Nectar is a viscous, sugar-rich liquid produced by glands in plants called nectaries, either within the flowers with which it attracts pollinating animals, or by extrafloral nectaries, which provide a nutrient source to animal mutualists, which in turn provide herbivore protection. Common nectar-consuming pollinators include mosquitoes, hoverflies, wasps, bees, butterflies and moths, hummingbirds, honeyeaters and bats.
Nectar is an economically important substance as it is the sugar source for honey.
Nectar is also useful in agriculture and horticulture because the adult stages of some predatory insects feed on nectar. For example, a number of predacious or parasitoid wasps (e.g., the social wasp species Apoica flavissima) rely on nectar as a primary food source. In turn, these wasps then hunt agricultural pest insects as food for their young.
Nectar is most often associated with flowering plants (angiosperms), but it is also produced by other groups, including ferns.
Contents
Etymology
Nectar is derived from Greek νέκταρ, the fabled drink of eternal life. Some derive the word from νε- or νη- "not" plus κτα- or κτεν- "kill", meaning "unkillable", thus "immortal". The common use of the word "nectar" to refer to the "sweet liquid in flowers", is first recorded in AD 1600.
Floral nectaries
A nectary or honey gland is floral tissue found in different locations in the flower and is one of several secretory floral structures, including elaiophores and osmophores, producing nectar, oil, and scent respectively. The function of these structures is to attract potential pollinators, which may include insects, including bees and moths, and vertebrates such as hummingbirds and bats. Nectaries can occur on any floral part, but they may also represent a modified part or a novel structure. The different types of floral nectaries include:
receptacle (receptacular: extrastaminal, intrastaminal, interstaminal)
hypanthium (hypanthial)
tepals (perigonal, tepal)
sepals (sepal)
petal (petal, corolla)
stamen (staminal, androecial: filament, anther, staminodal)
pistil (gynoecial: stigmatic, stylar)
pistillodes (pistillodal, carpellodial)
ovaries (ovarian: non-septal, septal, gynopleural)
Most members of Lamiaceae have a nectariferous disc which surrounds the ovary base and derived from developing ovarian tissue. In most Brassicaceae, the nectary is at the base of the stamen filament. Many monocotyledons 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. 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. The different types of floral nectaries coevolved depending on the pollinator that feeds on the plant's nectar. Nectar is secreted from epidermal cells of the nectaries, which have a dense cytoplasm, by means of trichomes or modified stomata. Adjacent vascular tissue conducts phloem bringing sugars to the secretory region, where it is secreted from the cells through vesicles packaged by the endoplasmic reticulum. The adjacent subepidermal cells may also be secretory. Flowers that have longer nectaries sometimes have a vascular strand in the nectary to assist in transport over a longer distance.
Secretion
Nectar secretion increases as the flower is visited by pollinators. After pollination, the nectar is frequently reabsorbed into the plant. The amount of nectar in flowers at any given time is variable due to many factors, including flower age, plant location, and habitat management.
Extrafloral nectaries
Extrafloral nectaries (also known as extranuptial nectaries) are specialised nectar-secreting plant glands that develop outside of flowers and are not involved in pollination, generally on the leaf or petiole (foliar nectaries) and often in relation to the leaf venation. They are highly diverse in form, location, size, and mechanism. They have been described in virtually all above-ground plant parts—including stipules, cotyledons, fruits, and stems, among others. They range from single-celled trichomes to complex cup-like structures that may or may not be vascularized. 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 tropical eudicots (e.g., Fabaceae) and magnoliids (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.
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. 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 any plant-eating insects around, thus functioning as "bodyguards". Foraging predatory insects show a preference for plants with extrafloral nectaries, particularly some species of ants and wasps, 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 insect herbivores. Among passion flowers, for example, extrafloral nectaries reduce herbivory by attracting ants, and by deterring two species of butterflies from laying eggs. In many carnivorous plants, extrafloral nectaries are also used to attract insect prey.
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". Instead, he believed that extrafloral nectaries were excretory in nature (hydathodes). Their defensive functions were first recognized by the Italian botanist Federico Delpino in his important monograph Funzione mirmecofila nel regno vegetale (1886). Delpino's study was inspired by a disagreement with Darwin, with whom he corresponded regularly.
Components
The main ingredients in nectar are sugars in varying proportions of sucrose, glucose, and fructose. In addition, nectars have diverse other phytochemicals serving to both attract pollinators and discourage predators. Carbohydrates, amino acids, and volatiles function to attract some species, whereas alkaloids and polyphenols appear to provide a protective function.
The Nicotiana attenuata, a tobacco plant native to the US state of Utah, uses several volatile aromas to attract pollinating birds and moths. The strongest such aroma is benzylacetone, but the plant also adds bitter nicotine, 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. Neurotoxins such as aesculin are present in some nectars such as that of the California buckeye. Nectar contains water, essential oils, carbohydrates, amino acids, ions, and numerous other compounds.
Similar attractive substances
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. In orchids, these have pheromone qualities. Osmophores have thick domed or papillate epidermis and dense cytoplasm. Platanthera bifolia produces a nocturnal scent from the labellum epidermis. Ophrys labella have dome-shaped, papillate, dark-staining epidermal cells forming osmophores. Narcissus emit pollinator specific volatiles from the corona.