A general overview on seed dormancy and methods of breaking it.Introduction
Seed has a pivotal role in human and animal nutrition and life. Many of the seeds, after distribution of mother plants or harvest do not germinate in optimal conditions due to a period of dormancy. Seed dormancy is a physiological phenomenon in wild and crop plants, and is more common in wild plants than the crop plants . Dormancy can be causing keep some plant species in particular environmental conditions, and one of the most important survival mechanisms in plants, is their ability to delay seed germination until conditions of the location and time be suitable for germination. Thus delay in seed germination is not a haphazard and in many seeds in dormancy some morphological and physiological changes should be occur for germination [14,26]. These changes are in environment natural conditions such as air, moisture, temperature, and light and seed can able to germination. Terms of ecological, seed dormancy is an advantage in keeping plants species. Geographic spread is more in seeds that have a dormancy period. This seeds depending on environmental conditions are able germinate at different times and places. Extent of dormancy in some crops (such as winter cereals) is desirable to prevent germination before harvesting, and keeping seed quality [17,14]. Seed dormancy causes the seed of many plants species may remain in soil for several years before germination, this issue, reveals reason the presence of unwanted plants and weeds in successive crops [22,9,17,26]. Lack of seed germination at certain times and suitable conditions are a big problem for seed researchers, botanists, and farmers. Seed dormancy in agricultural ecosystems due to use of inputs and energy is a negative features; Thus seed dormancy is a factor in the lack of a uniform seedbed and reduced yield. Seed consumption for sowing also increased, with seed dormancy. Using of suitable methods for breaking seed dormancy is essential for achieving a uniform cultivation and proper weed control .
Induction seed dormancy:
Induction dormancy is one of the primary dormancy, and occurs when necessary factors for seeds germination (such as water, light and temperature) does not provide, therefore, seed will not be able to germination. This type of dormancy is mainly related to physical properties of the seed coat. So this type of seed dormancy is controlled by external factors [22,26]. The following three factors are considered in relation to induction dormancy [10,26].
Impermeability seed coat to water is created by genetic and environmental factors; however, most researchers have shown that this feature is largely hereditary. Seed permeability also affects by environment conditions. Environmental conditions during seed development and maturation, is involved on seed impermeability. In general, climate and soil conditions have a significant impact on the final stages of seed maturation. In Seeds impermeable to water (hard-coated seeds), impenetrability may be due to a cuticle layer or developed a layer of strong epidermal cells. Hard-coated in legume seeds is due to the large reserves of suberin, lignin or cutin. Impenetrability to water in some seeds may be related to hilum building. Strophiole is an outgrowth of the hilum region which restricts water movement into and out of some seeds. Strophiole can be separated or moved by vigorous shaking, and thus the seeds are permeable to water. This procedure is called tapping; that may be used to break dormancy on sweet clover seed. In some seeds dormancy are broken with scratch or making pierce on coat of seed.
Oxygen is an atmospheric gas that is found in soil pore spaces; if a seed is buried too deeply within the soil or the soil is waterlogged, the seed can be oxygen starved. Oxygen is required by the germinating seed for metabolism. Oxygen is used in aerobic respiration, the main source of the seedling's energy until it grows leaves. Some seeds have impermeable seed coats that prevent oxygen from entering the seed, causing a type of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange and water uptake from the environment [24,27]. Seed internal membrane of cucumber, aspic seed pericarps and coffee seed endocarps are limit to penetration of oxygen. Cocklebur and apple seeds have impermeable coat to oxygen. There are limits to absorb oxygen at 20 [degrees]C, while permeability to oxygen increases at 4 [degrees]C; so maybe there is an interaction between oxygen permeability and temperature. Other seeds have different permeability to oxygen and carbon dioxide. For example permeability of seed internal membrane in cucumber to carbon dioxide is more than oxygen.
3- Mechanical resistance:
Physical limitations by seed coat during embryo development, caused seed dormancy. It seems that the pressure of water absorption and growth are not enough for seed coat cracking and germination. This type of dormancy can be seen on water plantain, pigweed, raspberry and cherry seeds; in addition, this seeds have inhibitor material in seed coat [1,26,30]
Methods of breaking induction dormancy:
In normal conditions, induction seed dormancy can be broken by soil melting and freezing, microorganism's activity, forest fires natural, soil activity, being eaten by animals, and other factors. Each of these activities may require several years for be completed, thus increases the time that required for germination. Broken seed dormancy by removing the limitations caused by seed coat is done in two mechanical and chemical ways; this method is called scarification [8,13,20,25]
Mechanical scarification is a method for overcoming the effect of an impermeable seed coat. Mechanical scarification can be done by rubbing seeds between two pieces of sandpaper, abrasive, sand or with a severe shaking. Heating, cooling, extreme changes in temperature, dipping seeds in boiling water for a short period of time, piercing the coat of seeds by needle, placing the seed exposed to certain and intermittent wavelengths are other techniques that cause seeds become permeable to air and water .
Seed coat may be removing by treated with chemical materials. Acid treatments are often used to break down especially thick impermeable seed coats. Since seeds placed in concentrated sulfuric acid ([H.sub.2]S[O.sub.4]) will become charcoal in time, the temperature of the acid and the length of time the seeds are soaked are very important. The acid should be used at room temperature for a period of a few minutes to several hours depending on the species. The seeds should be immersed in acid in a glass, china, or earthenware container, and should be stirred occasionally with a glass rod; however, too much stirring will cause the acid to heat undesirably. The seeds must be removed from the acid just before any acid penetrates the seed coats. When the allotted time is finished, the seeds should be removed promptly and washed thoroughly in several changes of water to neutralize completely all remaining acid. For some species the duration of the acid bath depends on the specific batch of seeds and can only be determined empirically. After treatment and a thorough washing, the seeds may be sown or dried and stored for several months [18,6]. In new techniques, selected enzymes in the seed coat, such as Cellulase and Pectinase are used for eliminate the seed coat. Also, organic solvents such as alcohol and acetone are used to eliminate seed dormancy.
Genetic dormancy is one of the most common seed dormancy which occurs by seeds intrinsic properties. Genetic dormancy is due to inhibitor material in the seed, and should be reduced or removed before germination. Physiological changes such as: embryos maturity, response to growth regulators, changes in temperature, exposure to light, is eliminating genetic dormancy. Environmental conditions existing at the time of seed development and maturity is effective on genetic dormancy duration .
Dormancy due to inhibitors is based upon the fact that germination and growth promoting enzymes and hormones can be inhibited, thus preventing germination. Inhibitors, such as Abscisic acid may be at sufficient level to counteract growth promoting enzymes, such as gibberellins. Usually it is the balance or ratio between inhibitors and promotors that needs to be tipped in the favor of those that will allow germination to proceed. These inhibitors are found in the endosperm, cotyledons, or other food storage tissue. Sometimes these chemicals are found in the outer coverings of the seed or fruit. Many of these chemicals are water soluble and can be leached from the seed, thus shifting the balance towards the growth promoting chemicals and allowing it to germinate. Others must be degraded into other forms or chemicals to reduce their concentration. With inhibitors that are found within the embryonic axis, it is temperature (and sometimes light) that generally controls this shift. Temperature may also favor the production of growth promoting hormones and enzymes in the embryonic axis. Cool temperatures generally shift the balance of promotors and inhibitors towards promoting germination .
Some certain metabolic pathways are blocked by some compounds in seeds. For example, cyanide is an inhibitor matter that found in apples and pears seeds. Each of these compounds prevents of germination through effect on respiration. Also phenolic compounds are considered as inhibitors of the germination, and due to extensive role are known as natural germination inhibitor substances. Coumarin, Duromine and Abscisic acid also are known as natural germination inhibitor substances. Abscisic acid has an opposite effect on Gibberellins and Cytokinins hormones [29,5].
Materials that create high osmotic pressure can prevent of germination. Sugars and salts can compete with seeds to absorb water and prevent water absorption by seeds, and thus prevent of germination. In this case, when osmotic inhibition was overcome, seed is able to germination.
Methods of breaking physiological dormancy:
One of the ways to eliminate the osmotic inhibition is seeds leaching. Inhibiting substances in the seed coat of sugar beet is destroyed through leaching.  observed that the maximum germination in Cassia angustifolia was obtained by leaching seeds in 24 hours at 20 [degrees]C. Also metabolic inhibitor materials are eliminating by use of scarification techniques. Scarification is done by using various methods such as acids or piercing coat seeds. In these way inhibitors materials are removed by the loss of the seed coat. Cabbage has inhibitors in seed coat, which disappears in dilute sulfuric acid in a few minutes [11,20]. In scarification techniques, physiological changes occur in the seeds that absorb water and are exposed to low temperatures. There is growing in scarified embryos. In scarified seeds, oxygen uptake and energy increases in the cell surface for embryo axis. Also increase enzymes such as catalase, phosphatase, lipase and peroxidase; therefore, this indicates that embryo in many stages of metabolism and development is affected by scarification. In scarified seeds, also occur hormonal changes, for example, amount of Abscisic acid (ABA) reduced in apple, pine, walnut and hazelnut seeds by scarification. Gibberellins can replace with scarification in some seeds, due to it stimulator role. Also observed increase in the amount of Gibberellin during the scarification operation. Thus, in addition to increased growth and metabolic activity in scarified seeds, changes in the amount of inhibitor and stimulator materials can also be effective on eliminating seed dormancy [4,3]. Also germination increased by placing seeds in daily alternating temperatures. Scarified seeds are placed under a temperature of 3 to 10 [degrees]C; however, in various plants certain temperatures, and durations that seeds are treated with this temperature is different [23,19] stated that the plant, pre-chilling for 7 and 15 days in the light and darkness had no affect on Echinacea angustifolia germination percentage, but the germination rate increased significantly. For most plant species, scarification temperature is the same absolute temperature that required for germination and the scarification time, is different depending on the type plant. Genetic dormancy decreases with increasing age of seed [21,26].
Light, wavelength, and day during have an impact on seeds germination, which have a physiological dormancy. Seed dormancy in lettuce is broken down with placing the seeds in red light (670 nm). Some plant species seeds, respond to short days and some react to long days . Continuous light may inhibit germination in some seeds such as onions and leeks.
Secondary dormancy occurs in some non-dormant and post dormant seeds that are exposed to conditions that are not favorable for germination, such as high temperatures, light and dark. Water, chemicals and gases can also lead to creation of secondary dormancy. It is caused by conditions that occur after the seed has been dispersed [2,12].
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(1) Sayed Roholla Mousavi, (1) Maryam Rezaei and (2) Aboutaleb Mousavi
(1) Aligoudarz Branch, Islamic Azad University, Aligoudarz, Iran
(2) Bardsir Branch, Islamic Azad University, Bardsir, Iran
Sayed Roholla Mousavi, Maryam Rezaei and Aboutaleb Mousavi: A General Overview on Seed Dormancy and Methods of Breaking It
Sayed Roholla Mousavi, Aligoudarz Branch, Islamic Azad University, Aligoudarz, Iran. Tell: +98-664-223-4731 E-mail: firstname.lastname@example.org