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From Wikipedia

Seed dormancy

Seed dormancy is a condition of plant seeds that prevents germinating under optimal environmental conditions. Living, non dormant seeds germinate when soil temperatures and moisture conditions are suited for cellular processes and division; dormant seeds do not.

One important function of most seeds is delayed germination, which allows time for dispersal and prevents germination of all the seeds at 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. Another form of delayed seed germination is seed quiescence, which is different than true seed dormancy and occurs when a seed fails to germinate because the external environmental conditions are too dry or warm or cold for germination. Many species of plants have seeds that delay germination for many months or years, and some seeds can remain in the soil seed bank for more than 50 years before germination. Some seeds have a very long viability period, and the oldest documented germinating seed was nearly 2000 years old based on radiocarbon dating.

Overview

True dormancy or innate dormancy is caused by conditions within the seed that prevent germination under normally ideal conditions. Often seed dormancy is divided into two major categories based on what part of the seed produces dormancy: exogenous and endogenous. There are three types of dormancy based on their mode of action: physical, physiological and morphological.

There have been a number of classification schemes developed to group different dormant seeds, but none have gained universal usage. Dormancy occurs because of a wide range of reasons that often overlap, producing conditions in which definitive categorization is not clear. Compounding this problem is that the same seed that is dormant for one reason at a given point may be dormant because of another reason at a later point. Some seeds fluctuate from periods of dormancy to non dormancy, and despite the fact that a dormant seed appears to be static or inert, in reality they are still receiving and responding to environmental cues.

Exogenous dormancy

Exogenous dormancy is caused by conditions outside the embryo and is often broken down into three subgroups:

Physical dormancy

Which occurs when seeds are impermeable to water or the exchange of gases. Legumes are typical examples of physically dormant seeds; they have low moisture content and are prevented from imbibing water by the seed coat. Chipping or cracking of the seed coat or any other coverings allows water intake. Impermeability is often caused by an outer cell layer which is composed of macrosclereid cells or the outer layer is composed of a mucilaginous cell layer. The third cause of seed coat impermeability is a hardened endocarp. Seed coats that are impermeable to water and gases form during the last stages of seed development.

Mechanical dormancy

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. These endogenous facts include physiologically dormancy cased by low embryo growth potential

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. Other chemicals that prevent germination are washed out of the seeds by rainwater or snow melt.

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.

Physiological dormancy

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 is indicated when an increase in germination rate occurs after an application of gibberellic acid (GA3) or after Dry after-ripening or dry storage. It is also indicated when dormant seed embryos are excised and produce healthy seedlings: or when up to 3 months of cold (0-10°C) or warm (=15°C) stratification increases germination: or when dry after-ripening shortens the cold stratification period required. In some seeds physiological dormancy is indicated when scarification increases germination.

Physiological dormancy is broken when inhibiting chemicals are broken down or are no longer produced by the seed; often by a period of cool moist conditions, normally below (+4C) 39F, or in the case of many species in Ranunculaceaeand a few others,(-5C) 24F.Abscisic acid is usually the growth inhibitor in seeds and its production can be affected by light. Some plants like Peony species have multiple types of physiological dormancy, one affects radicle (root) growth while the other affects plumule (shoot) growth. Seeds with physiological dormancy most often do not germinate even after the seed coat or other structures that interfere with embryo growth are removed. Conditions that affect physiological dormancy of seeds include:

  • 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 a lower moister content before germination can begin. If the seeds remain moist after dispersal, germination can be delayed for many months or even years. Many herbaceous plants from temperate climate zones have physiological dormancy that disappears with drying of the seeds. Other species will germinate after dispersal only under very narrow temperature ranges, but as the seeds dry they are able to germinate over a wider temperature range.
  • Photodormancy or light sensitivity affects germination of some seeds. These photoblastic seeds need a period of darkness or light to germinate. In species with thin seed coats, light may be able to penetrate into the dormant embryo. The presence of light or the absence of light may trigger the germination process, inhibiting germination in some seeds buried too deeply or in others not buried in the soil.
  • Thermodormancy is seed sensitivity to heat or cold. Some seeds including cocklebur and amaranth germinate only at high temperatures (30C or 86F) many plants that have seed that germinate in early to mid summer have thermodormancy and germinate only when the soil temperature is warm. Other seeds need cool soils to germinate, while others like celery are inhibited when soil temperatures are too warm. O


From Encyclopedia

Seed Germination and Dormancy Seed Germination and Dormancy

The embryo, contained within the seed, is the next generation of plant. Thus successful seed germination is vital for a species to perpetuate itself. By definition, germination commences when the dry seed, shed from its parent plant, takes up water (imbibition), and is completed when the embryonic root visibly emerges through the outer structures of the seed (usually the seed or fruit coat). Thereafter, there is seedling establishment, utilizing reserves stored within the seed, followed by vegetative and reproductive growth of the plant, supported by photosynthesis. The seed is metabolically inactive (quiescent) in the mature, dry state and can withstand extremes of drought and cold. For example, dry seeds can be stored over liquid nitrogen at -150 degrees Celsius (-238 degrees Fahrenheit) for many years without harm. Upon hydration of a seed, metabolism commences as water enters its cells, using enzymes and structural components present when the seed was dry. Respiration to provide energy has been observed within minutes of water uptake. Mitochondria that were stored in the dry seed are involved, although initially they are somewhat inefficient because of damage sustained during drying and rehydration. During germination they are repaired and also new organelles are synthesized. Protein synthesis also commences rapidly in the imbibing seed. Early during germination, stored messenger ribonucleic acids (mRNAs) are used as templates for protein synthesis, but later in germination these are replaced with newly transcribed messages, some of which code for a different set of proteins. Although the pattern of seed protein synthesis changes during germination, no proteins have been identified as being essential for this event to be completed. Elongation of cells of the radicle (embryonic root) is responsible for its emergence from the seed. This is a turgor -driven process and is achieved through increased elasticity of the radicle cell walls, by a process which is not known. Cell division and deoxyribonucleic acid (DNA) replication occur after germination, as the radicle grows, and reserves of protein, carbohydrate, and oil, stored in the dry seed, are used to support seedling growth. Mature seeds of some species are incapable of germinating, even under ideal conditions of temperature and hydration, unless they receive certain environmental stimuli; such seeds are dormant. Breaking of this dormancy may be achieved in several ways, depending upon the species. Frequently, dormancy is lost from seeds as they are stored in the dry state for several weeks to years, a phenomenon called dry after-ripening. But many seeds remain dormant in a fully imbibed state; they are as metabolically active as nondormant seeds, but yet fail to complete germination. Dormancy of these seeds may be broken by one or more of the following: (1) light, sunlight being the most effective; (2) low temperatures (1 to 5 degrees Celsius [33.8 to 41 degrees Fahrenheit]) for several weeks; (3) day/night fluctuating temperatures of 1 to 10 degrees Celsius (41 to 50 degrees Fahrenheit); (4) chemicals, such as nitrate in the soil, or applied hormones (gibberellins) in the laboratory; and (5) fire. Dormancy mechanism operate to control the germination of seeds in their natural environment and to optimize the conditions under which the resultant seedling can become established. Dormant seeds that require light will not germinate unless they are close to the soil surface; hence germinated seeds will not expend their stored reserves before they can reach the surface and become photosynthetically independent seedlings. This is particularly important for small, wind-dispersed weed seeds. The light-perception mechanism in light-requiring seeds involves a receptor protein, phytochrome, which is activated by red wavelengths of light and inactivated by far-red (near-infrared). Far-red light from sunlight penetrates farther into soil than does red, but also light penetrating through a leaf canopy is richer in farred than red, since the latter is absorbed by photosynthetic pigments in the leaf. Hence, germination of light-sensitive seeds is advantageously inhibited under a leaf canopy and helps explain why germination and subsequent plant growth is so profuse in forest clearings. Seeds that need a period of low temperature cannot germinate immediately after dispersal in the summer or early autumn but will do so after being subjected to the cold of winter, conditions that may cause the parent plant to die, and thus remove competition for space in the spring. The requirement for alternating temperatures will prevent germination of seeds beneath dense vegetation because the latter dampens the day/night temperature fluctuations; these seeds will germinate only when there is little vegetation cover, again reducing competition with established plants. Seed dormancy is also important in relation to agricultural and horticultural crops. Its presence causes delayed and sporadic germination, which is undesirable. On the other hand, the absence of dormancy from cereals, for example, can result in germination of the seed on the ear, causing spoilage of the crop. Thus having mild dormancy to prevent this, which is lost during storage of the seed (dry after-ripening), is desirable. see also Fire Ecology; Reproduction in Plants; Seeds J. Derek Bewley Bewley, J. Derek. "Seed Germination and Dormancy." Plant Cell 9 (1997): 1055–1066. ——, and Michael Black. Seeds: Physiology of Development and Germination, 2nd ed. New York: Plenum Press, 1994.


From Yahoo Answers

Question:pls help me with my investigatory project...my topic is all about factors affecting seed germination....

Answers:The factors affecting seed germination can be Abiotic or Biotic . A ) Abiotic factors that control or affect seed germination = 1 ) Temperature = Low or cold temp. is not favorable for seed germination . They prefer higher temp. To put it differently -' The rate of seed germination is directly proportionate to the rise in temp ( Up to a limit !! you don't expect seed germination at 100 degree Celsius! ) 2 ) Moisture or water = Dry seeds do not germinate . They must first imbibe water to trigger off the process of germination . 3) soil = It is not required initially , but as the seedlings grow they require mineral elements for further growth . this requirement can be fulfilled only by soil. 4 ) Light = for germination only light is not required but later on it plays a dominant role . B ) Biotic factors that control seed germination == 1 ) Viability of the seeds = After the seeds are produced they remain viable ( have potential to germinate ) up to certain period that varies from plant to plant or seed to seed .( It is like expiry date of a medicine! ). Lotus seeds have viability period of 800 years !!!!! Once that period expires , the seed germination is difficult . They are as good as dead . 2 ) Dormancy period = Many seeds do not germinate immediately after they are produced . The reason is they require a resting period during which they remain dormant and once that period is over they are ready to germinate . Do not confuse viability period with Dormancy or resting period .

Question:Hi, I am conducting a science experiment centred around the effect of environmental factors on phenotype. I am wondering whether I should begin the experiment using seedlings and record their initial height and other observations, or germinate the seeds myself. Which would be the faster and more effective option?

Answers:Germinate them yourself so every factor of your experiment is consistent. It might take a week longer, but will help eliminate one source of experimental error.

Question:I know that low pH levels have various detrimental effects on seed growth and development, but what about high pH levels? I have not found any information about the effect of high pH levels on germination. I am doing an experimental design lab on the affect of pH levels on green bean seed germination. Help would be much appreciated! Thanks

Answers:Plant species vary in their pH requirements. Some like an acid soil, some like it alkaline. Legumes like beans will grow in both but high pH levels, above around 7.8, quite often mean low available trace mineral content like zinc so they affect plant growth. Beans like a bit of zinc and they prefer slightly acid soils, around pH6. http://www.ext.colostate.edu/pubs/crops/00539.html . . . . http://www.ncsu.edu/sustainable/profiles/ppbean.html . . . . . If your tests are carefully controlled and have full scientific credibility with all the other factors controlled properly then they may be written up as a paper and used to add to the store of knowledge of bean germination. That's how all the other figures got into the literature. If there are no figures available you do the tests to get some yourself and then publish them so other people don't have to go through it all again. PhD's are earned that way. Find a topic that needs new results or a new technique, get it done and well written up with loads of graphs and charts and proper writing...no cheap journalistic stuff...and get it reviewed. You get a supervisor all the way to help you along. It has to be good though. Totally done right in street talk. Best of luck. Hope you get some good results.

Question:Whay do non-germinated seeds have greater rate of cellular respiration rate than germinated seeds? what is the device that uses total infrared radiation absorption to measure the amount of CO2 present in a flask? Thanks

Answers:Respiratory substrate is more in non-germinated seeds than germinated seeds. So naturally cellular respiration will be more in non-germinated seeds. IRGA-Infra red gas analyser measure photosynthetic rate and also the CO2 content.

From Youtube

Germination of a seed :Children are often found curious to know how a seed germinate this is for those young researchers.

Seed Germination :As seeds are crucial to the reproduction to plants, and therefore an integral part of our world, our video focuses on seed germination. We began by depicting the various parts of the seed in a drawing, showing where they are in the seed, as well as their functions. We then assumed the roles of the seed parts and acted out their processes while allowing the viewer to get an idea of how plant growth begins. The narration provides a clear and comprehensible explanation of the germination process. Seed germination begins when water enters the seed coat and activates the embryo. The embryo then sends a hormonal message to the aleurone layer to release enzymes. These enzymes enter the endosperm and break down starch into glucose. The glucose provides energy for the embryo to germinate. We represent this process in a simple yet informative manner. Our music selection sets a lighthearted mood.