Seed and bud dormancy
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Plant dormancy is a survival mechanism where growth is temporarily suspended. There are two main types - seed dormancy and bud dormancy. Seed dormancy prevents germination during unsuitable conditions and allows for dispersal. It can be caused by hard seed coats or environmental factors. Bud dormancy suspends growth in perennial plants during winter. Hormones like ABA promote dormancy while gibberellins and cytokinins break dormancy. Temperature also influences seed germination and dormancy breaking. Dormancy ensures plant survival during unfavorable periods and promotes species survival.
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Seed and bud dormancy
- 1. Plant Growth and Development Seed and Bud Dormancy Arnold V. Damaso MS Crop Science – Agronomy Central Luzon State University
- 2. • Dormancy is a mechanism to prevent germination during unsuitable ecological conditions, when the probability of seedling survival is low. Dormancy
- 3. • There are two types of dormancy – predictive dormancy – consequential dormancy cont...
- 4. • One important function of most seeds is delayed germination, which allows time for dispersal and prevents germination of all the seeds at the same time. • The staggering of germination safeguards some seeds and seedlings from suffering damage or death from short periods of bad weather or from transient herbivores; it also allows some seeds to germinate when competition from other plants for light and water might be less intense.
- 5. Importance and Significance • 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.
- 6. • In plant physiology, dormancy is a period of arrested plant growth. It is a survival strategy exhibited by many plant species it may be imposed by certain environmental factors or internal factors or genetic factors included.
- 7. • 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
- 9. • Vulnerability and Adaptation
- 10. Embryonic development ending in seed dormancy
- 11. Seed Dormancy • Angiosperms produce seeds within the ovary and ovary itself develops into a fruit. In Gymnosperms only seeds develop. • The development of ovary into fruit and seed go hand in hand.
- 12. Gene expression during dormancy and breaking dormancy
- 13. Role of GA and ABA in antagonistic manner during dormancy and breaking dormancy
- 14. • ABA promotes seed dormancy. ABA levels are high when seeds mature, promoting lowered metabolism and synthesis of proteins needed to withstand the dehydration associated with dormancy. • Seeds germinate when ABA is degraded by some environmental action. Desert seeds must have the ABA washed out of the seed coat; temperate are plants have ABA degraded by light stimulated enzymes.
- 15. Analysis of mRNAs and proteins during different stages of seed dormancy.
- 16. Categories and Types of Dormancy • Exogenous dormancy • Endogenous dormancy – Physical – Physiological – Morphological
- 17. Exogenous dormancy • Exogenous dormancy is caused by conditions outside the embryo and is often broken down into three subgroups: – Physical dormancy – Mechanical dormancy – Chemical dormancy
- 18. Endogenous dormancy • Endogenous dormancy is caused by conditions within the embryo itself, and it is also often broken down into three subgroups: – physiological dormancy, – morphological dormancy and – combined dormancy, (each of these groups may also have subgroups)
- 19. Duration of seed dormancy • The duration of dormancy in seeds varies from species to species and it is species specific.
- 20. 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.
- 21. 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.
- 22. Causes for seed dormancy
- 23. • Prevents water uptake: Some plants produce seeds with hard seed coat which is also waxy. • Prevents oxygen: Some seeds though they are capable of imbibing water, they are incapable . • Prevents the growth of the embryo. In this condition, the seeds may remain for months or years until the said seed coat gets cracked or loosened of taking in atmospheric air.
- 24. • 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. • Chemical Scarification. The hard seed coats can be loosened by strong acids or solvent treatments where the hard coat is rendered soft.
- 25. Temperature Effect • 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
- 26. 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.
- 27. Effect of Light • Besides initiating many other photobiological processes, radiant energy has a profound influence on seed dormancy and germination.
- 28. 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.
- 29. Chemical compounds that break seed dormancy • Plants have unique properties in synthesizing compounds which can induce dormancy as well as break the dormancy. • Gibberellins • Cytokinins • Ethylene • Chlorohydrins • Theourea
- 30. Bud Dormancy • 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.
- 31. How plants prepare for winter?
- 34. Dormant cabbage
- 35. 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.
- 36. Mechanism of Induction of Dormancy in Buds • 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.
- 37. 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.
- 38. • The photoperiodic effect either in breaking the dormancy or induction of dormancy is explained on the basis of phytochrome involvement.
- 39. Summary • Dormancy is the state in which a plant or plant part exhibits little or no growth and in which most, if not all, metabolic activity ceases for a period of time. • The vast majority of plant life functions best when there is ample water and temperatures are well above freezing throughout the year
- 40. • Dormancy evolved as a means of surviving unfavorable environmental conditions. In the temperate zones, buds normally form from spring to midsummer.
- 41. • Seed dormancy can also be caused by a number of different factors. For several reasons, the presence of a hard seed coat will very often result in dormancy of the seed. • Bud dormancy is a suspension of most physiological activity and growth that can be reactivated.
- 42. • Seed germination is also dependent on temperature. • The seeds of almost all species have a minimal temperature below which they will not germinate.
- 43. • Gibberellins, cytokinins, and ethylene, all natural plant hormones, have been shown to be involved in breaking seed dormancy, and the gibberellins and other substances, such as thiourea, are used to germinate seeds commercially.
- 44. End
Hinweis der Redaktion
- predictive dormancy- is due to decrease in temperature and changes in photoperiod. consequential dormancy where plant enters into dormancy after adverse conditions step into. Sudden changes in environment can induce such dormancy.
- This is the Hormonal Regulation of Dormancy Plant Growth Regulator like GA, ABA and Other Hormones (occurred Biosynthesis Transport from Horrmonome) to Adaptation Responses of Dormancy Germination and Stress. However, reverse form a Gene Discovery regulates a physiology based and mutants transcriptome chemicals occured also a dormancy germination and stress.
- Vulnerability and adaptation
- 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 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.
- Left) Seeds of Arabidopsis thaliana Cvi that have lost dormancy (G) are characterized 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.
- 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 signaling (GA catabolism) dominates the dormant state, whereas, GA synthesis and signaling (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 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.
- In other cases breaking dormancy is relative to the ration of ABA (which keeps seeds in dormancy) and gibberellins (which promote germination).
- Often seed dormancy is divided into two major categories based on what part of the seed produces dormancy: exogenous and endogenous and there are three types of dormancy based on their mode of action: physical, physiological and morphological
- Physical dormancy is the result of impermeable layer(s) that develops during maturation and drying of the seed or fruit Mechanical dormancy occurs when seed coats or other coverings are too hard to allow the embryo to expand during germination. In the past this mechanism of dormancy was ascribed to a number of species that have been found to have endogenous factors for their dormancy instead Chemical dormancy - Includes growth regulators etc., that are present in the coverings around the embryo. They may be leached out of the tissues by washing or soaking the seed, or deactivated by other means.
- Physiological dormancy prevents embryo growth and seed germination until chemical changes occur. These chemicals include inhibitors that often retard embryo growth to the point where it is not strong enough to break through the seed coat or other tissues. Physiological Dormancy have a subgroups: Drying (Drying; some plants including a number of grasses and those from seasonally arid regions need a period of drying before they will germinate, the seeds are released but need to have lower moisture content before germination can begin) Photodormancy (Photodormancy or light sensitivity affects germination of some seeds. These photoblastic seeds need a period of darkness or light to germinate) Thermodormancy (Thermodormancy is seed sensitivity to heat or cold. Some seeds including cocklebur and amaranth germinate only at high temperatures (30C or 86F). In Morphological dormancy, the embryo is underdeveloped or undifferentiated. Some seeds have fully differentiated embryos that need to grow more before seed germination, or the embryos are not differentiated into different tissues at the time of fruit ripening. In Combined dormancy Seeds have both morphological and physiological dormancy (Morpho-physiological or morphophysiological dormancy occurs when seeds with underdeveloped embryos, also have physiological components to dormancy)
- 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.
- In spite of favorable 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. According to 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 is invaluable for human beings are viable for 1 to 2 years.
- With time, they can be dispersed to longer distances; still they are capable of germination under favorable 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.
- 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 diffuse in which are the most important factors favorable for germination.
- Explanation (Prevents water uptake) 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. Explanation (prevents oxygen) 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.
- Mechanical Scarification…. This greatly facilitates the emergence of the embryo out of the road coat. Chemical Scarification…. However the duration of treatment has to be determined for every kind of seed, otherwise the treatment may cause damage to the young embryos.
- It is only then the seeds overcome 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.
- 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.
- 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.
- 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.
- 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.
- because of extreme variations in the temperature, especially cold conditions.
- they do it to go through dormancy.
- 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.
- By inhibiting the synthesis of proteins, RNA and other metabolic processes ABA imposes dormancy on meristematic tissues of the plant body.
- 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.
- Dormancy mechanisms are drivers, reasons why seed pools form in the soil (or not); much of the material on dormancy mechanisms overlaps with seed pools and recruitment. Soil seed pools are the inevitable consequence of dormancy mechanisms. Recruitment, seed germination and seedling emergence/establishment are also inevitable consequences of dormancy mechanisms. Dormancy mechanisms, seed pools and recruitment are of a whole. Except for those in moist, tropical regions, however, plants are exposed to dry periods and temperatures below freezing for varying lengths of time during the year. Plants, unlike animals, do not have the luxury of body insulation or locomotion.
- Hence, plants cannot seek shelter or use other active ways to survive water shortages and cold weather. Consequently, many plants become dormant to avoid unfavorable environmental conditions. In dormancy, their metabolic activity either ceases or is drastically reduced.
- In many cases, the seed coat is impermeable to water, Because water is required for the germination process, the impermeable nature of the seed coat will serve as an effective inducer of dormancy. It may be a response to environmental conditions such as seasonality or extreme heat, drought, or cold. The exit from bud dormancy is marked by the resumed growth of the bud.
- The exact mechanism by which low temperature causes dormancy is poorly understood, but it appears that the temperature alters membrane structure, which somehow prevents the seed from germinating.