Germination sometimes occurs early in the development process; the mangrove (Rhizophora) embryo develops within the ovule, pushing out a swollen rudimentary root through the still-attached flower. In peas and corn (maize) the cotyledons (seed leaves) remain underground (e.g., hypogeal germination), while in other species (beans, sunflowers, etc.) the hypocotyl (embryonic stem) grows several inches above the ground, carrying the cotyledons into the light, in which they become green and often leaflike (e.g., epigeal germination).
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.
Dormancy is brief for some seeds—for example, those of certain short-lived annual plants. After dispersal and under appropriate environmental conditions, such as suitable temperature and access to water and oxygen, the seed germinates, and the embryo resumes growth.
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.
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.
Md. Salim Azad , in Exotic Fruits , 2018
Seed germination is a transit process when an active plant with photosynthesis grows from a quiescent embryo, generated in the fertilized ovule. The process of seed germination includes the following five changes or steps: imbibition, respiration, effect of light on seed germination, mobilization of reserves during seed germination, and role of growth regulators and development of the embryo axis into a seedling. All five of these stages result from a interplay of several metabolic and cellular events, coordinated by a complex regulatory network that includes seed dormancy, an intrinsic ability to temporarily block radicle elongation to optimize the timing of germination. The primary plant hormones including abscisic acid (ABA) and gibberellin (GA) antagonistically regulate seed dormancy and germination [8–10] . ABA is synthesized during seed maturation and decreased before the onset of germination; it plays key roles in inhibiting germination and establishing and maintaining seed dormancy  . In contrast to ABA, GA significantly increases to promote germination by causing the secretion of hydrolytic enzymes that weaken the structure of the seed testa [12, 13] .
Praveen K. Kathare , Enamul Huq , in Reference Module in Life Sciences , 2020
3.1 Seed Germination and Seedling Growth
Seed germination is one of the most crucial phases in the plant growth and development, and is under the tight regulation of phy-mediated light signaling as well as two hormones, abscisic acid (ABA) and gibberellic acid (GA), that function antagonistically. Under unfavorable conditions, ABA maintains seed dormancy, while GA promotes the seed germination in response to favorable environmental conditions. Light-dependent activation of phys promotes biosynthesis of GA and represses ABA biosynthesis ( Paik and Huq, 2019 ). PIF1 and PIF8, two of the downstream signaling components of phys, are repressors of seed germination under dark conditions. PIF1 represses the seed germination either directly or indirectly through inhibition of GA signaling pathway. However, in response to light illumination, phys translocate to the nucleus and degrade PIF1 and PIF8, relieving the repression, which results in seed germination ( Oh et al., 2020; Legris et al., 2019 ). The light-induced promotion of seed germination ensures that seeds are germinated under favorable environmental conditions for increased survival.
Seed germination is the first phase of the growth cycle in plants ( Parihar et al., 2015 ). Salinity adversely affects seed germination, excess amount of soluble salt content into the soil reduces the water potential into the soil. As water moves from higher water potential to lower water potential, seeds are unable to take water from saline soil and causes hormonal imbalance ( Khan and Rizvi, 1994 ), reduces protein metabolism ( Dantas et al., 2007 ), nucleic acid metabolism ( Gomes-Filho et al., 2008 ) and ultimately reduces the utilization of seed reserves ( Othman et al., 2006 ). There are some evident that salinity drastically affects the seed germination in various plants like Oryza saliva ( Xu et al., 2011 ), Triticum aestivum ( Akbarimoghaddam et al., 2011 ), Zea mays ( Khodarahmpour et al., 2012 ), Brassicaspp. ( Akram and Jamil, 2007 ). Bybordi (2010) reported that with the increasing salt concentration the rate of seed germination decreases in Brassica napus ( Bybordi, 2010 ).
Y.A. Nanja Reddy , . S. Sanjeev Krishna , in Millets and Pseudo Cereals , 2021
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.
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.
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.
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.
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.
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.