seed coat which either prevents the entry of water or oxygen or the seed coat does not crack and prevents the emergence of growing embryo. The only mechanism by which such seeds can be induced to germinate is to break the seed coat and make it weak so that the seeds can take up water and oxygen and facilitate the embryo to emerge out of the seed coat. This can be achieved by a process called scarification which can be done by following methods.
1. Mechanical Scarification: Shaking the seed with abrasives or nicking the seed coat with sharp edged metals or chewing the seed coats without damaging the embryo makes the seed coat to crack open. This greatly facilitates the emergence of the embryo out of the road coat.
2. Chemical Scarification: The hard seed coats can be loosened by strong acids or solvent treatments where the hard coat is rendered soft. However the duration of treatment has to be determined for every kind of seed, otherwise the treatment may cause damage to the young embryos.
Immature Seeds and after ripening effects:
Many plants shed their seeds before the embroyos are fully mature. In such cases, germination fails to occur till the embryos reach a maturity state. Maturation of such seeds can be achieved by storing them under favorably conditions. But some seeds do not germinate even under such favorable conditions. For that the storing has to be prolonged for some more time, only then the seeds germinate. This behavior is often called ‘after ripening’ effect. This behavior has been explained as due to certain changes in its metabolic activities during storage. The maturation of such seeds, noetheless can be augmented or accelerated by keeping layers of seeds alternating with moist sand or moss at low temperatures. But the mechanism by which layering accelerates the maturation of embryos is not known. But one probable explanation is that by keeping the seeds under moist conditions many inhibitors are supposed to leach out.
A large number of plants produce seeds which germinate under normal temperatures. But some do not germinate if they are stored at room temperatures. They require chilling treatment for a period of time. It is only then the seeds over come dormancy and germinate. Probably cold treatment destroys the inhibitors present in the seed coat or in the embryo. This effect is almost like vernalization, but the mechanism is different about which we don’t know much.
Effect of Light:
Besides initiating many other photobiological processes, radiant energy has a profound influence on seed dormancy and germination. Among the innumerable species of plant, some are in sensitive to light radiations. Based on the above said property seeds have been classified into three kinds, i.e. positive photoblastic types, negative photoblastic types and non photoblastic types.
In the case of positive photoblastic types when seeds are exposed to one or two cycles of intense source of white light, they germinate. On the other hand, the negative photoblastic varieties do not respond to white light treatment, but they germinate if they are maintained in complete dark conditions. The non photoblastic types are insensitive to light and they germinate irrespective of the presence or absence of light.
The effective spectrum of the visible light that induces dormancy or germination in light sensitive seeds is red light and far red light. The seeds of Grand Rapids and Pepper grass germinate promptly if they are exposed to red light. On the contrary, far red light inhibits their germination and seeds remain dormant. However the red and far red light effects on seed germination can be reversed. The table given below indicates that with the increase in the number of alternate cycles of red and far red light treatments ending with red light, the percentage of seed germination enhances significantly. But the treatment with far red as the last of the cycle, the percentage of inhibition is more or less 90-93%.
The effectiveness of red light and far red light in inducing or inhibiting the germination further suggests the involvement of phytochromes in this process. In fact, Ikuma and Thimann, way back, demonstrated the role of phytochromes in inducing seed dormancy or germination. However, the effectiveness of phytochromes depends upon other factors like water, temperature, pH, and age of seeds, duration of light treatment and other chemicals present in seeds.
Phytochromes, as described elsewhere, exist in two forms which are interconvertible in the presence of red and far red lights (PR and PfR).
The PfR produced under red light conditions undergoes reverse reaction in dark to produce PR form of the pigment. However, in some cases it has been found that the PR form is converted to PfR form by specific enzyme medicated reactions which require the input of energy. Furthermore, it is also known that PfR requires another unknown factor called X to be in active state. (PfR-X).
The PfR-X form of pigment complex is believed to be very effective in breaking the dormancy in red light requiring positive photoblastic seeds. The only difference between the positive photoblastic seeds and negative photoblastic seeds is that the former requires higher concentration of PfR –X complex than the latter one; where as the negative photoblastic seeds require a minimum amount of PfR complex to be highly effective. If the concentration of PfR is more in negative photoblastic seeds their do not germinate. Correspondingly, the non-photoblastic varieties germinate without light treatment, none the less PfR has a promotive effect. In spite of all these studies, the exact mechanism by which the phytochrome pigments bring about dormancy or break the dormancy is not clear.
Inductions of Dormancy by Chemicals:
Search for substances that induce dormancy in seeds resulted in the discovery of a host of compounds like Coumerin, Para ascorbic acid, Hydrogen cyanide, Abscisic acid, etc. Most of them have been isolated from the seed coats, endosperm pulp and the juice of fruits of embryos. In those species which exhibits dormancy during the maturation of seeds and fruits some of the above said compounds are actively synthesized and stored in different parts of seeds and fruits.
The above said substances are non toxic but induce dormancy. Until and unless the said compounds are either destroyed or leached out, seeds remain dormant.
In recent years, the effect of ABA on seed drying and inducing dormancy in seeds has attracted a lot of attention. In many fleshy fruits the ripening process is also initiated by ethylene. But ethylene inturn can induce the synthesis of ABA. The abscissins thus produced bring about the induction of few enzymes which not only facilitate fruit ripening but also induce changes in membrane permeability and further they influences transcriptional activity as well as translational activity. Such changes ultimately slow down most of the metabolic activities in the embryo. At the same time, the water content of the embryonic cells is greatly reduced, resulting in the suspension of all metabolic activities of the embryo, thus seeds become dormant. The degree of dormancy depends upon the viable concentration of ABA stored in seeds.
Chemical compounds that break seed dormancy:
Plants have unique properties in synthesizing compounds which can induce dormancy as well as break the dormancy. Of course the synthesis of such compounds takes place at different environmental conditions. Some of the compounds produced by plants are as effective as light and temperature in breaking the seed dormancy. The compounds that are known to overcome dormancy are gibberellins, cytokinins, ethylene, chlorohydrins, theourea, etc.
Interplay of GA and ABA is seed Dormancy:
The action of Gibberellins is breaking seed dormancy is interesting because they are very effective on seeds that require light treatment for germination. It is now known that both GA and ABA are synthesized in the same plastids and their synthetic pathway starts from the common precursor called Mevalonate. Furthermore, it can be demonstrated how the synthesis of GA and ABA is correlated to red light and far red light mediated phytochrome activity
In red light requiring seeds on exposure to the red light more of PfR pigments accumulate. Once the concentration of these pigments reaches a threshold value, they probably initiate or favor the pathway of GA synthesis and also they facilitate the release of GA. The release of GA inturn activates the respiratory activity and metabolic activities and activate certain genes (via transcription and translation) resulting in the production of various components required for the active growth and development of embryo. Thus GA overcomes the dormancy and induces germination. On the contrary, short day or far red induced dormancy is due to the accumulation of more of PR form of pigments. These pigments in fact favor the pathway of synthesis and release of ABA from plastids. Then ABA brings about the inhibitory effect on cellular metabolism and imposes seed dormancy. It is also speculated that each of these components produced in response to different light treatments can’t bring about feed back inhibition on each other’s pathway. The interplay of GA and ABA in response to different light periods, either in breaking the dormancy or in imposing the dormancy, is very interesting.