Nectar

Nectar is a 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 antiherbivore protection. Common nectar-consuming pollinators include mosquitoes, hoverflies, wasps, bees, butterflies and moths, hummingbirds, honeyeaters and bats. Nectar plays an important role in the foraging economics and overall evolution of nectar-eating species; for example, nectar and its properties are responsible for the differential evolution of the African honey bee, A. m. scutellata and the western honey bee. Nectar is an ecologically important item, 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, the social wasp species Apoica flavissima relies on nectar as a primary food source. In turn, these wasps then hunt agricultural pest insects as food for their young. For example, thread-waisted wasps (genus Ammophila) are known for hunting caterpillars that are destructive to crops. Caterpillars however, do eventually become butterflies and moths, which are very important pollinators. Nectar secretion increases as the flower is visited by pollinators. After pollination, the nectar is frequently reabsorbed into the plant. Nectar is derived from Greek nektar, the fabled drink of Greek gods. The word is derived as a compound of nek, meaning death, and tar, meaning the ability to overcome. The common use of nectar refers to the 'sweet liquid in flowers', first recorded in AD 1600. A nectary is floral tissue found in different locations in the flower. The different types of floral nectaries include sepal nectaries, petal nectaries, staminal nectaries found on the stamen, and gynoecial nectaries found on the ovary tissue. The nectaries also may 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 by means of trichomes or modified stomata. The nectar comes from phloem with additional sugars that are secreted from the cells through vesicles packaged by the endoplasmic reticulum. Flowers that have longer nectaries sometimes have a vascular strand in the nectary to assist in transport over a longer distance. Floral nectaries are used by plants to attract pollinators such as insects, hummingbirds, and other vertebrates. The 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. 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. Different characteristics of the spur, such as its length or position in the flower, may determine the type of pollinator that visits the flower. 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. 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. 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 to provide a reward for. In flowers that are generally pollinated by 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.