There is no gainsaying in the fact that sexual reproduction is not only essential for the continuation of life, it is also important in ensuring genetic diversity. Differences in the genetic components of individuals of same species will ensure survival through the process of natural selection.
Sexual reproduction usually involves seeds which could mean different things depending on the organism involved. In some plants and and virtually all animals, seeds represent sperms that are used to fertilize ovules during reproduction. However, in seed producing plants; seeds are products of fertilization of the ovule. Typically, a seed is defined as a ripened, fertilized ovule.
When a seed is planted and the conditions are right, it will typically germinates in few days, weeks or months as the case maybe. During this period, a lot of processes take place which are not usually visible to the opened eyes. This article is aimed at simplifying the various physiological processes that takes place during germinations and what usually happens when a seed refuses to germinate.
Seed and the germination process
A typical seed is made up of the outer covering known as the seed coat which could of one or multiple layers, a source of stored food for the nutrition of the embryo otherwise known as the endosperm and the embryo itself which is a young multicellular organism that eventually grows out during germination.
The embryo is made up of the plumule which is a structure that grows to become the shoot, the radicle which eventually grows to become the root and then one or two cotyledons depending on whether the seed is from a monocotyledonous plant or dicotyledonous plant. While the endosperm nourishes the embryo in monocot seeds, the cotyledons (which happens to contain the endosperm) nourishes the embryo in dicot seeds.
A typical seed stores carbohydrates, lipids and proteins either in endosperm of monocots or in the cotyledons of the embryo as in the dicots. Usually, lipids in form of oils from the major food reserves of the seed except in the legume and the grass familes of plants where starch is the major food reserve. Legume and grass families form the principal crops of humans and such, the bulk of our carbohydrates is from them. Legumes in addition are also rich in protein; hence, they are a major source of proteins for both humans and livestocks. In addition, seeds contain high levels of minerals especially phosphorus, as well as normal cytoplasmic constituents such as nucleic acids and vitamins.
Germination of seeds generally starts when the various environmental factors necessary for germinations are in place. The seed absorbs water through imbibition and osmosis leading to the hydration of the embryo and activation of respiratory enzymes. Other germination related enzymes are newly-synthesized through the use of amino acids provided by the metabolic digestion of protein.
A typical germinating seed has two centers of activities which includes the food storage center (the endosperm and or the cotyledons) and the growth center which is the embryo. In the storage center, catabolic breakdown of food materials takes place with concurrent synthesis of enzymes. The digestion of food reserves takes place by hydrolysis and the soluble products of digestion are tranlocated to the growth regions of the embryo.
The sugars, fatty acids and glycerol that constitutes the products of hydrolysis are used to provide substrates for respiration in both storage and growth centers while they may also be used for anabolic reactions in the growth center. The glucose is majorly used in the synthesis of cellulose and other cell wall materials while the amino acids are utilized in the process of synthesizing protein, an important product in enzymatic functions and a structural component of protoplasm.
During germination, both the storage as well as the growth centers obtain their energy requirements through respiration. The respiratory process involves the oxidation of a substrate, usually sugar, to carbon dioxide and water. A net loss in dry mass of the seed therefore occurs since carbon dioxide which has a greater mass than than oxygen which is taken in, is lost as a gas. The water product from the respiration does not contribute to the dry weight of the seed. Hence, the loss in weight continues until the seedling produces green leaves and starts to synthesize its own food through photosynthesis.
The embro grows by cell division, enlargement and differentiation. While the amount of proteins, cellulose, nucleic acids etc. increases steadily in the growing regions, the dry mass of food storage decreases. The first visible sign of germination is the emrgence of the radicle which grows in obedience to gravity and anchors the seed to become the root. Thereafter, the plumule emerges with its negative obedience to gravity, hence it grows upward and above the ground to become the shoot.
In the dicot seeds, the two cotyledon leaves are carried above the soil along with the emergine shoot. The cotyledons in most cases posses the ability to carryout the photosynthetic function of leaves in the continuation of nourishing the growing plant before eventually dropping-off at some stage. This type of germination is peculiar to dicots and is otherwise known as epigeal germination. However in monocots, the single cotyledon remains underground and the kind of germination exhibited is known as hypogeal germination.
In grasses, the plumules are usually protected by a sheath known as coleoptile. The structure grows in the direction of light and against the force of gravity. The first leaf grows out through the coleoptile and unrolls in response to light.
When seeds refuse to germinate
Ordinarily, one would expect a planted seed to germinate at the appropriate time depending on the kind of seed. As long as the conditions necessary for germination are met, a normal seed should germinate. However, even when the right temperature, moisture, light and some other requisite factors for germination are met, some seeds still refuse to germinate. Consequently, such seeds are said to be dormant.
Seed dormancy is defined as a state in which seeds are prevented from germinating even under environmental conditions normally favorable for germination. source
Seed dormancy is quite important for some reasons. It is one of the adaptive mechanisms exhibited by plants to ensure survival and dispersal within the environment. As a result of dormancy, seeds can escape a period of unfavourable climatic condition that may have result in death if they had germinated. It also ensures that seeds can be stored for a long time and still be suitable for the use of man, both agricultural and consumption wise.
Varieties of ways exist through which seeds can get dormant. A seed with an immature embryo would not germinate. Metabolic processes continues within the seed until the embryo becomes mature after which the seed would be ready for germination. Hence, one of the reasons the seeds you planted are not germinating is immature embryo.
A seed that requires after-ripening processes before the embryo could germinate would not germinate if planted ordinarily. While some seeds would germinate if planted immediately after harvesting at maturity, some seeds require an extra period of drying before they can germinate. If such seeds are planted immediately after maturity without drying, either they will not germinate at all or die off at tender age. More information on after-ripening can be found here
Aside from immature embryo and after-ripening process, some seeds may refuse to germinate due to seed coat's impermeability to water and gases (two of the most important factors for germination) and seed coat hardedness. The hardedness stuffs up the embryo and act as a barrier against the its emmergence, a process that is largely a characteristic of the dormancy found in most leguminous plants/seeds.
Moreover, many species of plant produce seeds that contain chemical growth inhibitors which prevent germination upon planting. Abscisic acid, phenolic acid, ferulic acid, coumarin, short fatty acids and cyanogenic chemicals are few of the chemicals that have been implicated in the induction of dormancy in variety of seeds.
Breaking seed's dormancy
Depending on the mechanism that prevents a particular seed from germinating, a variety of methods exist to overcome the dormancy. A dormancy that results from hard seed coats can sometimes be broken through an administration of physical damages to the seed. This process is generally termed scarification.
Scarification of seed coats can be achieved artificially by removing the seed coat, scratching it off a rough surface, soaking seeds in acids or by simply pricking the seed coat with pin. Under natural circumstances, bacteria or passage through the gut of an animal may have the same scarification effects while the seeds of some species are stimulated to germinate by fire.
Seeds that refuse to grow as a result of the presence of growth inhibitors might have their dormancy broken through soaking of the seeds in water or the introduction of growth promoters such as gibberellin. While the water works by diluting the concentrations of the growth inhibitors, the growth promoter works by overriding the effects of the growth inhibitors.
Dormancy that results from immature embryo in seeds can be broken by giving such seeds sufficient time to allow for maturity of the embryo, a period that corresponds to after-ripening. The dormancy of some seeds can aslo be broken by light after water uptake, a process known as photochrome-controlled response. This process is usually associated with a rise in gibberellin levels within the seed and commonly be observed in some lettuce species.
Some seeds commonly require a cold period in order for their dormancy to be broken. This cold period is known as pre-chilling. It is usually common among cereals and members of the rose family such as plum, cherry and apple. It is associated with a rise in gibberellin activity and sometimes a reduction in growth inhibitors. It ensures that seeds must pass through a cold spell of a particular length before germination and are less likely to during a warm period in winter.
Thank you for reading.
References
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