Germination
Germination is the process by which an
Seed plants
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 of radicle and plumule. The seed of a vascular plant is a small package produced in a fruit or cone after the union of male and female reproductive cells. All fully developed seeds contain an embryo 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]
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 right
- 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 cellular chemicals.[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 point photosynthesisprovides 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 for atmospheric gas that is found in soilpore 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 as rice, anaerobic germination can occur in waterlogged conditions. The seed produces a hollow coleoptile that acts like a 'snorkel', providing the seed with access to oxygen.[4]
- Temperature affects cellular metabolic 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 (forest firewhich cracks their seed coats; this is a type of physical dormancy.
Most common annual vegetables have optimal germination temperatures between 75–90 F (24–32 C), though many species (e.g. radishes or spinach) 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 many photoblastic 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]
- digestive tract to weaken the seed coat enough to allow the seedling to emerge.[2]
Dormancy
Some live seeds are dormant 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, notably abscisic acid, which inhibits germination, and gibberellin, which ends seed dormancy. In brewing, barley seeds are treated with gibberellin to ensure uniform seed germination for the production of barley malt.[2]
Seedling establishment
In some definitions, the appearance of the radicle 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 of phytotoxicity 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.
Germination rate and germination capacity
In agriculture and gardening, the germination rate describes how many seeds of a particular plant species, 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 of sowing. On the other hand, the number of seed able to complete germination in a population (i.e. seed lot) is referred to as germination capacity.
Repair of DNA damage
Seed quality deteriorates with age, and this is associated with accumulation of genome damage.[7] During germination, repair processes are activated to deal with accumulated DNA damage.[8] In particular, single- and double-strand breaks in DNA can be repaired.[9] The DNA damage checkpoint kinase ATM has a major role in integrating progression through germination with repair responses to the DNA damages accumulated by the aged seed.[10]
Dicot germination
The part of the plant that first emerges from the seed is the embryonic root, termed the radicle or primary root. It allows the seedling to become anchored in the ground and start absorbing water. After the root absorbs water, an embryonic shoot emerges from the seed. This shoot comprises three main parts: the cotyledons (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
Hypogeal
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. Peas, chickpeas and mango, for example, germinate this way.[11]
Monocot germination
In
Precocious germination
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.
Pollen germination
Another germination event during the life cycle of gymnosperms and flowering plants is the germination of a pollen grain after pollination. 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 a tube cell. Once the pollen grain lands on the stigma of a receptive flower (or a female cone in gymnosperms), it takes up water and germinates. Pollen germination is facilitated by hydration on the stigma, as well as by the structure and physiology of the stigma and style.[2] Pollen can also be induced to germinate in vitro (in a petri dish or test tube).[12][13]
During germination, the tube cell elongates into a pollen tube. In the flower, the pollen tube then grows towards the ovule where it discharges the sperm produced in the pollen grain for fertilization. The germinated pollen grain with its two sperm cells is the mature male microgametophyte of these plants.[2]
Self-incompatibility
Since most plants carry both male and female reproductive organs in their flowers, there is a high risk of self-pollination and thus
Spore germination
Germination can also refer to the emergence of cells from
Resting spores
In
Ferns and mosses
In
Bacteria
Bacterial spores can be
Light-stimulated germination
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.[20] 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 of PIF1. PIF1, phytochrome-interaction-factor-1, negatively regulates germination by increasing the expression of proteins that repress the synthesis of gibberellin (GA), a major hormone in the germination process.[21] Another factor that promotes germination is HFR1 which accumulates in light in some way and forms inactive heterodimers with PIF1.[22]
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.[20] Bethke et al. (2006) exposed dormant Arabidopsis 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 of Abscisic acid (ABA), a plant hormone largely responsible for seed dormancy.[23] 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.[24] 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.
See also
- Lily seed germination types
- Oldest viable seed
- Pot farm
- Pyrophyte, for germination after fire
- Seed tray
- Seedling
- Sprouting
- Urban horticulture
- Vivipary, when seeds or embryos begin to develop inside or before they detach from the parent
References
- ^ Forensic Botany. Wiley-Blackwell. 2012. p. 10.
- ^ ISBN 978-0-7167-1007-3.
- .
- PMID 18660495. Retrieved 27 March 2022.
- ISBN 9780124166837.
- ISBN 978-0-85199-723-0.
- PMID 25750428.
- PMID 11321247.
- PMID 20584150.
- PMID 27503884.
- ISBN 978-81-224-0065-6.
- PMID 16658085.
- PMID 7460877.
- S2CID 1196223.
- PMID 15587065.
- PMID 15879525.
- ISBN 978-0-08-086087-9.
- ISBN 978-0-538-74125-5.
- ISBN 978-0-85229-787-2.
- ^ PMID 28898656.
- PMID 26905653.
- PMID 29248678.
- PMID 17220360.
- PMID 26095078.
Further reading
- Rajjou L, Duval M, Gallardo K, Catusse J, Bally J, Job C, Job D (2012). "Seed germination and vigor" (PDF). Annual Review of Plant Biology. 63: 507–33. PMID 22136565.
- Deno NC (1980). Seed Germination: Theory and Practice. State College, PA. )
External links
- Sowing Seeds, a survey of seed sowing techniques
- Germination time-lapse, ≈1 minute HD video of mung bean seeds germinating over 10 days, on YouTube