To break seed dormancy by cutting or softening the hard wall of a seed.
Producing seeds that require scarification is one of many plant strategies for reproducing successfully. Understanding this process, its need, and its effectiveness will help the Wikipedia community understand a life history trait of plants and the reasons it might be selected over other seed production and dispersal strategies.
Many plants have dormant seeds as a biological mechanism to ensure that seeds will germinate at a time and under conditions that are favorable for the growth and survival of the next generation (Leadem 1997). There are two types of dormancy: exogenous (seed coat dormancy) and endogenous (embryo dormancy). Some species seeds have hard seed coats preventing imbibition of water and the exchanges of gases. Without imbibition and gas exchange, germination would be impossible. Physical seed coat dormancy occurs frequently in species adapted to alternating dry and wet seasons, including leguminous genera (Willan 1985). There are several treatments to break dormancy for the purpose of inducing germination. This page will focus on scarification
techniques that break seed coat dormancy.
Pretreatment, a process to overcome physical seed coat dormancy, is designed to soften, puncture, scratch, or split the seed coat in order to make the seed permeable to water and gases without damaging the embryo and endosperm within the seed. This treatment, called scarification, consists of physical and biological methods, dry heating, or soaking in water or chemical solutions (Willan 1985). In nature, scarification often occurs by seeds falling. Freezing temperatures or microbial activities modify the seed coat during the winter. Seeds are also scarified as they pass through the digestive tract of animals (Blazich and Evans 1999).
Species react to differently to various kinds of scarification treatments. Listed below are the most common scarification techniques and examples of species that use them.
One of the simplest and most direct physical methods is to cut, drill or file a small hole in the seed coat before sowing. This method has been found to be successful in many legumes. Sand paper may also be used to reduce seed coat thickness by abrasion, especially on hard-coated species. Manual treatment of individual seeds is slow but can be safe and effective. This process is best suited for larger seeds. Soaking in cold water before sowing reinforces physical scarification (Willan 1985).
Another method of physical scarification, percussion, has been used effectively to improve seed germination. Percussion is the repeated striking of a seed against a hard surface. Khadduri et. al showed this as an effective way to improve germination for Locust trees (Robinia spp.
) over the traditional hot water soak and acid scarification methods (2002). The percussion weakens the area of natural water entry into the seed without causing damage to the embryo. Multiple methods of scarification may improve a seed's germination, but for Locusts and other plant families, a physical method like percussion is the most effective.
Soaking in Water:
A number of treatments involve soaking seeds in water or other liquids. These treatments may soften hard seed coats or leach out chemical inhibitors. Hot water treatment has yielded beneficial results with a number of leguminous seeds. The seeds are usually placed into boiling water, immediately removed from the heat source, and left to cool gradually while the seeds remaining in the water for about 12 hours. Soaking in boiling water makes the seed coats permeable to water (Phartyal et al. 2005). They imbibe and swell as the water cools. It has been shown that the initial water temperature has a bigger effect on germination rate than the periods of soaking and cooling of the seed (Willan 1985). This treatment is recommended for species that have little resistance to germination. Prescriptions for hot water must be applied carefully without killing the seeds though excessive heating.
Acid treatments mimic part of a biological method of seed scarification. Seed coats can be effectively broken down by chemical means whether that is stomach acid or concentrated sulfuric acid, the chemical most commonly used to break seed coat dormancy. Seeds that have been dormant for a long period may require longer soaking periods in the acid than fresh seeds. Because of the toughness of the seed coat varies between species and within species, the optimum period of immersion in acid for each lot may be determined by treating a small sample for different periods and the soaking the lots in water at room temperature for 1 to 5 days. Over soaking may pit the seed and even expose the endosperm. Insufficient soaking leaves the seed coat of most species glossy. Coats of correctly treated seeds are dull and not deeply pitted. Sulfuric acid treatment has been effective for several temperate and subtropical species for example Gleditsia triacanthos (Willan 1985).
When looking for the best scarification method for Sky-blue lupine (lupinus diffusus
), a plant with "erratic" germination rates, Dehgan et al. tested acid treatments, hot water soaks, abrasion, and no scarification to improve germination rates (2003). Most of the treatments had some effect on germination, but chemical acid soaking was the only method that significantly improved germination. This study is another example showing that method of scarification when breaking seed coat dormancy can be as important to know as the fact that a plant requires scarification.
In nature, the frost, rain action and soil microbes carry out natural scarification. Animals and microorganisms are an important factor in the breakdown of seed coat impermeability. Seed coats are broken down as they pass through the digestive system track of animals because of the action of the strong digestive chemicals. Termites are an important agent for breaking down seed coat dormancy in many parts of the tropics. This treatment is efficient for certain tropical fruits like teak fruit (Pterocarpus angolensis
). Seeds are spread on the ground in a 5 cm thick layer immediately after collection and covered with cardboard. After five weeks, the termites will remove the exocarp, improving germination rates (Willan 1985). Partial fermentation, which could be damaging to many seeds, is beneficial to others to overcome seed coat dormancy. Fruits of Tectona grandis
are laid on the ground through the rainy season and then are collected and stratified (Willan 1985).
When looking at the effects of seeds being passed through bird guts, Traveset et al. found that seeds had lost mass in the process (2001). They explained this by saying that seeds had been mechanically scarified in the birds' gizzards. The digestive chemicals of the birds tested had also acted the seed coats on (Traveset et al. 2001). These actions did not reduce the diameter of the seed, but reduced the mass of the coat. For one of the species studied, passage through bird digestive track significantly increased germination rates (Traveset et al. 2001). This suggests that the plant relies on birds to eat its fruit and defecate its seeds in order to maximize reproductive potential.
Dry heat and fire:
In the seasonal wet and dry tropics, fire is a powerful natural factor in the removal of seed coat dormancy. Fire treatment has been implemented successfully for seeds of species such as Tectona
and _Aleurites moluccana. The seeds are spread evenly on the ground and then covered with grass, which is set on fire. As soon as the grass in burned, the seeds are placed in cold water. The quick temperature change causes the seeds to crack open (Willan 1985). The dry heat cracks the seed coat, which accelerates imbibition (Schelin et al. 2004). Seeds that experience too high of temperatures or high temperatures that last too long germinate less frequently. For this reason, care needs to be taken when dry heating is used as a treatment method.
Ecological Role/Trade Offs
Scarification is needed by many plant species to break seed coat dormancy. Methods of scarification vary. While all methods may break a seed coat allowing water to infiltrate, most species have a preferred method. Rosner et al. tested the effect of hydrogen peroxide treatment as a less abrasive method of chemical scarification (2003). Sulfuric acid may be too strong of a treatment for seeds with weaker seed coats and may damage seed embryos. This group found that hydrogen peroxide broke down the seed coat without harming the embryo and improved seed germination. Finding the preferred method of scarification to most effectively break seed coat dormancy, may improve rates of germination in trees. Understanding why plants have developed particular methods of scarification and the manner in which this occurs in nature may help ecologists manage an ecosystem for increased seed germination for a desired species.
Seeds that require some form of scarification in order to germinate have paid for this strategy at cost in carbon, time from seed production to germination, and potential to not have scarification needs met. The carbon put into strengthening the seed coat could be allocated elsewhere. Plants could have greater seed production if not for scarification requirements. Plants could allocate more energy to growing if their energy requirements for seed production were not so high. Many seeds have to wait for conditions to be right in order to germinate. The time could be as short as the time it takes a seed to run through a digestive tract to as long as the time it takes for the next fire to move through an area. Plants requiring seed scarification may also lose their viability before conditions for germination are met. Plants make these trade offs in order to better protect their seeds and ensure that conditions are good for growth when a seed does germinate.
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Dehgan, Bijan, Jeffrey G. Norcini, Steven M. Kabat, and Hector E Pérez. (2003). Effect of seed scarification and gibberellic acid treatment on seedling emergence of sky-blue lupine (lupinus diffusus). Journal of Environmental Horticulture. 2003. 21(2):64-67.
Khaduduri, Nabil Y., John T. Harrington, Lee S. Rosner, and David R. Dreesen. (2002). Percussion as an alternative scarification for New Mexico locust and black locust seeds. USDA Forest Service, Rocky Mountain Research Station Proceedings. Report: RMRS-P-24. 370.
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Phartyal, S.S., J.M. Baskin, C.C. Baskin, and R.C. Thapliyal. (2005). Physical dormancy in seeds of Dodonaea viscosa (Sapindaceae)
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Schelin, Maria, Mulualem Tigabu, Ingalill Eriksson, Louis Sawadogo, and Per Christer Oden. (2004). Perdispersal seed predation in Acacia macrostachya,
its impact on seed viability, and germincation responses to scarification and dry heat treatments. New Forest.
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Sedbrook, Judy. Denver County Cooperative Extension.
Gardening and Horticulture. "S". Retrieved on Nov. 25, 2006. Available from: http://www.colostate.edu/Depts/CoopExt/4DMG/Glossary/s.htm
Traveset, A; N. Riera, and R. E. Mas. (2001) Passage through Bird Guts Causes Interspecific Differences in Seed Germination Characteristics. Functional Ecology
, Vol. 15, No. 5. (Oct. 2001), pp. 669-675.
Willan, R. L. (1985). A guide to forest seed handling. Food and Agriculture Organization of the United Nations. Rome, Italy. Available from http://www.fao.org/docrep/006/ad232e/ad232e00.htm
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