Nectar in Flowering Plants and Its Role in Pollination

Nectar is a sweet viscous secretion from the glands or nectaries in the plant blossoms, leaves, and stems. Mainly, nectar is a watery solution of the sugars glucose, sucrose, and fructose, but it also contains traces of salts, proteins, essential oils, and acids. Sugar content differs from 3 to 80%, based on such factors as plant species, air conditions, and soil. Nectar production as a food reward for animals is a coevolution’s classic example.

About the Nectar Serving

In flowers, the new nectar serves chiefly to attract the pollinators, such as hummingbirds, fruit-eating bats, insects, and sunbirds. The nectaries are usually found at the base of flower stamens, where they attract animal visitors and enable pollen to be transferred. Most of the flowers only secrete relatively fewer amounts of nectar that encourages cross-pollination since the animals should visit many flowers in order to receive a full meal.

Generally, a few insects, which are known as nectar robbers, bypass the flower’s sexual organs to obtain nectar by penetrating the outside of the flower often rather than entering it. In this manner, nectar robbers “steal” the nectar reward without facilitating the pollination.

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Use of Nectar

Nectar is the raw material used by honeybees to make honey, in addition to being eaten as food. Honeybees collect nectar mostly from blossoms, and nectar with less than 15% sugar content is seldom collected. Within three minutes of buzzing bees vibrating the flower, at least one plant species, Oenothera drummondii, will increase the sugar content of its nectar.

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Nectar on the Vegetative Plant

The new nectar on the vegetative plant structures may serve to attract animals, which defend the plant. Members of the genus Costus, for example, are thought to attract nectar-eating ants, which then defend the plants from herbivorous insects. Several carnivorous pitcher plant species, on the other hand, use nectar in their traps to lure prey to their deaths.

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Floral Nectaries

A nectarine or nectary is a floral tissue that is found in multiple locations in flower, and it is one of many secretory floral structures, including osmophores and elaiophores, producing nectar, scent, and oil, respectively. This structure’s function is to attract potential pollinators that may include insects, including moths and bees, and vertebrates such as bats and hummingbirds. Nectaries can take place on any floral part, but also, they may represent a novel or a modified part structure. Different types of floral nectaries can be given as:

• petal (petal, corolla),

• sepals (sepal),

• stamen (androecial, staminal: anther, filament, staminodal)

The nectariferous disc that covers the ovary base and is derived from developing ovarian tissue is found in most Lamiaceae members. In most of the Brassicaceae, the nectary lies at the stamen filament’s base. Several monocotyledons contain septal nectaries that are at the carpels’ fused margins. These exude nectar from the gynoecium's surface's small pores. Nectaries can also differ in number, symmetry, and colour.

Also, the nectaries can be categorized as either structural or non-structural. Structural nectaries refer to the specific tissue areas that exude nectar, such as the types of floral nectaries. At the same time, non-structural nectaries secrete nectar infrequently from the non-differentiated tissues. Different types of floral nectaries coevolved based on the pollinator, which feeds on the nectar of the plant.

Nectar is secreted from the nectaries’ epidermal cells that contain a dense cytoplasm by means of modified stomata or trichomes. Adjacent vascular tissue conducts the phloem by bringing sugars to the secretory region, where it is secreted from the cells via vesicles packaged by the endoplasmic reticulum. Also, the adjacent subepidermal cells may be secretory. Flowers that contain longer nectaries at times have a vascular strand in the nectary to assist in transport over a longer distance.

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Extrafloral Nectaries

Extrafloral nectaries (which are also called extranuptial nectaries) are the specialized nectar-secreting plant glands, which develop outside of flowers and are not involved in the pollination, in general, on the leaf or petiole (which means foliar nectaries) and often in relation to the venation of the leaf. They are highly diverse in location, form, mechanism, and size. They've been found in almost every part of the plant above ground, including cotyledons, stipules, roots, and fruits, to name a few.

They are the ones that range from single-celled trichomes to complex cup-like structures, which either may or may not be vascularized. These are made up of groups of glandular trichomes (such as Hibiscus spp.) or elongated secretory epidermal cells, similar to floral nectaries. Often, the latter is associated with the underlying vascular tissue.

Also, they can be associated with specialized pockets (such as domatia), raised regions (for example, Euphorbiaceae), or pits. Some of the magnoliids (for example, Piperaceae) and tropical dicot leaves (for example, Fabaceae) possess bodies or pearl glands which are globular trichomes specialized in attracting the ants. They also secrete matter, which is specifically rich in proteins, lipids, and carbohydrates.

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The above figure illustrates the Extrafloral nectaries on present the petiole of wild cherry (which is Prunus avium) leaf.