Nectar is derived fromGreekνέκταρ, the fabled drink of eternal life.[3] Some derive the word from νε- or νη- "not" plus κτα- or κτεν- "kill"[citation needed], meaning "unkillable", thus "immortal". The common use of the word "nectar" to refer to the "sweet liquid in flowers", is first recorded inAD 1600.[3]
A nectary or honey gland is floraltissue found in different locations in the flower and is one of several secretory floral structures, includingelaiophores and osmophores, producing nectar, oil and scent respectively. The function of these structures is to attract potentialpollinators, which may include insects, includingbees andmoths, and vertebrates such ashummingbirds andbats. Nectaries can occur on any floral part, but they may also represent a modified part or a novel structure.[4] The different types of floral nectaries include:[5]
Most members ofLamiaceae have a nectariferous disc which surrounds the ovary base and derived from developing ovarian tissue. In mostBrassicaceae, the nectary is at the base of the stamen filament. Manymonocotyledons have septal nectaries, which are at the unfused margins of the carpels. These exude nectar from small pores on the surface of the gynoecium. Nectaries may also vary in color, number, and symmetry.[6] Nectaries can also be categorized as structural or non-structural. Structural nectaries refer to specific areas of tissue that exude nectar, such as the types of floral nectaries previously listed. Non-structural nectaries secrete nectar infrequently from non-differentiated tissues.[7] The different types of floral nectariescoevolved depending on the pollinator that feeds on the plant's nectar. Nectar is secreted fromepidermal cells of the nectaries, which have a densecytoplasm, by means oftrichomes or modifiedstomata. Adjacent vascular tissue conductsphloem bringingsugars to the secretory region, where it is secreted from the cells throughvesicles packaged by theendoplasmic reticulum.[8] The adjacent subepidermal cells may also be secretory.[4] Flowers that have longer nectaries sometimes have avascular strand in the nectary to assist in transport over a longer distance.[9][4]
Pollinators feed on the nectar and depending on the location of the nectary the pollinator assists infertilization andoutcrossing of the plant as they brush against the reproductive organs, thestamen andpistil, of the plant and pick up or depositpollen.[10] Nectar from floral nectaries is sometimes used as a reward to insects, such asants, that protect the plant from predators. Many floral families have evolved a nectarspur. These spurs are projections of various lengths formed from different tissues, such as the petals or sepals. They allow for pollinators to land on the elongated tissue and more easily reach the nectaries and obtain the nectar reward.[6] Different characteristics of the spur, such as its length or position in the flower, may determine the type of pollinator that visits the flower.[11]
Defense fromherbivory is often one of the roles of extrafloral nectaries. Floral nectaries can also be involved in defense. In addition to thesugars found in nectar, certainproteins may also be found in nectar secreted by floral nectaries. Intobacco plants, these proteins haveantimicrobial andantifungal properties and can be secreted to defend thegynoecium from certain pathogens.[12]
Floral nectaries haveevolved and diverged into the different types of nectaries due to the various pollinators that visit the flowers. InMelastomataceae, different types of floral nectaries have evolved and been lost many times. Flowers that ancestrally produced nectar and had nectaries may have lost their ability to produce nectar due to a lack of nectar consumption by pollinators, such as certain species ofbees. Instead they focused on energy allocation topollen production. Species ofangiosperms that have nectaries use the nectar to attract pollinators that consume the nectar, such asbirds andbutterflies.[13] InBromeliaceae, septal nectaries (a form of gynoecial nectary) are common in species that are insect or bird pollinated. In species that are wind pollinated, nectaries are often absent because there is no pollinator.[14] In flowers that are generally pollinated by a long-tongued organism such as certainflies,moths, butterflies, and birds, nectaries in the ovaries are common because they are able to reach the nectar reward when pollinating. Sepal and petal nectaries are often more common in species that are pollinated by short-tongued insects that cannot reach so far into the flower.[15]
Nectar secretion increases as the flower is visited by pollinators. After pollination, the nectar is frequently reabsorbed into the plant.[16] The amount of nectar in flowers at any given time is variable due to many factors, including flower age,[17] plant location,[18] and habitat management.[19]
Extrafloral nectaries with droplets of nectar on the petiole of a wild cherry (Prunus avium) leaf
Extrafloral nectaries on a red stinkwood (Prunus africana) leaf
Extrafloral nectaries (also known as extranuptial nectaries) are specialised nectar-secreting plant glands that develop outside of flowers and are not involved inpollination, generally on theleaf orpetiole (foliar nectaries) and often in relation to theleaf venation.[20][21] They are highly diverse in form, location, size, and mechanism. They have been described in virtually all above-ground plant parts—includingstipules,cotyledons,fruits, andstems, among others. They range from single-celledtrichomes to complex cup-like structures that may or may not bevascularized. Like floral nectaries, they consist of groups of glandular trichomes (e.g.,Hibiscus spp.) or elongated secretory epidermal cells. The latter are often associated with underlying vascular tissue. They may be associated with specialised pockets (domatia), pits or raised regions (e.g.,Euphorbiaceae). The leaves of some tropicaleudicots (e.g.,Fabaceae) andmagnoliids (e.g.,Piperaceae) possess pearl glands or bodies which are globular trichomes specialised to attract ants. They secrete matter that is particularly rich in carbohydrates, proteins and lipids.[20][22][23]
While their function is not always clear, and may be related to regulation of sugars, in most cases they appear to facilitate plant insect relationships.[20] In contrast to floral nectaries, nectar produced outside the flower generally has a defensive function. The nectar attracts predatory insects which will eat both the nectar and anyplant-eating insects around, thus functioning as "bodyguards".[24] Foraging predatory insects show a preference for plants with extrafloral nectaries, particularly some species ofants andwasps, which have been observed to defend the plants bearing them.Acacia is one example of a plant whose nectaries attract ants, which protect the plant from other insectherbivores.[20][21] Amongpassion flowers, for example, extrafloral nectaries prevent herbivores by attracting ants and deterring two species of butterflies from laying eggs.[25] In manycarnivorous plants, extrafloral nectaries are also used to attract insectprey.[26]
Charles Darwin understood that extrafloral nectar "though small in quantity, is greedily sought by insects" but believed that "their visits do not in any way benefit the plant".[27] Instead, he believed that extrafloral nectaries wereexcretory in nature (hydathodes). Their defensive functions were first recognized by theItalianbotanistFederico Delpino in his importantmonographFunzione mirmecofila nel regno vegetale (1886). Delpino's study was inspired by a disagreement with Darwin, with whom he corresponded regularly.[27]
Extrafloral nectaries have been reported in over 3941 species ofvascular plants belonging to 745genera and 108families, 99.7% of which belong toflowering plants (angiosperms), comprising 1.0 to 1.8% of all known species. They are most common amongeudicots, occurring in 3642 species (of 654 genera and 89 families), particularly amongrosids which comprise more than half of the known occurrences. The families showing the most recorded occurrences of extrafloral nectaries areFabaceae, with 1069 species,Passifloraceae, with 438 species, andMalvaceae, with 301 species. The genera with the most recorded occurrences arePassiflora (322 species, Passifloraceae),Inga (294 species, Fabaceae), andAcacia (204 species, Fabaceae).[22] Other genera with extrafloral nectaries includeSalix (Salicaceae),Prunus (Rosaceae) andGossypium (Malvaceae).[25]
Foliar nectaries have also been observed in 101 species offerns belonging to eleven genera and six families, most of them belonging toCyatheales (tree ferns) andPolypodiales.[28][22] Fern nectaries appear to have evolved around 135 million years ago, nearly simultaneously with angiosperms. However, fern nectaries did not diversify remarkably until nearly 100 million years later, in theCenozoic, with weak support for a role played by arthropod herbivore diversifications.[29][30] They are absent inbryophytes,gymnosperms, earlyangiosperms,magnoliids, and members ofApiales among the eudicots.[22]Phylogenetic studies and the wide distribution of extrafloral nectaries among vascular plants point to multiple independentevolutionary origins of extrafloral nectaries in at least 457 independent lineages.[22]
The main ingredients in nectar are sugars in varying proportions ofsucrose,glucose, andfructose.[31] In addition, nectars have diverse otherphytochemicals serving to both attract pollinators and discourage predators.[32][7]Carbohydrates,amino acids, andvolatiles function to attract some species, whereasalkaloids andpolyphenols appear to provide a protective function.[32]TheNicotiana attenuata, a tobacco plant native to the US state ofUtah, uses severalvolatile aromas to attract pollinating birds and moths. The strongest such aroma isbenzylacetone, but the plant also adds bitternicotine, which is less aromatic, so may not be detected by the bird until after taking a drink. Researchers speculate the purpose of this addition is to discourage the forager after only a sip, motivating it to visit other plants, therefore maximizing the pollination efficiency gained by the plant for a minimum nectar output.[7][33]Neurotoxins such asaesculin are present in some nectars such as that of theCalifornia buckeye.[34] Nectar contains water,essential oils,carbohydrates,amino acids,ions, and numerous other compounds.[16][7][35]
Some insect pollinated plants lack nectaries, but attract pollinators through other secretory structures. Elaiophores are similar to nectaries but are oil secreting. Osmophores are modified structural structures that produce volatile scents. Inorchids, these havepheromone qualities. Osmophores have thick domed orpapillate epidermis and dense cytoplasm.Platanthera bifolia produces a nocturnal scent from thelabellum epidermis.Ophrys labella have dome-shaped, papillate, dark-staining epidermal cells forming osmophores.Narcissus emit pollinator specific volatiles from thecorona.[4]
^Wallace, Gary D. (February 1977). "Studies of the Monotropoideae (Ericaceae). Floral Nectaries: Anatomy and Function in Pollination Ecology".American Journal of Botany.64 (2):199–206.doi:10.1002/j.1537-2197.1977.tb15719.x.
^Thornburg, R. W.; Carter, C.; Powell, A.; Mittler, R.; Rizhsky, L.; Horner, H. T. (May 2003). "A major function of the tobacco floral nectary is defense against microbial attack".Plant Systematics and Evolution.238 (1–4):211–218.Bibcode:2003PSyEv.238..211T.doi:10.1007/s00606-003-0282-9.S2CID19339791.
^Sajo, M. G.; Rudall, P. J.; Prychid, C. J. (August 2004). "Floral anatomy of Bromeliaceae, with particular reference to the evolution of epigyny and septal nectaries in commelinid monocots".Plant Systematics and Evolution.247 (3–4): 215.Bibcode:2004PSyEv.247..215S.doi:10.1007/s00606-002-0143-0.S2CID20457047.
Baker, H.G. and Baker, I. (1975) Studies of nectar-constitution and pollinator-plant coevolution. In Coevolution of animals and plants. Gilbert, L.E. and Raven, P.H. ed. Univ. of Texas Press, Austin, 100–140.
Esau, K. (1977) Anatomy of seed plants. John Wiley & Sons, New York.
Baker, H.G. and Baker, I. (1981) Chemical constituents of nectar in relation to pollination mechanisms and phylogeny. In Biochemical aspects of evolutionary biology. 131–171.
Beutler, R. (1935) Nectar. Bee World 24:106–116, 128–136, 156–162.
Carter, C.; Graham, R.; Thornburg, R.W. (1999). "Nectarin I is a novel, soluble germin-like protein expressed in the nectar of Nicotiana sp".Plant Mol. Biol.41 (2):207–216.doi:10.1023/A:1006363508648.PMID10579488.S2CID18327851.
Deinzer, M.L.; Tomson, P.M.; Burgett, D.M.; Isaacson, D.L. (1977). "Pyrrolizidine alkaloids: Their occurrence in honey from tansy ragwort (Senecio jacobaea L.)".Science.195 (4277):497–499.Bibcode:1977Sci...195..497D.doi:10.1126/science.835011.PMID835011.
Ecroyd, C.E.; Franich, R.A.; Kroese, H.W.; Steward, D. (1995). "Volatile constituents of Dactylanthus taylorii flower nectar in relation to flower pollination and browsing by animals".Phytochemistry.40 (5):1387–1389.Bibcode:1995PChem..40.1387E.doi:10.1016/0031-9422(95)00403-t.
Ferreres, Federico; Andrade, Paula; Gil, Maria I.; Tomás-Barberán, Francisco A. (1996). "Floral nectar phenolics as biochemical markers for the botanical origin of heather honey".Zeitschrift für Lebensmittel-Untersuchung und Forschung.202 (1):40–44.doi:10.1007/BF01229682.S2CID84713582.
Frey-Wyssling, A. (1955) The phloem supply to the nectaries. Acta Bot. Neerl. 4:358–369.
Griebel, C.; Hess, G. (1940). "Der C-Vitamingehalt des Blütennektars bestimmter Labiaten".Zeitschrift für Untersuchung der Lebensmittel.79 (1–2):168–171.doi:10.1007/BF01662427.
Heinrich, G (1989). "Analysis of cations in nectars by means of a laser microprobe mass analyser (LAMMA)".Beitr. Biol. Pflanz.64:293–308.
Smith, L.L.; Lanza, J.; Smith, G.C. (1990). "Amino acid concentrations in extrafloral nectar of Impatiens sultani increase after simulated herbivory".Ecol. Publ. Ecol. Soc. Am.71 (1):107–115.Bibcode:1990Ecol...71..107S.doi:10.2307/1940251.JSTOR1940251.
Vogel, S. (1969) Flowers offering fatty oil instead of nectar. Abstracts XIth Internatl. Bot. Congr. Seattle.
Websites
Thornburg, Robert (4 June 2001)."Nectar".Nectary Gene Expression Index. Department of Biochemistry, Biophysics and Molecular Biology,Iowa State University. Archived fromthe original on 10 September 2003. Retrieved11 January 2020.
