Editing Seed (section)

== Germination ==
{{Main|Seedling|Germination}}
[[File:Sunflower seedlings.jpg|thumb|Germinating [[sunflower]] seedlings]]

Seed germination is a process by which a seed embryo develops into a seedling. It involves the reactivation of the metabolic pathways that lead to growth and the emergence of the radicle or seed root and plumule or shoot. The emergence of the seedling above the soil surface is the next phase of the plant's growth and is called seedling establishment.<ref name="the_encyclopedia_of_seeds">{{cite book  | last1 = Black | first1 = Michael H. | last2 = Halmer | first2 = Peter | title = The encyclopedia of seeds: science, technology and uses | url = https://archive.org/details/encyclopediaofse00mich | url-access = limited | year = 2006 | publisher = CABI | location = Wallingford, UK  | isbn = 978-0-85199-723-0 | page = [https://archive.org/details/encyclopediaofse00mich/page/n251 224]  }}</ref>

Three fundamental conditions must exist before germination can occur. (1) The embryo must be alive, called seed viability. (2) Any dormancy requirements that prevent germination must be overcome. (3) The proper environmental conditions must exist for germination.

Far red light can prevent germination.<ref>{{cite book |title=Photobiology: The Science of Life and Light |date=26 December 2007 |publisher=Springer |url=https://books.google.com/books?id=bRWd5bGhXM4C |page=147|isbn=9780387726557 }}</ref>

Seed viability is the ability of the embryo to germinate and is affected by a number of different conditions. Some plants do not produce seeds that have functional complete embryos, or the seed may have no embryo at all, often called empty seeds. Predators and pathogens can damage or kill the seed while it is still in the fruit or after it is dispersed. Environmental conditions like flooding or heat can kill the seed before or during germination. The age of the seed affects its health and germination ability: since the seed has a living embryo, over time cells die and cannot be replaced. Some seeds can live for a long time before germination, while others can only survive for a short period after dispersal before they die.{{fact|date=May 2024}}

'''Seed vigor''' is a measure of the quality of seed, and involves the viability of the seed, the germination percentage, germination rate, and the strength of the seedlings produced.<ref>[http://extension.osu.edu/~seedsci/svvt01.html Seed Vigor and Vigor Tests] {{webarchive|url=https://web.archive.org/web/20060912050649/http://extension.osu.edu/~seedsci/svvt01.html |date=2006-09-12 }}</ref>

The '''germination percentage''' is simply the proportion of seeds that germinate from all seeds subject to the right conditions for growth. The '''germination rate''' is the length of time it takes for the seeds to germinate. Germination percentages and rates are affected by seed viability, dormancy and environmental effects that impact on the seed and seedling. In agriculture and horticulture quality seeds have high viability, measured by germination percentage plus the rate of germination. This is given as a percent of germination over a certain amount of time, 90% germination in 20 days, for example. 'Dormancy' is covered above; many plants produce seeds with varying degrees of dormancy, and different seeds from the same fruit can have different degrees of dormancy.<ref>International Seed Testing Association. 1973. {{ISSN|0251-0952}}. pp. 120–121.''Seed science and technology''. Wageningen?: International Seed Testing Association.</ref> It's possible to have seeds with no dormancy if they are dispersed right away and do not dry (if the seeds dry they go into physiological dormancy). There is great variation amongst plants and a dormant seed is still a viable seed even though the germination rate might be very low.

Environmental conditions affecting seed germination include; water, oxygen, temperature and light.

Three distinct phases of seed germination occur: water imbibition; lag phase; and [[radicle]] emergence.

In order for the seed coat to split, the embryo must imbibe (soak up water), which causes it to swell, splitting the seed coat. However, the nature of the seed coat determines how rapidly water can penetrate and subsequently initiate [[germination]]. The rate of imbibition is dependent on the permeability of the seed coat, amount of water in the environment and the area of contact the seed has to the source of water. For some seeds, imbibing too much water too quickly can kill the seed. For some seeds, once water is imbibed the germination process cannot be stopped, and drying then becomes fatal. Other seeds can imbibe and lose water a few times without causing ill effects, but drying can cause secondary dormancy.

=== Repair of DNA damage ===
During seed [[dormancy]], often associated with unpredictable and stressful environments, [[DNA repair#DNA damage|DNA damage]] accumulates as the seeds age.<ref name=Cheah>{{cite journal |author1=Cheah KS |author2=Osborne DJ |title=DNA lesions occur with loss of viability in embryos of ageing rye seed |journal=Nature |volume=272 |issue=5654 |pages=593–599 |date=April 1978  |pmid=19213149 |doi= 10.1038/272593a0|bibcode=1978Natur.272..593C |s2cid=4208828 }}</ref><ref name=Koppen>{{cite journal |author1=Koppen G |author2=Verschaeve L |title=The alkaline single-cell gel electrophoresis/comet assay: a way to study DNA repair in radicle cells of germinating ''Vicia faba'' |journal=Folia Biol. (Praha) |volume=47 |issue=2 |pages=50–54 |year=2001 |pmid=11321247 }}</ref><ref name="pmid16313635">{{cite journal |author1=Bray CM |author2=West CE |title=DNA repair mechanisms in plants: crucial sensors and effectors for the maintenance of genome integrity |journal=New Phytol. |volume=168 |issue=3 |pages=511–528 |date=December 2005  |pmid=16313635 |doi=10.1111/j.1469-8137.2005.01548.x |doi-access= |bibcode=2005NewPh.168..511B }}</ref> In [[rye]] seeds, the reduction of DNA integrity due to damage is associated with loss of seed viability during storage.<ref name=Cheah /> Upon germination, seeds of ''[[Vicia faba]]'' undergo [[DNA repair]].<ref name=Koppen /> A plant DNA [[ligase]] that is involved in repair of single- and double-strand breaks during seed germination is an important determinant of seed longevity.<ref name="pmid20584150">{{cite journal |author1=Waterworth WM |author2=Masnavi G |author3=Bhardwaj RM |author4=Jiang Q |author5=Bray CM |author6=West CE |title=A plant DNA ligase is an important determinant of seed longevity |journal=Plant J. |volume=63 |issue=5 |pages=848–860 |date=September 2010  |pmid=20584150 |doi=10.1111/j.1365-313X.2010.04285.x |doi-access=free }}</ref> Also, in [[Arabidopsis]] seeds, the activities of the DNA repair enzymes [[Poly ADP ribose polymerase]]s (PARP) are likely needed for successful germination.<ref name="pmid17587307">{{cite journal |author1=Hunt L |author2=Holdsworth MJ |author3=Gray JE |title=Nicotinamidase activity is important for germination |journal=Plant J. |volume=51 |issue=3 |pages=341–351 |date=August 2007  |pmid=17587307 |doi=10.1111/j.1365-313X.2007.03151.x }}</ref> Thus DNA damages that accumulate during [[dormancy]] appear to be a problem for seed survival, and the enzymatic repair of DNA damages during germination appears to be important for seed viability.

=== Inducing germination ===
A number of different strategies are used by gardeners and horticulturists to break [[seed dormancy]].

'''[[Scarification (botany)|Scarification]]''' allows water and gases to penetrate into the seed; it includes methods to physically break the hard seed coats or soften them by chemicals, such as soaking in hot water or poking holes in the seed with a pin or rubbing them on sandpaper or cracking with a press or hammer. Sometimes fruits are harvested while the seeds are still immature and the seed coat is not fully developed and sown right away before the seed coat become impermeable. Under natural conditions, seed coats are worn down by rodents chewing on the seed, the seeds rubbing against rocks (seeds are moved by the wind or water currents), by undergoing freezing and thawing of surface water, or passing through an animal's digestive tract. In the latter case, the seed coat protects the seed from [[digestion]], while often weakening the seed coat such that the embryo is ready to sprout when it is deposited, along with a bit of fecal matter that acts as fertilizer, far from the parent plant. [[Microorganism]]s are often effective in breaking down hard seed coats and are sometimes used by people as a treatment; the seeds are stored in a moist warm sandy medium for several months under nonsterile conditions.

'''[[Stratification (botany)|Stratification]]''', also called moist-chilling, breaks down physiological dormancy, and involves the addition of moisture to the seeds so they absorb water, and they are then subjected to a period of moist chilling to after-ripen the embryo. Sowing in late summer and fall and allowing to overwinter under cool conditions is an effective way to stratify seeds; some seeds respond more favorably to periods of oscillating temperatures which are a part of the natural environment.

'''Leaching''' or the soaking in water removes chemical inhibitors in some seeds that prevent germination. [[Rain]] and melting [[snow]] naturally accomplish this task. For seeds planted in gardens, running water is best&nbsp;– if soaked in a container, 12 to 24 hours of soaking is sufficient. Soaking longer, especially in stagnant water, can result in oxygen starvation and seed death. Seeds with hard seed coats can be soaked in hot water to break open the impermeable cell layers that prevent water intake.

Other methods used to assist in the germination of seeds that have dormancy include prechilling, predrying, daily alternation of temperature, light exposure, potassium nitrate, the use of plant growth regulators, such as gibberellins, cytokinins, ethylene, [[thiourea]], sodium hypochlorite, and others.<ref>Hartmann, Hudson Thomas, and Dale E. Kester. 1983. ''Plant propagation principles and practices''. Englewood Cliffs, NJ: Prentice-Hall. {{ISBN|0-13-681007-1}}. pp. 175–177.</ref> Some seeds germinate best after a fire. For some seeds, fire cracks hard seed coats, while in others, chemical dormancy is broken in reaction to the presence of smoke. Liquid smoke is often used by gardeners to assist in the germination of these species.<ref>{{cite journal|title=Trace Gas Emissions and Smoke-Induced Seed Germination|author=Jon E. Keeley and Fotheringham|issue=5316|pages=1248–1250|journal=Science|doi=10.1126/science.276.5316.1248|volume=276|date=1997-05-23|citeseerx=10.1.1.3.2708}}</ref>

=== Sterile seeds ===
Seeds may be sterile for few reasons: they may have been irradiated, unpollinated, cells lived past expectancy, or bred for the purpose.

=== Evolution and origin of seeds ===
The issue of the origin of seed plants remains unsolved. However, more and more data tends to place this origin in the middle [[Devonian]]. The description in 2004 of the proto-seed ''Runcaria heinzelinii'' in the [[Givetian]] of [[Belgium]] is an indication of that ancient origin of seed-plants. As with modern ferns, most land plants before this time reproduced by sending into the air [[spore]]s that would land and become whole new plants.

Taxonomists have described early "true" seeds from the upper Devonian, which probably became the theater of their true first [[evolutionary radiation]]. With this radiation came an [[evolution of seed size]], shape, dispersal and eventually the radiation of gymnosperms and angiosperms and [[monocotyledon]]s and [[dicotyledon]]s. Seed plants progressively became one of the major elements of nearly all ecosystems.

=== True to the seed ===

Also called growing true, refers to plants whose seed will yield the same type of plant as the original plant. Open pollinated plants, which include heirlooms, will almost always grow true to seed if another variety does not cross-pollinate them.