Pollen is a powdery substance produced by most types of flowers ofseed plants for the purpose of sexual reproduction.[1] It consists of pollen grains (highly reducedmicrogametophytes), which produce malegametes (sperm cells).
Pollen grains have a hard coat made ofsporopollenin that protects the gametophytes during the process of their movement from thestamens to thepistil of flowering plants, or from the malecone to the female cone ofgymnosperms. If pollen lands on a compatible pistil or female cone, itgerminates, producing apollen tube that transfers thesperm to theovule containing the female gametophyte. Individual pollen grains are small enough to require magnification to see detail. The study of pollen is calledpalynology and is highly useful inpaleoecology,paleontology,archaeology, andforensics.
Pollen in plants is used for transferringhaploid male genetic material from theanther of a single flower to thestigma of another in cross-pollination.[2] In a case of self-pollination, this process takes place from the anther of a flower to the stigma of the same flower.[2]
Pollen is infrequently used as food andfood supplement. Because of agricultural practices, it is often contaminated by agricultural pesticides.[3]
Pollen itself is not the male gamete.[4] It is agametophyte, something that could be considered an entire organism, which then produces the male gamete. Each pollen grain contains vegetative (non-reproductive) cells (only a single cell in most flowering plants but several in other seed plants) and a generative (reproductive) cell. In flowering plants the vegetative tube cell produces thepollen tube, and the generative cell divides to form the two sperm nuclei.
Pollen grains come in a wide variety of shapes, sizes, and surface markings characteristic of the species (seeelectron micrograph, right). Pollen grains ofpines,firs, andspruces are winged. The smallest pollen grain, that of theforget-me-not (Myosotis spp.),[which?] is 2.5–5 μm (0.005 mm) in diameter.[5] Corn pollen grains are large, about 90–100 μm.[6] Most grass pollen is around 20–25 μm.[5] Some pollen grains are based ongeodesic polyhedra like asoccer ball.[7]
Pollen microspores ofLycopersicon esculentum at coenocytic tetrad stage of development observed through oil immersion microscope; the chromosomes of what will become four pollen grains can be seen.
In angiosperms, during flower development the anther is composed of a mass of cells that appear undifferentiated, except for a partially differentiated dermis. As the flower develops, fertile sporogenous cells, thearchespore, form within the anther. The sporogenous cells are surrounded by layers of sterile cells that grow into the wall of the pollen sac. Some of the cells grow into nutritive cells that supply nutrition for the microspores that form by meiotic division from the sporogenous cells. The archespore cells divide by mitosis and differentiate to formpollen mother cells (microsporocyte,meiocyte).
In a process calledmicrosporogenesis, four haploidmicrospores are produced from each diploid pollen mother cell, aftermeiotic division. After the formation of the four microspores, which are contained bycallose walls, the development of the pollen grain walls begins. The callose wall is broken down by an enzyme called callase and the freed pollen grains grow in size and develop their characteristic shape and form a resistant outer wall called the exine and an inner wall called the intine. The exine is what is preserved in the fossil record.
Two basic types of microsporogenesis are recognised, simultaneous and successive. In simultaneous microsporogenesis meiotic steps I and II are completed beforecytokinesis, whereas in successive microsporogenesis cytokinesis follows. While there may be a continuum with intermediate forms, the type of microsporogenesis has systematic significance. The predominant form amongst themonocots is successive, but there are important exceptions.[8]
During microgametogenesis, the unicellular microspores undergo mitosis and develop into maturemicrogametophytes containing the gametes.[9] In some flowering plants,[which?]germination of the pollen grain may begin even before it leaves the microsporangium, with the generative cell forming the two sperm cells.
Tulipanther with many grains of pollenCloseup image of a cactus flower and its stamensPollen storm inJasper National Park
Except in the case of some submerged aquatic plants, the mature pollen grain has a double wall. The vegetative and generative cells are surrounded by a thin delicate wall of unalteredcellulose called theendospore orintine, and a tough resistant outer cuticularized wall composed largely ofsporopollenin called theexospore orexine. The exine often bears spines or warts, or is variously sculptured, and the character of the markings is often of value for identifying genus, species, or even cultivar or individual.
The spines may be less than a micron in length (spinulus, plural spinuli) referred to asspinulose (scabrate), or longer than a micron (echina, echinae) referred to asechinate. Various terms also describe the sculpturing such asreticulate, a net like appearance consisting of elements (murus, muri) separated from each other by a lumen (plural lumina). These reticulations may also be referred to as brochi.
The pollen wall protects the sperm while the pollen grain is moving from the anther to the stigma; it protects the vital genetic material from drying out and solar radiation. The pollen grain surface is covered with waxes and proteins, which are held in place by structures called sculpture elements on the surface of the grain. The outer pollen wall, which prevents the pollen grain from shrinking and crushing the genetic material during desiccation,[citation needed] is composed of two layers. These two layers are the tectum and the foot layer, which is just above the intine. The tectum and foot layer are separated by a region called the columella, which is composed of strengthening rods. The outer wall is constructed with a resistant biopolymer called sporopollenin.
Pollen apertures are regions of the pollen wall that may involve exine thinning or a significant reduction in exine thickness.[10] They allow shrinking and swelling of the grain caused by changes in moisture content. The process of shrinking the grain is called harmomegathy.[11] Elongated apertures or furrows in the pollen grain are called colpi (singular: colpus) or sulci (singular:sulcus). Apertures that are more circular are called pores. Colpi, sulci and pores are major features in the identification of classes of pollen.[12] Pollen may be referred to asinaperturate (apertures absent) oraperturate (apertures present).
The aperture may have a lid (operculum), hence is described asoperculate.[13] However the term inaperturate covers a wide range of morphological types, such as functionally inaperturate (cryptoaperturate) and omniaperturate.[8] Inaperaturate pollen grains often have thin walls, which facilitatespollen tube germination at any position.[10] Terms such asuniaperturate andtriaperturate refer to the number of apertures present (one and three respectively). Spiraperturate refers to one or more apertures being spirally shaped.
The orientation of furrows (relative to the original tetrad of microspores) classifies the pollen assulcate orcolpate. Sulcate pollen has a furrow across the middle of what was the outer face when the pollen grain was in its tetrad.[14] If the pollen has only a single sulcus, it is described asmonosulcate, has two sulci, asbisulcate, or more, aspolysulcate.[15][16] Colpate pollen has furrows other than across the middle of the outer faces, and similarly may be described aspolycolpate if more than two.Syncolpate pollen grains have two or more colpi that are fused at the ends.[17][14]Eudicots have pollen with three colpi (tricolpate) or with shapes that are evolutionarily derived from tricolpate pollen.[18] The evolutionary trend in plants has been from monosulcate to polycolpate or polyporate pollen.[14]
Additionally,gymnosperm pollen grains often have air bladders, or vesicles, calledsacci. The sacci are not actually balloons, but are sponge-like, and increase thebuoyancy of the pollen grain and help keep it aloft in the wind, as most gymnosperms areanemophilous. Pollen can bemonosaccate, (containing one saccus) orbisaccate (containing two sacci). Modernpine,spruce, andyellowwood trees all produce saccate pollen.[19]
The transfer of pollen grains to the female reproductive structure (pistil in angiosperms) is calledpollination. Pollen transfer is frequently portrayed as a sequential process that begins with placement on the vector, moves through travel, and ends with deposition.[20] This transfer can be mediated by the wind, in which case the plant is described asanemophilous (literally wind-loving). Anemophilous plants typically produce great quantities of very lightweight pollen grains, sometimes with air-sacs.
Non-flowering seed plants (e.g., pine trees) are characteristically anemophilous. Anemophilous flowering plants generally have inconspicuous flowers.Entomophilous (literally insect-loving) plants produce pollen that is relatively heavy, sticky andprotein-rich, for dispersal byinsectpollinators attracted to their flowers. Many insects and somemites are specialized to feed on pollen, and are calledpalynivores.
In non-flowering seed plants, pollen germinates in the pollen chamber, located beneath themicropyle, underneath the integuments of the ovule. Apollen tube is produced, which grows into thenucellus to provide nutrients for the developing sperm cells. Sperm cells ofPinophyta andGnetophyta are withoutflagella, and are carried by the pollen tube, while those ofCycadophyta andGinkgophyta have many flagella.
When placed on thestigma of a flowering plant, under favorable circumstances, a pollen grain puts forth apollen tube, which grows down the tissue of the style to theovary, and makes its way along theplacenta, guided by projections or hairs, to the micropyle of anovule. The nucleus of the tube cell has meanwhile passed into the tube, as does also the generative nucleus, which divides (if it has not already) to form two sperm cells. The sperm cells are carried to their destination in the tip of the pollen tube. Double-strand breaks in DNA that arise during pollen tube growth appear to be efficientlyrepaired in the generative cell that carries the malegenomic information to be passed on to the next plant generation.[21] However, the vegetative cell that is responsible for tube elongation appears to lack thisDNA repair capability.[21]
Thesporopollenin outer sheath of pollen grains affords them some resistance to the rigours of the fossilisation process that destroy weaker objects; it is also produced in huge quantities. There is an extensive fossil record of pollen grains, often disassociated from their parent plant. The discipline of palynology is devoted to the study of pollen, which can be used both forbiostratigraphy and to gain information about the abundance and variety of plants alive — which can itself yield important information about paleoclimates. Also, pollen analysis has been widely used for reconstructing past changes in vegetation and their associated drivers.[22]Pollen is first found in thefossil record in the lateDevonian period,[23][24] but at that time it is indistinguishable from spores.[23] It increases in abundance until the present day.
Nasal allergy to pollen is calledpollinosis, and allergy specifically to grass pollen is calledhay fever. Generally, pollens that cause allergies are those of anemophilous plants (pollen is dispersed by air currents.) Such plants produce large quantities of lightweight pollen (because wind dispersal is random and the likelihood of one pollen grain landing on another flower is small), which can be carried for great distances and are easily inhaled, bringing it into contact with the sensitive nasal passages.
Pollen allergies are common in polar and temperate climate zones, where production of pollen is seasonal. In the tropics pollen production varies less by the season, and allergic reactions less.In northern Europe, common pollens for allergies are those ofbirch andalder, and in late summerwormwood and different forms ofhay. Grass pollen is also associated withasthma exacerbations in some people, a phenomenon termedthunderstorm asthma.[25]
In the US, people often mistakenly blame the conspicuousgoldenrod flower for allergies. Since this plant is entomophilous (its pollen is dispersed by animals), its heavy, sticky pollen does not become independently airborne. Most late summer and fall pollen allergies are probably caused byragweed, a widespread anemophilous plant.[26]
Arizona was once regarded as a haven for people with pollen allergies, although several ragweed species grow in the desert. However, as suburbs grew and people began establishingirrigated lawns and gardens, more irritating species of ragweed gained a foothold and Arizona lost its claim of freedom from hay fever.
Anemophilous spring blooming plants such asoak,birch,hickory,pecan, and early summergrasses may also induce pollen allergies. Most cultivated plants with showy flowers are entomophilous and do not cause pollen allergies.
Symptoms of pollen allergy includesneezing, itchy, or runny nose,nasal congestion, red, itchy, and watery eyes. Substances, including pollen, that cause allergies can trigger asthma. A study found a 54% increased chance ofasthma attacks when exposed to pollen.[27]
The number of people in the United States affected by hay fever is between 20 and 40 million, including around 6.1 million children[28][29] and such allergy has proven to be the most frequentallergic response in the nation. Hay fever affects about 20% of Canadians and the prevalence is increasing.[30] There are certain evidential suggestions pointing out hay fever and similar allergies to be ofhereditary origin. Individuals who suffer fromeczema or areasthmatic tend to be more susceptible to developing long-term hay fever.[31]
Since 1990, pollen seasons have gotten longer and more pollen-filled, and climate change is responsible, according to a new study.[32] The researchers attributed roughly half of the lengthening pollen seasons and 8% of the trend in pollen concentrations to climate changes driven by human activity.[33]
InDenmark, decades of rising temperatures cause pollen to appear earlier and in greater amounts, exacerbated by the introduction of new species such as ragweed.[34]
The most efficient way to handle a pollen allergy is by preventing contact with the material. Individuals carrying the ailment may at first believe that they have a simple summer cold, but hay fever becomes more evident when the apparent cold does not disappear. The confirmation of hay fever can be obtained after examination by ageneral physician.[35]
Antihistamines are effective at treating mild cases of pollinosis; this type of non-prescribed drugs includesloratadine,cetirizine andchlorpheniramine. They do not prevent the discharge ofhistamine, but it has been proven that they do prevent a part of the chain reaction activated by thisbiogenic amine, which considerably lowers hay fever symptoms.
Allergy immunotherapy (AIT) treatment involves administering doses of allergens to accustom the body to pollen, thereby inducing specific long-term tolerance.[36] Allergy immunotherapy can be administered orally (as sublingual tablets or sublingual drops), or by injections under the skin (subcutaneous). Discovered by Leonard Noon and John Freeman in 1911, allergy immunotherapy represents the only causative treatment for respiratory allergies.
Most major classes ofpredatory andparasiticarthropods contain species that eat pollen, despite the common perception thatbees are the primary pollen-consuming arthropod group. ManyHymenoptera other than bees consume pollen as adults, though only a small number feed on pollen aslarvae (including someant larvae).Spiders are normally consideredcarnivores but pollen is an important source of food for several species, particularly forspiderlings, which catch pollen on theirwebs.[37] Somepredatory mites also feed on pollen, with some species being able to subsist solely on pollen, such asEuseius tularensis, which feeds on the pollen of dozens of plant species. Members of some beetle families such asMordellidae andMelyridae feed almost exclusively on pollen as adults, while various lineages within larger families such asCurculionidae,Chrysomelidae,Cerambycidae, andScarabaeidae are pollen specialists even though most members of their families are not (e.g., only 36 of 40,000 species ofground beetles, which are typically predatory, have been shown to eat pollen—but this is thought to be a severe underestimate as the feeding habits are only known for 1,000 species). Similarly,Ladybird beetles mainly eat insects, but many species also eat pollen, as either part or all of their diet.Hemiptera are mostlyherbivores oromnivores but pollen feeding is known (and has only been well studied in theAnthocoridae). Many adult flies, especiallySyrphidae, feed on pollen, and three UK syrphid species feed strictly on pollen (syrphids, like allflies, cannot eat pollen directly due to the structure of their mouthparts, but can consume pollen contents that are dissolved in a fluid).[38] Some species of fungus, includingFomes fomentarius, are able to break down grains of pollen as a secondary nutrition source that is particularly high in nitrogen.[39] Pollen may be valuable diet supplement fordetritivores, providing them with nutrients needed for growth, development and maturation.[40] It was suggested that obtaining nutrients from pollen, deposited on the forest floor during periods of pollen rains, allows fungi to decompose nutritionally scarce litter.[40]
Some species ofHeliconius butterflies consume pollen as adults, which appears to be a valuable nutrient source, and these species are more distasteful to predators than the non-pollen consuming species.[41][42]
Bee pollen for human consumption is marketed as afood ingredient and as adietary supplement. The largest constituent iscarbohydrates, with protein content ranging from 7 to 35 percent depending on the plant species collected by bees.[43]
Honey produced by bees from natural sources contains pollen derivedp-coumaric acid,[44] anantioxidant and naturalbactericide that is also present in a wide variety of plants and plant-derived food products.[45]
TheU.S. Food and Drug Administration (FDA) has not found any harmful effects of bee pollen consumption, except for the usual allergies. However, FDA does not allow bee pollen marketers in the United States to make health claims about their produce, as no scientific basis for these has ever been proven. Furthermore, there are possible dangers not only from allergic reactions but also from contaminants such as pesticides[3] and from fungi and bacteria growth related to poor storage procedures. A manufacturers's claim that pollen collecting helps the bee colonies is also controversial.[46]
Pine pollen (송화가루;Songhwa Garu) is traditionally consumed in Korea as an ingredient in sweets and beverages.[47]Māori of precolonial New Zealand would gather pollen ofTypha orientalis to make a special bread calledpungapunga.[48]
The growing industries in pollen harvesting for human and bee consumption rely on harvesting pollen baskets from honey bees as they return to their hives using apollen trap.[49] When this pollen has been tested for parasites, it has been found that a multitude of viruses and eukaryotic parasites are present in the pollen.[50][51] It is currently unclear if the parasites are introduced by the bee that collected the pollen or if it is from the flower.[51][52] Though this is not likely to pose a risk to humans, it is a major issue for the bumblebee rearing industry that relies on thousands of tonnes of honey bee collected pollen per year.[53] Several sterilization methods have been employed, though no method has been 100% effective at sterilisation without reducing the nutritional value of the pollen[54]
AnSEM micrograph ofredbud pollen. Scanning electron microscopes are major instruments in palynology.
Inforensic biology, pollen can tell a lot about where a person or object has been, because regions of the world, or even more particular locations such a certain set of bushes, will have a distinctive collection of pollen species.[55] Pollen evidence can also reveal the season in which a particular object picked up the pollen.[56] Pollen has been used to trace activity at mass graves from theBosnian War,[57] catch a burglar who brushed against aHypericum bush during a crime,[58] and has even been proposed as an additive for bullets to enable tracking them.[59]
In someNative American religions, pollen was used inprayers and rituals to symbolize life and renewal bysanctifying objects, dancing grounds, trails, andsandpaintings. It may also be sprinkled over heads or in mouths. ManyNavajo people believed the body becameholy when it traveled over a trail sprinkled with pollen.[60]
For agricultural research purposes, assessing the viability of pollen grains can be necessary and illuminating. A very common, efficient method to do so is known as Alexander's stain. This differential stain consists ofethanol,malachite green,distilled water,glycerol,phenol,chloral hydrate,acid fuchsin,orange G, andglacial acetic acid.[61] (A less-toxic variation omits the phenol and chloral hydrate[62].) In angiosperms and gymnosperms non-aborted pollen grain will appear red or pink, and aborted pollen grains will appear blue or slightly green.
^abTosi, S.; Costa, C.; Vesco, U.; Quaglia, G.; Guido, G. (2018). "A survey of honey bee-collected pollen reveals widespread contamination by agricultural pesticides".The Science of the Total Environment.615:208–218.doi:10.1016/j.scitotenv.2017.09.226.PMID28968582.S2CID19956612.
^Singh, Gurcharan (2004)."Palynology".Plant Systematics: An Integrated Approach. Science Publishers. p. 142.ISBN978-1-57808-351-0. Retrieved23 January 2014.
^Sanford, Malcolm T."Producing Pollen". 17 May 2006. Archived fromthe original on January 13, 2007. Retrieved2015-07-15., University of Florida, Institute of Food and Agricultural Sciences; citing P. Witherell, "Other Products of the Hive," Chapter XVIII,The Hive and the Honey Bee, Dadant & Sons, Inc., Hamilton, IL, 1975.
^Sanford, Malcolm T."Producing Pollen". University of Florida, Institute of Food and Agricultural Sciences. Archived fromthe original on 2001-04-29. Retrieved2007-08-30. Document ENY118. Original publication date November 1, 1994. Revised February 1, 1995. Reviewed May 1, 2003.
^Neill, Lindsay; Sturny, Arno (Aug 2022). "Pāraoa Rēwena: The Relegation of Aotearoa New Zealand's Indigenous Bread".Sites: A Journal of Social Anthropology and Cultural Studies.19 (1): 65.doi:10.11157/sites-id505.
^Graystock, Peter; Yates, Kathryn; Evison, Sophie E. F.; Darvill, Ben; Goulson, Dave; Hughes, William O. H. (July 2013). "The Trojan hives: pollinator pathogens, imported and distributed in bumblebee colonies".Journal of Applied Ecology.50 (5):1207–1215.Bibcode:2013JApEc..50.1207G.doi:10.1111/1365-2664.12134.S2CID3937352.
^D. Mildenhall (2006). "Hypericum pollen determines the presence of burglars at the scene of a crime: An example of forensic palynology".Forensic Science International.163 (3):231–235.doi:10.1016/j.forsciint.2005.11.028.PMID16406430.
^Hirshfelder, Arlene (2000).Encyclopedia of Native American Religions. Facts on File, Inc. p. 225.ISBN978-0-8160-3949-4.
^Alexander, M. P. (1 January 1969). "Differential Staining of Aborted and Nonaborted Pollen".Stain Technology.44 (3):117–122.doi:10.3109/10520296909063335.PMID4181665.
Society for the Promotion of Palynological Research in Austria (2021)."Illustrated Pollen Terms".PalDat - Palynological Database.University of Vienna. Division of Structural and Functional Botany. Retrieved16 January 2021.
Davis, Owen (1999)."Palynology — Pollen". University of Arizona. Department of Geosciences. Archived fromthe original on 2005-12-22. Retrieved2009-02-19.
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