Here is an essay on ‘Seed Dormancy’ for class 11 and 12. Find paragraphs, long and short essays on ‘Seed Dormancy’ especially written for school and college students.
Essay on Seed Dormancy
Essay # 1. Meaning of Seed Dormancy:
Dormancy, a suspended state of growth or rest, is a condition that may persist for an indefinite period despite conditions favoring germination. Technically a seed is dormant at the point of physical or physiological separation from the mother plant. This immediate dormancy ceases, however, with a new set of conditions favoring germination.
Quiescence is a more descriptive term for the seed rest that results from an environment non-supportive for germination (e.g., on the maturing plant or in storage). More aptly stated, dormant seeds are those that fail to germinate when placed in an environment that supports germination of non-dormant members of the seed population.
Selection pressures during thousands of years of domestication have virtually eliminated crop plant dormancy. Most crop seeds germinate readily after maturation and desiccation, or drying down. In rainy weather germination of quiescent seeds in the inflorescence of the standing crop is not uncommon.
On the other hand, seeds from wild species (e.g., weeds and trees, including fruit trees) generally exhibit intense dormancy. Crop plants with a short domestication history often exhibit dormancy to some degree and require special conditions or more storage time for germination (e.g., many hard-seeded forage legumes and many grass species with physiological dormancy, including Sorghum, Poa, and Festuca).
The fact that dormancy is ubiquitous in wild species suggests its ecological significance in species survival. Natural selection pressures during evolution have resulted in plants with dormant seeds and/or dormant buds as an adaptation to periods of environmental adversity such as those found in temperate climates.
If germination or bud growth were not synchronized with the incidence of suitable climatic conditions for growth and reproduction, the species could not persist. Dormancy is a major principle in the success of weeds, which survive and find an ecological niche despite the all-out war directed against them. Seeds of many weedy species remain viable and ultimately germinate despite severe stresses from temperature, water, fire, cultivation, and animal and bird ingestion.
Essay # 2. Types of Seed Dormancy:
Amen (1968) classified dormancy mechanisms of certain species as follows:
1. Immature Embryo:
Orchideacae spp.
2. Impermeable Seed Coats:
Leguminoseae (to water), Gramineae (to O2)
3. Mechanically Resistant Seed Coats:
Certain species of Gramineae and species with nuts as seeds.
4. Physiological:
A wide range of species with seeds that contain growth inhibitors or a supply of growth promoters in the embryo sac, seed coats, or hulls insufficient to initiate the vital processes of germination.
The process of becoming germinative, commonly referred to as after- ripening, may be accomplished by maturation on the mother plant, desiccation in storage, or merely ageing in dry storage. On the other hand, after-ripening in some species may require prolonged cold treatment or a more complex set of conditions, such as alternating temperatures, cycles of radiation, presence of salts, leaching, or hull removal. These treatments are effective only on imbibed seeds.
Immature embryos, common in parasitic seed plants, such as witch weed (Striga lutea), require a host-provided stimulus. Cytokinin from the host plant has been suggested as the necessary stimulus. In some species embryo maturation can occur in storage or during germination.
A hard seed coat is the principal dormancy mechanism in legume seeds. Water impermeability of legume seeds results from two factors:
(1) A seed coat with a densely compacted layer of scleroid Malpighian cells at right angles to the surface of the testa plus phenolic, or other water- repellent compounds, such as are common in legume seed coats;
(2) Closure of the natural openings in the seed coat, which include the micfopyle, funicle, and pieurogram (a depression below the micropyle and funicle). Olvera et al. (1982) concluded that the main factor responsible for the hard seed in Leucaena (legume) is pieurogram closure. These structures close as the moisture level outside the seed becomes lower than inside, allowing moisture to leave but not to enter.
A large number of after-ripening treatments are effective in breaking hard seed dormancy. Strong acids or alkalis are highly effective but can also damage the seeds. Heat at 100°C for 1, 5 minutes, as delivered by a 250-W infrared lamp or hot water, are effective in reducing hard seed content. Hot water (100°C) for 5 to 20 sec caused the pieurogram in Leucaena to open and a resultant germination of 95 to 100%, depending on variety.
Scarification (mechanical abrasion, acid, or hot water treatment of the seed coat) can remove hilum plugs and increase permeability. Hard seeds in moderate amounts are believed to be of value to forage seeds, so scarification is not always advisable. After-ripening is accomplished naturally by freezing and thawing, wetting and drying, animal ingestion, microbial action, and/or enough ageing in storage.
The hulls of many grass seeds and weedy species, such as green needle- grass and Indian rice grass, are impervious to O2. In green needle grass the lemma and palea (seed hulls) act as a barrier. Germination peaked at 12% 7 yrs after harvest. Laboratory chilling and KNO3 solution pretreatments induced nearly complete germination. Cocklebur and wild oats are classical examples of seed dormancy that results from an O2-impermeable seed coat.
The cocklebur seed is actually a spiney, dry, non-dehiscent fruit containing two seeds. The lower seed may germinate readily, while the upper one remains dormant for several years because of the low O2 tension surrounding it. Hulls of wild oats also impose a low O2 tension. Removal of the hulls of the seeds of both these plants greatly improves germination.
Mechanically resistant seed coats can imbibe water readily, unlike hard seeds, but resist swelling and embryo protrusion. Seeds of some grasses and most species with hard-seeded fruits (nuts) as seeds have seed coats mechanically resistant to embryo emergence. A prolonged period of wet storage can weaken the hard covering and accomplish after-ripening.
In the case of black walnut (Jugians nigra) several weeks of cold (2-5°C) wet storage (stratification) are required; this species appears to have at least a double dormancy mechanism: a mechanically resistant seed coat and an unripe (physiologically immature) embryo. It is also known that walnut husks contain a strong growth inhibitor, juglone.
This substance, probably lost by leaching during wet storage, seems to constitute even a third dormancy mechanism in black walnut. The ecology of nut-bearing species is generally closely associated with small animals burying nuts during fall, which accomplishes seed dispersal, softening of the seed coat, and stratification during the overwintering period.
Essay # 3. Physiological Seed Dormancy:
Physiological dormancy is often referred to as embryo dormancy and has been called deep dormancy. A physiologically immature embryo is considered a physiological dormancy.
The presence of growth inhibitors, a deficiency of growth-promoting substances, or a lack of proper balance between the two hormones has been postulated as the factor causing embryo dormancy. Abscisic acid (ABA), coumarin, and other inhibitors have been shown to induce dormancy, but these factors may be in the hull, seed coat, aleurone, or embryo. Growth-promoting substances (GAs and cytokinins) release dormancy in a wide variety of species.
A theoretical model of growth promoter-growth inhibitor balance in germination is illustrated in Fig. 9.15. According to the model, germination can occur when a critical hormone balance is reached, either by elevation of growth-promoting substances or by depression of growth inhibitors. Amen (1963) stated that most dormancy mechanisms can be broken by growth- promoting substances.
The fact that GA treatment replaces the light requirement in numerous photoblastic seeds (lettuce, tobacco) and the cold requirement in species requiring stratification (wild oat, many tree species) supports this conclusion.
Growth-promoting substances often decline during seed ontogeny, whereas growth inhibitors such as ABA increase, the result being dormancy at seed maturity due to hormone imbalance. Various conditions during postharvest generally cause the reversal of the above process, which explains the loss of light and stratification requirements in certain species during dry storage.
Coumarin is a common natural chemical inhibitor in physiological dormancy, but abscisic acid (ABA, or dormin), unsaturated lactones, alkaloids, phenols, ethylene, ammonia, essential oils, hydrocyanic acid, and organic acids also have been reported to cause dormancy. Growth inhibitors controlling dormancy may be in the embryo, as in a number of grasses; in the hull, as in lettuce and buckwheat; or in the fruit, as in apple and tomato.
Varying degrees of dormancy found in ten varieties of wheat were due to water- or methanol-soluble growth inhibitors, which disappeared after a month or so of dry, warm storage. Leaching or hull removal increases germination of certain grass species. In sorghum, dormancy has been associated with brown pericarp fused with the testa.
The dormancy factor was removed by scarification or by hot water treatments. The endosperm and aleurone of certain species also contained dormancy factors. The embryo and pericarp of wild rice contained inhibitory levels of ABA, which were removed by more than 100 days of cold water (3°C) storage.
Seed dormancy is extremely complex, involving several seed structures, growth stimuli of the environment, endogenous growth substances, exogenous chemicals, and all possible interactions of these factors.
If only four factors are operative —for example, seed coat, temperature, endogenous substance A, and exogenous substance B —12 main effects, 72 primary interactions, or 84 possible causative factors may influence dormancy.
The after-ripening of freshly harvested lettuce is illustrative:
(1) Due to a secondary dormancy (acquired postharvest), germination occurred in a narrow range of low temperatures (15-20°C).
(2) Germination was rapid at a wide temperature range after an adequate period of dry storage, in which dormancy was lost.
(3) Imbibed germinative seeds became dormant (secondary dormancy) if exposed to high temperatures (30-35°C).
(4) The induced (secondary) dormancy did not break down to allow germination unless returned to a narrow range of cool temperatures.
(5) Seeds with secondary, or even true, dormancy germinated readily if exposed to R radiation or GA3.
(6) Germination occurred if seed hulls were removed.
Treatment of imbibed lettuce seeds with coumarin induced dormancy in germinative seeds, just as a high temperature caused secondary dormancy. Primary dormancy (initially developed during seed ontogeny), secondary dormancy or relative dormancy (environmentally imposed in mature seed but germinable only at a narrow temperature range, 15-20°C), or true dormancy, as defined by Borriss (1949) (no germination even at the optimum temperature), are all internally controlled but may be imposed by the environment.
No comments yet.