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seed germination

Seed germination is the most important stage in a plants life cycle. Water, air, temperature and light are all essential for the seed germination process starting from imbibition, activation and succeeding manifestation. Rice seed germination is affected greatly by temperature. Temperatures colder than the favorable range (18–33°C) retards the germination process. Cold temperatures slow down the diffusion process which causes disrupted imbibition and escape of solutes from the seeds. The effect of cold stress is more pronounced at the imbibing phase which is regarded as the most sensitive phase. The exposure of rice seeds to cold stress during this phase causes an increased escape of solutes from the seeds. The standard temperature for rice seed germination is considered to be 30°C. The minimum critical temperature of rice germination is considered as 10°C ( Yoshida, 1981 ). Soil temperatures below 10°C can result in complete failure of germination ( Yoshida, 1981 ). Temperatures below 20°C decrease both the speed and percentage of seed germination ( Yoshida, 1981 ), lower crop stands, and consequently reduce grain yield ( da Cruz and Milach, 2004 ; Cruz et al., 2006 ; Sharifi, 2010 ). Germination speed is related to seedling vigor and it could be a significant determinant of good field performance ( da Cruz and Milach, 2004 ).

Seed germination and seedling establishment are highly sensitive to deficit soil moisture conditions. Although management practices can mitigate such stress, it would be appropriate to develop varieties with intrinsic stress tolerance through rapid imbibition rates. Seed germination in finger millet takes 2–3 days both for laboratory germination and field emergence under adequate moisture conditions. During seed germination under rainfed monsoon conditions, the average soil evaporation would be nearly 3–4 mm d − 1 and seed is placed 2 cm deep, within 2 days after sowing, soil moisture in top layer will be depleted; hence rapid imbibition is necessary for seed germination. The rate of imbibition in rainfed soil will be low and seed germination will be inhibited, hence, it is necessity to determine the optimum soil moisture content required for establishment of crop stand for a given soil. Although, dry conditions for sowing can be managed by way of transplanting, under rainfed conditions, transplanting shock will be high and practically difficult when large area need to be planted, hence direct sowing under optimal conditions would be apt for rainfed situations. Therefore, identification of genotypes suitable to rainfed conditions can be identified both under in vitro and field conditions. The simulated drought conditions can be provided using polyethylene glycol (6000 or 8000 MW) which prevents the entry of water into cell wall of the seed coat, thereby creates drought condition. Further, gravimetric approach using pot culture would be more appropriate that simulate field conditions. Identification of specific trait of rapid seed germination through higher imbibition rates, solute concentration of seed, and incorporation of such traits into ruling varieties would be relevant for drought escape.

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Seed germination , which determines when the plant enters natural or agricultural ecosystems, is a crucial process in the seed plant life cycle and the basis for crop production. The germination of freshly produced seeds is inhibited by primary dormancy, which helps the seeds equip for environments with unfavorable conditions [1–3] . The seeds will enter a germinating state from the dormant state at an appropriate time when the dormancy is lost through moist chilling (stratification) or after-ripening [4] . Therefore, seed germination is a accurately timed checkpoint to avoid unsuitable weather and unfavorable environments during plant establishment and reproductive growth [5] . Finally, seed germination in crops will affect seedling survival rates and vegetative growth, which are accordingly associated with ultimate yield and quality. Considering agronomic production, crop cultivars must be prepared for rapid and uniform germination at sowing, which will improve the crop yield and quality; however, this selection during crop breeding usually results in weak dormancy, which is one of the factors leading to PHS in the rainy season, which tends to overlap with the harvest season [6, 7] . Hence, to improve crop agronomic performance, the crop cultivars during breeding must be prepared for uniform and rapid germination at sowing while preventing PHS [7a] .

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N. Priyanka , . Perumal Venkatachalam , in Advances in Phytonanotechnology , 2019

In some plants, the seeds are able to germinate as soon as they have matured on the plant, as demonstrated by papaya and by wheat, peas, and beans in a very rainy season. Certain mangrove species normally form foot-long embryos on the trees; these later drop down into the mud or sea water. Such cases, however, are exceptional. The lack of dormancy in cultivated species, contrasting with the situation in most wild plants, is undoubtedly the result of conscious selection by humans.

In plants whose seeds ripen and are shed from the mother plant before the embryo has undergone much development beyond the fertilized egg stage (orchids, broomrapes, ginkgo, ash, winter aconite, and buttercups), there is an understandable delay of several weeks or months, even under optimal conditions, before the seedling emerges.

Lack of dormancy

There are at least three ways in which a hard testa may be responsible for seed dormancy: it may (1) prevent expansion of the embryo mechanically, (2) block the entrance of water, or (3) impede gas exchange so that the embryos lack oxygen. Resistance of the testa to water uptake is most widespread in the bean family, the seed coats of which, usually hard, smooth, or even glassy, may, in addition, possess a waxy covering. In some cases water entry is controlled by a small opening, the strophiolar cleft, which is provided with a corklike plug; only removal or loosening of the plug will permit water entry. Similar seeds not possessing a strophiolar cleft must depend on abrasion, which in nature may be brought about by microbial attack, passage through an animal, freezing and thawing, or mechanical means. In horticulture and agriculture, the coats of such seeds are deliberately damaged or weakened by humans ( scarification). In chemical scarification, seeds are dipped into strong sulfuric acid, organic solvents such as acetone or alcohol, or even boiling water. In mechanical scarification, they may be shaken with some abrasive material such as sand or be scratched with a knife.

Dormancy has at least three functions: (1) immediate germination must be prevented even when circumstances are optimal so as to avoid exposure of the seedling to an unfavourable period (e.g., winter), which is sure to follow; (2) the unfavourable period has to be survived; and (3) the various dispersing agents must be given time to act. Accordingly, the wide variation in seed and diaspore longevity can be appreciated only by linking it with the various dispersal mechanisms employed as well as with the climate and its seasonal changes. Thus, the downy seeds of willows, blown up and down rivers in early summer with a chance of quick establishment on newly exposed sandbars, have a life span of only one week. Tropical rainforest trees frequently have seeds of low life expectancy also. Intermediate are seeds of sugarcane, tea, and coconut palm, among others, with life spans of up to a year. Mimosa glomerata seeds in the herbarium of the Muséum National d’Histoire Naturelle in Paris were found viable after 221 years. In general, viability is better retained in air of low moisture content. Some seeds, however, remain viable underwater—those of certain rush (Juncus) species and Sium cicutaefolium for at least 7 years. Salt water can be tolerated for years by the pebblelike but floating seeds of Guilandina bonduc, which in consequence possess an almost pantropical distribution. Seeds of the sacred lotus ( Nelumbo nucifera) found in a peat deposit in Manchuria and estimated by radioactive-carbon dating to be 1,400 ± 400 years old rapidly germinated (and subsequently produced flowering plants) when the seeds were filed to permit water entry. In 1967, seeds of the arctic tundra lupine ( Lupinus arcticus) found in a frozen lemming burrow with animal remains established to be at least 10,000 years old germinated within 48 hours when returned to favourable conditions. The problem of differential seed viability has been approached experimentally by various workers, one of whom buried 20 species of common Michigan weed seeds, mixed with sand, in inverted open-mouthed bottles for periodic inspection. After 80 years, 3 species still had viable seeds. See also soil seed bank.

Frequently seed coats are permeable to water yet block entrance of oxygen; this applies, for example, to the upper of the two seeds normally found in each burr of the cocklebur plant. The lower seed germinates readily under a favourable moisture and temperature regime, but the upper one fails to do so unless the seed coat is punctured or removed or the intact seed is placed under very high oxygen concentrations.

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Until it becomes nutritionally self-supporting, the seedling depends upon reserves provided by the parent sporophyte. In angiosperms these reserves are found in the endosperm, in residual tissues of the ovule, or in the body of the embryo, usually in the cotyledons. In gymnosperms food materials are contained mainly in the female gametophyte. Since reserve materials are partly in insoluble form—as starch grains, protein granules, lipid droplets, and the like—much of the early metabolism of the seedling is concerned with mobilizing these materials and delivering, or translocating, the products to active areas. Reserves outside the embryo are digested by enzymes secreted by the embryo and, in some instances, also by special cells of the endosperm.

Seed dormancy

In many seeds the embryo cannot germinate even under suitable conditions until a certain period of time has lapsed. The time may be required for continued embryonic development in the seed or for some necessary finishing process—known as afterripening—the nature of which remains obscure.

The seeds of many plants that endure cold winters will not germinate unless they experience a period of low temperature, usually somewhat above freezing. Otherwise, germination fails or is much delayed, with the early growth of the seedling often abnormal. (This response of seeds to chilling has a parallel in the temperature control of dormancy in buds.) In some species, germination is promoted by exposure to light of appropriate wavelengths. In others, light inhibits germination. For the seeds of certain plants, germination is promoted by red light and inhibited by light of longer wavelength, in the “far red” range of the spectrum. The precise significance of this response is as yet unknown, but it may be a means of adjusting germination time to the season of the year or of detecting the depth of the seed in the soil. Light sensitivity and temperature requirements often interact, the light requirement being entirely lost at certain temperatures.

Active growth in the embryo, other than swelling resulting from imbibition, usually begins with the emergence of the primary root, known as the radicle, from the seed, although in some species (e.g., the coconut) the shoot, or plumule, emerges first. Early growth is dependent mainly upon cell expansion, but within a short time cell division begins in the radicle and young shoot, and thereafter growth and further organ formation (organogenesis) are based upon the usual combination of increase in cell number and enlargement of individual cells.