Sunflower seedlings, three days after germinationSunflower time lapse with soil. Cross section, showing how the root and the upper part of the plant grow
A seed pot used in horticulture for sowing and taking plant cuttings and growingplugsGermination glass (glass sprouter jar) with a plasticsieve-lidBrassica campestris germinating seedsTime-lapse video of mung bean seeds germinating
Germination is usually the growth of a plant contained within a seed resulting in the formation of the seedling. It is also the process of reactivation of metabolic machinery of the seed resulting in the emergence ofradicle andplumule. The seed of avascular plant is a small package produced in afruit orcone after the union of male and femalereproductive cells. All fully developed seeds contain anembryo and, in most plant species some store of food reserves, wrapped in a seed coat. Dormant seeds are viable seeds that do not germinate because they require specific internal or environmental stimuli to resume growth. Under proper conditions, the seed begins to germinate and the embryo resumes growth, developing into a seedling.[clarification needed]
Step 1: Water imbibition, the uptake of water, results in rupture of seed coat. Step 2: The imbibition of the seed coat results in emergence of theradicle (1) and theplumule (2); thecotyledons are unfolded (3). Step 3: This marks the final step in the germination of the seed, where the cotyledons are expanded, which are the true leaves. Note: Temperature must be kept at an optimum level.
Disturbance of soil can result in vigorous plant growth by exposing seeds already in the soil to changes in environmental factors where germination may have previously been inhibited by depth of the seeds or soil that was too compact. This is often observed at gravesites after a burial.[1]
Seed germination depends on both internal and external conditions. The most important external factors include righttemperature,water,oxygen orair and sometimeslight ordarkness.[2] Various plants require different variables for successful seed germination. Often this depends on the individual seed variety and is closely linked to theecological conditions of a plant'snatural habitat. For some seeds, their future germination response is affected by environmental conditions during seed formation; most often these responses are types ofseed dormancy.
Water is required for germination. Mature seeds are often extremely dry and need to take in significant amounts of water, relative to the dry weight of the seed, before cellularmetabolism and growth can resume. Most seeds need enough water to moisten the seeds but not enough to soak them. The uptake of water by seeds is calledimbibition, which leads to the swelling and the breaking of the seed coat. When seeds are formed, most plants store a food reserve with the seed, such asstarch,proteins, oroils. This food reserve provides nourishment to the growing embryo. When the seed imbibes water,hydrolytic enzymes are activated which break down these stored food resources into metabolically usefulchemicals.[2] After the seedling emerges from the seed coat and starts growing roots and leaves, the seedling's food reserves are typically exhausted; at this pointphotosynthesis provides the energy needed for continued growth and the seedling now requires a continuous supply of water, nutrients, and light.
Oxygen is required by the germinating seed formetabolism.[3] Oxygen is used inaerobic respiration, the main source of the seedling's energy until it grows leaves.[2] Oxygen is anatmospheric gas that is found insoil pore spaces; if a seed is buried too deeply within the soil or the soil is waterlogged, the seed can be oxygen starved. Some seeds have impermeable seed coats that prevent oxygen from entering the seed, causing a type of physical dormancy which is broken when the seed coat is worn away enough to allow gas exchange and water uptake from the environment.
In a small number of plants, such asrice, anaerobic germination can occur in waterlogged conditions. The seed produces a hollowcoleoptile that acts like a 'snorkel', providing the seed with access to oxygen.[4]
Temperature affects cellular metabolism and growth rates. Seeds from different species and even seeds from the same plant germinate over a wide range of temperatures. Seeds often have a temperature range within which they will germinate, and they will not do so above or below this range. Many seeds germinate at temperatures slightly above 60–75 F (16–24 C) [room-temperature in centrally heated houses], while others germinate just above freezing and others germinate only in response to alternations in temperature between warm and cool. Some seeds germinate when the soil is cool 28–40 F (-2 – 4 C), and some when the soil is warm 76–90 F (24–32 C). Some seeds require exposure to cold temperatures (vernalization) to break dormancy. Some seeds in a dormant state will not germinate even if conditions are favorable. Seeds that are dependent on temperature to end dormancy have a type of physiological dormancy. For example, seeds requiring the cold of winter are inhibited from germinating until they take in water in the fall and experience cooler temperatures. Coldstratification is a process that induces the dormancy breaking prior to light emission that promotes germination .[5] Four degrees Celsius is cool enough to end dormancy for most cool dormant seeds, but some groups, especially within the familyRanunculaceae and others, need conditions cooler than -5 C. Some seeds will only germinate after hot temperatures during aforest fire which cracks their seed coats; this is a type of physical dormancy.
Most common annualvegetables have optimal germination temperatures between 75–90 F (24–32 C), though many species (e.g.radishes orspinach) can germinate at significantly lower temperatures, as low as 40 F (4 C), thus allowing them to be grown from seeds in cooler climates. Suboptimal temperatures lead to lower success rates and longer germination periods.
Light or darkness can be an environmental trigger for germination and is a type of physiological dormancy. Most seeds are not affected by light or darkness, but manyphotoblastic seeds, including species found in forest settings, will not germinate until an opening in the canopy allows sufficient light for the growth of the seedling.[2]
Scarification mimics natural processes that weaken the seed coat before germination. In nature, some seeds require particular conditions to germinate, such as the heat of a fire (e.g., many Australian native plants), or soaking in a body of water for a long period of time. Others need to be passed through an animal'sdigestive tract to weaken the seed coat enough to allow the seedling to emerge.[2]
Some live seeds aredormant and need more time, and/or need to be subjected to specific environmental conditions before they will germinate.Seed dormancy can originate in different parts of the seed, for example, within the embryo; in other cases the seed coat is involved. Dormancy breaking often involves changes in membranes, initiated by dormancy-breaking signals. This generally occurs only within hydrated seeds.[6] Factors affecting seed dormancy include the presence of certain plant hormones, notablyabscisic acid, which inhibits germination, andgibberellin, which ends seed dormancy. Inbrewing, barley seeds are treated with gibberellin to ensure uniform seed germination for the production of barleymalt.[2]
In some definitions, the appearance of theradicle marks the end of germination and the beginning of "establishment", a period that utilizes the food reserves stored in the seed. Germination and establishment as an independent organism are critical phases in the life of a plant when they are the most vulnerable to injury, disease, and water stress.[2] The germination index can be used as an indicator ofphytotoxicity in soils. The mortality between dispersal of seeds and completion of the establishment can be so high that many species have adapted to produce large numbers of seeds.[citation needed]
Inagriculture andgardening, thegermination rate describes how many seeds of a particularplantspecies, variety or seedlot are likely to germinate over a given period. It is a measure of germination time course and is usually expressed as a percentage, e.g., an 85% germination rate indicates that about 85 out of 100 seeds will probably germinate under proper conditions over the germination period given. Seed germination rate is determined by the seed genetic composition, morphological features and environmental factors.[citation needed] The germination rate is useful for calculating the number of seeds needed for a given area or desired number of plants. For seed physiologists and seed scientists "germination rate" is the reciprocal of time taken for the process of germination to complete starting from time ofsowing. On the other hand, the number of seed able to complete germination in a population (i.e. seed lot) is referred to asgermination capacity.
Soil salinity is one of the stress factors that can limit the germination rate. Environmental stress activates some stress-related activities [CuZn-superoxide dismutase (SOD), Mn-SOD,L-ascorbate oxidase (AO),DNA polymerase Delta 1 (POLD)-1, Chaperon (CHAPE) and heat shock protein (HSP)-21], genetic template stability and photosynthetic pigment activation.[7] Application of exogenicglutamine limiting this process. Research carried out ononion seeds shows a reduction in the mean germination time, an increase in the coefficient of germination velocity, the germination index and germination percentage after administration of exogenous glutamine to plants.[7]
Seed quality deteriorates with age, and this is associated with accumulation of genome damage.[8] During germination,repair processes are activated to deal with accumulatedDNA damage.[9] In particular, single- and double-strand breaks in DNA can be repaired.[10] The DNA damage checkpoint kinaseATM has a major role in integrating progression through germination with repair responses to the DNA damages accumulated by the aged seed.[11]
The stages of germination of a pea plant: A. seed coat, B. radicle, C. primary root, D. secondary root, E. cotyledon, F. plumule, G. leaf, H. tap root
The part of the plant that first emerges from the seed is the embryonic root, termed theradicle or primary root. It allows the seedling to become anchored in the ground and start absorbing water. After the root absorbs water, an embryonicshoot, termed the plumule emerges from the seed. This shoot comprises three main parts: thecotyledons (seed leaves), the section of shoot below the cotyledons (hypocotyl), and the section of shoot above the cotyledons (epicotyl). The way the shoot emerges differs among plant groups.[2]
Epigeal germination (or epigeous germination) is a botanical term indicating that the germination takes place above the ground. In epigeal germination, thehypocotyl elongates and forms a hook, pulling rather than pushing thecotyledons andapical meristem through the soil. Once it reaches the surface, it straightens and pulls the cotyledons and shoot tip of the growing seedlings into the air.Beans, tamarind, and papaya are examples of plants that germinate this way.[2]
Germination can also be done by hypogeal germination (or hypogeous germination), where the epicotyl elongates and forms the hook. In this type of germination, the cotyledons stay underground where they eventually decompose. For example: peas,chickpeas and mango germinate this way.[12]
Inmonocot seeds, the embryo's radicle and cotyledon are covered by acoleorhiza andcoleoptile, respectively. The coleorhiza is the first part to grow out of the seed, followed by the radicle. The coleoptile is then pushed up through the ground until it reaches the surface. There, it stops elongating and the first leaves emerge.[2]
When a seed germinates without undergoing all four stages of seed development, i.e., globular, heart shape, torpedo shape, and cotyledonary stage, it is known as precocious germination.[citation needed]
Another germination event during the life cycle ofgymnosperms andflowering plants is the germination of a pollen grain afterpollination. Like seeds,pollen grains are severely dehydrated before being released to facilitate their dispersal from one plant to another. They consist of a protective coat containing several cells (up to 8 in gymnosperms, 2–3 in flowering plants). One of these cells is atube cell. Once the pollen grain lands on thestigma of a receptiveflower (or a femalecone in gymnosperms), it takes up water and germinates. Pollen germination is facilitated byhydration on the stigma, as well as by the structure andphysiology of the stigma and style.[2] Pollen can also be induced to germinatein vitro (in apetri dish or test tube).[13][14]
During germination, the tube cell elongates into apollen tube. In the flower, the pollen tube then grows towards theovule where it discharges thesperm produced in the pollen grain for fertilization. The germinated pollen grain with its two sperm cells is the mature malemicrogametophyte of these plants.[2]
Since most plants carry both male and female reproductive organs in their flowers, there is a high risk of self-pollination and thusinbreeding. Some plants use the control of pollen germination as a way to prevent this self-pollination. Germination and growth of the pollen tube involve molecular signaling between stigma and pollen. Inself-incompatibility in plants, the stigma of certain plants can molecularly recognize pollen from the same plant and prevent it from germinating.[15]
Conidia are asexual reproductive (reproduction without the fusing of gametes) spores of fungi which germinate under specific conditions. A variety of cells can be formed from the germinating conidia. The most common are germ tubes which grow and develop into hyphae. The initial formation and subsequent elongation of the germ tube in the fungusAspergillus niger has been captured in 3D usingholotomography microscopy. Another type of cell is a conidial anastomosis tube (CAT); these differ from germ tubes in that they are thinner, shorter, lack branches, exhibit determinate growth and home toward each other. Each cell is of a tubular shape, but the conidial anastomosis tube forms a bridge that allows fusion between conidia.[16][17]
3D-visualization ofAspergillus niger spore germination. This image has been captured usingholotomography microscopy.
Inresting spores, germination involves cracking the thick cell wall of the dormant spore. For example, inzygomycetes the thick-walled zygosporangium cracks open and thezygospore inside gives rise to the emerging sporangiophore. Inslime molds, germination refers to the emergence ofamoeboid cells from the hardened spore. After cracking the spore coat, further development involves cell division, but not necessarily the development of a multicellular organism (for example in the free-living amoebas of slime molds).[2]
Inplants such asbryophytes,ferns, and a few others, spores germinate into independentgametophytes. In the bryophytes (e.g.,mosses andliverworts), spores germinate intoprotonemata, similar to fungal hyphae, from which the gametophyte grows. Inferns, the gametophytes are small, heart-shapedprothalli that can often be found underneath a spore-shedding adult plant.[2]
Bacterial spores can beexospores orendospores which are dormant structures produced by a number of different bacteria. They have no or very low metabolic activity and are formed in response to adverse environmental conditions.[18] They allow survival and are not a form of reproduction.[19] Under suitable conditions the spore germinates to produce a viable bacterium. Endospores are formed inside the mother cell, whereas exospores are formed at the end of the mother cell as a bud.[20]
As mentioned earlier,light can be an environmental factor that stimulates the germination process. The seed needs to be able to determine when is the perfect time to germinate and they do that by sensing environmental cues. Once germination starts, the stored nutrients that have accumulated during maturation start to be digested which then supports cell expansion and overall growth.[21] Within light-stimulated germination,phytochrome B (PHYB) is the photoreceptor that is responsible for the beginning stages of germination. When red light is present, PHYB is converted to its active form and moves from the cytoplasm to the nucleus where it upregulates the degradation ofPIF1. PIF1, phytochrome-interaction-factor-1, negatively regulates germination by increasing the expression of proteins that repress the synthesis ofgibberellin (GA), a major hormone in the germination process.[22] Another factor that promotes germination is HFR1 which accumulates in light in some way and forms inactive heterodimers with PIF1.[23]
Although the exact mechanism is not known,nitric oxide (NO) plays a role in this pathway as well. NO is thought to repress PIF1 gene expression and stabilises HFR1 in some way to support the start of germination.[21] Bethke et al. (2006) exposed dormantArabidopsis seeds to NO gas and within the next 4 days, 90% of the seeds broke dormancy and germinated. The authors also looked at how NO and GA effects the vacuolation process of aleurone cells that allow the movement of nutrients to be digested. A NO mutant resulted in inhibition of vacuolation but when GA was later added the process was active again leading to the belief that NO is prior to GA in the pathway. NO may also lead to the decrease in sensitivity ofabscisic acid (ABA), a plant hormone largely responsible for seed dormancy.[24] The balance between GA and ABA is important. When ABA levels are higher than GA then that leads to dormant seeds and when GA levels are higher, seeds germinate.[25] The switch between seed dormancy and germination needs to occur at a time when the seed has the best chances of surviving and an important cue that begins the process of seed germination and overall plant growth is light.[citation needed]
^Baskin CC, Baskin JM (2014).Variation in Seed Dormancy and Germination within and between Individuals and Populations of a Species. Seeds: Ecology, Biogeography, and, Evolution of Dormancy and Germination. Burlington: Elsevier Science. pp. 5–35.ISBN9780124166837.
^Koppen G, Verschaeve L (2001). "The alkaline single-cell gel electrophoresis/comet assay: a way to study DNA repair in radicle cells of germinating Vicia faba".Folia Biologica.47 (2):50–4.PMID11321247.
^Li R, Jia Y, Yu L, Yang W, Chen Z, Chen H, Hu X (February 2018). "Nitric oxide promotes light-initiated seed germination by repressing PIF1 expression and stabilizing HFR1".Plant Physiology and Biochemistry.123:204–212.Bibcode:2018PlPB..123..204L.doi:10.1016/j.plaphy.2017.11.012.PMID29248678.
Deno NC (1980).Seed Germination: Theory and Practice. State College, PA.OCLC918148836.An extensive study of the germination rates of a huge variety of seeds under different experimental conditions, including temperature variation and chemical environment{{cite book}}: CS1 maint: location missing publisher (link)
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