Nectar

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Nectar of camellia Nectar.jpg
Nectar of camellia
Orange-yellow nectaries and greenish nectar in buckwheat flowers Buckwheat flowers macro.jpg
Orange-yellow nectaries and greenish nectar in buckwheat flowers
An Australian painted lady feeding on a flower's nectar Australian painted lady feeding closeup.jpg
An Australian painted lady feeding on a flower's nectar
Gymnadenia conopsea flowers with nectar-filled spur 0rchideen 04.jpg
Gymnadenia conopsea flowers with nectar-filled spur

Nectar is a viscous, sugar-rich liquid produced by plants in glands 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. It 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. [1]

Contents

Nectar is most often associated with flowering plants angiosperms, but it is also produced by other groups, including ferns. [2]

Etymology

Nectar is derived from Greek νέκταρ, 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 in AD 1600. [3]

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. [4] The different types of floral nectaries include: [5]

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. [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 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. [8] The adjacent subepidermal cells may also be secretory. [4] Flowers that have longer nectaries sometimes have a vascular 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 in fertilization and outcrossing of the plant as they brush against the reproductive organs, the stamen and pistil, of the plant and pick up or deposit pollen. [10] Nectar from floral nectaries is sometimes used as a reward to insects, such as ants, that protect the plant from predators. Many floral families have evolved a nectar spur. 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 from herbivory is often one of the roles of extrafloral nectaries. Floral nectaries can also be involved in defense. In addition to the sugars found in nectar, certain proteins may also be found in nectar secreted by floral nectaries. In tobacco plants, these proteins have antimicrobial and antifungal properties and can be secreted to defend the gynoecium from certain pathogens. [12]

Floral nectaries have evolved and diverged into the different types of nectaries due to the various pollinators that visit the flowers. In Melastomataceae, 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 of bees. Instead they focused on energy allocation to pollen production. Species of angiosperms that have nectaries use the nectar to attract pollinators that consume the nectar, such as birds and butterflies. [13] In Bromeliaceae, 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 certain flies, 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]

Secretion

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

Extrafloral nectaries of a Wild Cherry.jpg
Extrafloral nectaries with droplets of nectar on the petiole of a wild cherry ( Prunus avium ) leaf
Prunus africana nectaries.jpg
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 in pollination, generally on the leaf or petiole (foliar nectaries) and often in relation to the leaf venation. [20] [21] 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. [20] [22] [23]

Ants on extrafloral nectaries in the lower surface of a young Drynaria quercifolia frond Ants on Drynaria.jpg
Ants on extrafloral nectaries in the lower surface of a young Drynaria quercifolia frond

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 any plant-eating insects around, thus functioning as "bodyguards". [24] 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. [20] [21] Among passion flowers, for example, extrafloral nectaries prevent herbivores by attracting ants and deterring two species of butterflies from laying eggs. [25] In many carnivorous plants, extrafloral nectaries are also used to attract insect prey. [26]

Loxura atymnus butterflies and yellow crazy ants consuming nectar secreted from the extrafloral nectaries of a Spathoglottis plicata bud Loxura atymnus-Kadavoor-2018-06-18-001.jpg
Loxura atymnus butterflies and yellow crazy ants consuming nectar secreted from the extrafloral nectaries of a Spathoglottis plicata bud

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 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. [27]

Extrafloral nectaries have been reported in over 3941 species of vascular plants belonging to 745 genera and 108 families, 99.7% of which belong to flowering plants (angiosperms), comprising 1.0 to 1.8% of all known species. They are most common among eudicots, occurring in 3642 species (of 654 genera and 89 families), particularly among rosids which comprise more than half of the known occurrences. The families showing the most recorded occurrences of extrafloral nectaries are Fabaceae, with 1069 species, Passifloraceae, with 438 species, and Malvaceae, with 301 species. The genera with the most recorded occurrences are Passiflora (322 species, Passifloraceae), Inga (294 species, Fabaceae), and Acacia (204 species, Fabaceae). [22] Other genera with extrafloral nectaries include Salix (Salicaceae), Prunus (Rosaceae) and Gossypium (Malvaceae). [25]

Nylanderia flavipes ant visiting extrafloral nectaries of Senna Nylanderia flavipes extrafloral nectary.jpg
Nylanderia flavipes ant visiting extrafloral nectaries of Senna

Foliar nectaries have also been observed in 101 species of ferns belonging to eleven genera and six families, most of them belonging to Cyatheales (tree ferns) and Polypodiales. [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 the Cenozoic, with weak support for a role played by arthropod herbivore diversifications. [29] [30] They are absent in bryophytes, gymnosperms, early angiosperms, magnoliids, and members of Apiales among the eudicots. [22] Phylogenetic studies and the wide distribution of extrafloral nectaries among vascular plants point to multiple independent evolutionary origins of extrafloral nectaries in at least 457 independent lineages. [22]

Components

The main ingredients in nectar are sugars in varying proportions of sucrose, glucose, and fructose. [31] In addition, nectars have diverse other phytochemicals serving to both attract pollinators and discourage predators. [32] [7] Carbohydrates, amino acids, and volatiles function to attract some species, whereas alkaloids and polyphenols appear to provide a protective function. [32] 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. [7] [33] Neurotoxins such as aesculin are present in some nectars such as that of the California buckeye. [34] Nectar contains water, essential oils, carbohydrates, amino acids, ions, and numerous other compounds. [16] [7] [35]

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. [4]

See also

Related Research Articles

<span class="mw-page-title-main">Pollinator</span> Animal that moves pollen from the male anther of a flower to the female stigma

A pollinator is an animal that moves pollen from the male anther of a flower to the female stigma of a flower. This helps to bring about fertilization of the ovules in the flower by the male gametes from the pollen grains.

<span class="mw-page-title-main">Iridaceae</span> Family of flowering plants comprising irises, gladioli, and crocuses

Iridaceae is a family of plants in order Asparagales, taking its name from the irises. It has a nearly global distribution, with 69 accepted genera with a total of c. 2500 species. It includes a number of economically important cultivated plants, such as species of Freesia, Gladiolus, and Crocus, as well as the crop saffron.

<span class="mw-page-title-main">Myrmecophyte</span> Plants that live in association with ants

Myrmecophytes are plants that live in a mutualistic association with a colony of ants. There are over 100 different genera of myrmecophytes. These plants possess structural adaptations in the form of domatia where ants can shelter, and food bodies and extrafloral nectaries that provide ants with food. In exchange for these resources, ants aid the myrmecophyte in pollination, seed dispersal, gathering of essential nutrients, and defense. Domatia adapted specifically to ants may be called myrmecodomatia.

<span class="mw-page-title-main">Entomophily</span> Form of pollination by insects

Entomophily or insect pollination is a form of pollination whereby pollen of plants, especially but not only of flowering plants, is distributed by insects. Flowers pollinated by insects typically advertise themselves with bright colours, sometimes with conspicuous patterns leading to rewards of pollen and nectar; they may also have an attractive scent which in some cases mimics insect pheromones. Insect pollinators such as bees have adaptations for their role, such as lapping or sucking mouthparts to take in nectar, and in some species also pollen baskets on their hind legs. This required the coevolution of insects and flowering plants in the development of pollination behaviour by the insects and pollination mechanisms by the flowers, benefiting both groups. Both the size and the density of a population are known to affect pollination and subsequent reproductive performance.

<span class="mw-page-title-main">Zoophily</span> Pollination by animals

Zoophily, or zoogamy, is a form of pollination whereby pollen is transferred by animals, usually by invertebrates but in some cases vertebrates, particularly birds and bats, but also by other animals. Zoophilous species frequently have evolved mechanisms to make themselves more appealing to the particular type of pollinator, e.g. brightly colored or scented flowers, nectar, and appealing shapes and patterns. These plant-animal relationships are often mutually beneficial because of the food source provided in exchange for pollination.

<i>Vachellia cornigera</i> Species of legume

Vachellia cornigera, commonly known as bullhorn acacia, is a swollen-thorn tree and myrmecophyte native to Mexico and Central America. The common name of "bullhorn" refers to the enlarged, hollowed-out, swollen thorns that occur in pairs at the base of leaves, and resemble the horns of a steer. In Yucatán it is called "subín", in Panamá the locals call them "cachito". The trees are commonly found in wet lowlands

A semiochemical, from the Greek σημεῖον (semeion), meaning "signal", is a chemical substance or mixture released by an organism that affects the behaviors of other individuals. Semiochemical communication can be divided into two broad classes: communication between individuals of the same species (intraspecific) or communication between different species (interspecific).

<span class="mw-page-title-main">Nectarivore</span> Animal in which nectar is a main source of nutrition in their diet

In zoology, a nectarivore is an animal which derives its energy and nutrient requirements from a diet consisting mainly or exclusively of the sugar-rich nectar produced by flowering plants.

<span class="mw-page-title-main">Palynivore</span> Group of herbivorous animals

In zoology, a palynivore /pəˈlɪnəvɔːɹ/, meaning "pollen eater" is an herbivorous animal which selectively eats the nutrient-rich pollen produced by angiosperms and gymnosperms. Most true palynivores are insects or mites. The category in its strictest application includes most bees, and a few kinds of wasps, as pollen is often the only solid food consumed by all life stages in these insects. However, the category can be extended to include more diverse species. For example, palynivorous mites and thrips typically feed on the liquid content of the pollen grains without actually consuming the exine, or the solid portion of the grain. Additionally, the list is expanded greatly if one takes into consideration species where either the larval or adult stage feeds on pollen, but not both. There are other wasps which are in this category, as well as many beetles, flies, butterflies, and moths. One such example of a bee species that only consumes pollen in its larval stage is the Apis mellifera carnica. There is a vast array of insects that will feed opportunistically on pollen, as will various birds, orb-weaving spiders and other nectarivores.

<span class="mw-page-title-main">Pollination syndrome</span> Flower traits that attract pollinators

Pollination syndromes are suites of flower traits that have evolved in response to natural selection imposed by different pollen vectors, which can be abiotic or biotic, such as birds, bees, flies, and so forth through a process called pollinator-mediated selection. These traits include flower shape, size, colour, odour, reward type and amount, nectar composition, timing of flowering, etc. For example, tubular red flowers with copious nectar often attract birds; foul smelling flowers attract carrion flies or beetles, etc.

<span class="mw-page-title-main">Nectar robbing</span> Foraging behavior

Nectar robbing is a foraging behavior used by some organisms that feed on floral nectar, carried out by feeding from holes bitten in flowers, rather than by entering through the flowers' natural openings. Nectar robbers usually feed in this way, avoiding contact with the floral reproductive structures, and therefore do not facilitate plant reproduction via pollination. Because many species that act as pollinators also act as nectar robbers, nectar robbing is considered to be a form of exploitation of plant-pollinator mutualism. While there is variation in the dependency on nectar for robber species, most species rob facultatively.

<span class="mw-page-title-main">Pouyannian mimicry</span> Evolutionary strategy

Pouyannian mimicry is a form of mimicry in plants that deceives an insect into attempting to copulate with a flower. The flower mimics a potential female mate of a male insect, which then serves the plant as a pollinator. The mechanism is named after the French lawyer and amateur botanist Maurice-Alexandre Pouyanne. The resemblance that he noted is visual, but the key stimuli that deceive the pollinator are often chemical and tactile.

<i>Isophysis</i> Genus of flowering plants

Isophysis is a genus of herbaceous, perennial and rhizomatous plants in the Iris family (Iridaceae). A monotypic genus formerly known as Hewardia, it contains a single species, Isophysis tasmanica is a Palaeoendemic found only in the south-west of Tasmania.

Marcgravia evenia is a species of flowering vine in the family Marcgraviaceae. Within this family it belongs to the Galetae group, which is characterized by a long inflorescence axis and boat shaped nectaries. The plant is endemic to Cuba. The inflorescence of M. evenia is extraordinary. At the upper end of the pendant inflorescence are several concave bracts set at an angle to reflect and focus sonar pulses from bats, helping the bats to locate the flowers. In the middle of the inflorescence is a discoid circle of about twenty tubular tetramerous flowers. Below these is a second set of bracts very different from the reflective ones These are modified into extrafloral nectaries which is why the bats are interested, and can be enlisted as pollinators. Inflorescences with two different kinds of bracts are quite rare. although the common poinsettia is one such.

<span class="mw-page-title-main">Tritrophic interactions in plant defense</span> Ecological interactions

Tritrophic interactions in plant defense against herbivory describe the ecological impacts of three trophic levels on each other: the plant, the herbivore, and its natural enemies. They may also be called multitrophic interactions when further trophic levels, such as soil microbes, endophytes, or hyperparasitoids are considered. Tritrophic interactions join pollination and seed dispersal as vital biological functions which plants perform via cooperation with animals.

<i>Hydnophytum formicarum</i> Species of plant

Hydnophytum formicarum, commonly called a "Baboon's head" or "Ant plant", is an epiphyte native to Southeast Asia and is considered critically endangered in Singapore. It is a myrmecophyte as ants live in its tuber, also known as a caudex, and pollinate its flowers. It resides in open-canopied areas, rainforests, and terrestrial regions of high elevation.

<span class="mw-page-title-main">Floral scent</span>

Floral scent, or flower scent, is composed of all the volatile organic compounds (VOCs), or aroma compounds, emitted by floral tissue. Other names for floral scent include, aroma, fragrance, floral odour or perfume. Flower scent of most flowering plant species encompasses a diversity of VOCs, sometimes up to several hundred different compounds. The primary functions of floral scent are to deter herbivores and especially folivorous insects, and to attract pollinators. Floral scent is one of the most important communication channels mediating plant-pollinator interactions, along with visual cues.

<span class="mw-page-title-main">Nectar spur</span> Nectar spur, secrets

A nectar spur is a hollow extension of a part of a flower. The spur may arise from various parts of the flower: the sepals, petals, or hypanthium, and often contain tissues that secrete nectar (nectaries). Nectar spurs are present in many clades across the angiosperms, and are often cited as an example of convergent evolution.

<span class="mw-page-title-main">Monocotyledon reproduction</span> Flowering plant reproduction system

The monocots are one of the two major groups of flowering plants, the other being the dicots. In order to reproduce they utilize various strategies such as employing forms of asexual reproduction, restricting which individuals they are sexually compatible with, or influencing how they are pollinated. Nearly all reproductive strategies that evolved in the dicots have independently evolved in monocots as well. Despite these similarities and their close relatedness, monocots and dicots have distinct traits in their reproductive biologies.

<i>Passiflora glandulosa</i> Plant species

Passiflora glandulosa, more commonly known as wild passion fruit, is a short deciduous climbing plant with stems that can grow up to 20 m (66 ft) long. The genus Passiflora is known for its colorful flowers, which is also prominent in the glandulosa species. Named by Antonio José Cavanilles in 1790, Passiflora glandulosa is native to many regions of the northeast coast of South America, having a neotropic distribution. In specific, Passiflora glandulosa is heavily populated in Guyana, Suriname, Brazil and French Guiana.

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  31. ↑ Chalcoff, Vanina (March 2006). "Nectar Concentration and Composition of 26 Species from the Temperate Forest of South America". Annals of Botany. 97 (3): 413–421. doi:10.1093/aob/mcj043. PMC   2803636 . PMID   16373370.
  32. 1 2 González-Teuber, M.; Heil, M. (2009). "Nectar chemistry is tailored for both attraction of mutualists and protection from exploiters". Plant Signaling & Behavior. 4 (9): 809–813. Bibcode:2009PlSiB...4..809G. doi:10.4161/psb.4.9.9393. PMC   2802787 . PMID   19847105.
  33. ↑ Everts, Sarah (1 September 2008). "Two-Faced Flowers". Chemical & Engineering News.
  34. ↑ C.Michael Hogan (2008) Aesculus californica, Globaltwitcher.com, ed. N. Stromberg
  35. ↑ Park & Thornburg 2009.

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