Nectar is a sweet, aqueous secretion produced by plants, serving primarily as a reward to attract animals that facilitate the transfer of pollen. This sugar-rich fluid is the energetic currency plants use to engage in mutualistic relationships with diverse pollinators, including insects like bees and butterflies, and vertebrates such as birds and bats. The composition of this liquid is highly specialized, acting as a tailored food source that influences the behavior and visitation patterns of the plant’s chosen partner.
The Primary Ingredient: Sugars
The bulk of nectar is water and dissolved carbohydrates, forming a solution that can range dramatically in concentration from 5% to over 80% sugar by weight. This concentration is a significant factor in determining which animals visit the flower, as different species have varying physiological needs and feeding capabilities. For instance, the nectar preferred by bees tends to be highly concentrated, often between 35% and 65%, providing a dense energy source for flight.
The sugar content is composed of three main molecules: the disaccharide sucrose and its two component monosaccharides, glucose and fructose. The ratio of these three sugars indicates the target pollinator. Nectars high in sucrose are often associated with flowers visited by hummingbirds, moths, and long-tongued bees. Conversely, nectars dominated by the hexoses (glucose and fructose) are favored by short-tongued bees, bats, and passerine birds, reflecting their different digestive enzymes and energetic requirements.
Beyond Sugar: Amino Acids and Trace Elements
While sugars provide energy, nectar also contains non-sugar components that contribute to the nutritional and ecological complexity of the reward. The second most abundant group of solutes is free amino acids, the building blocks of proteins, though they are present at concentrations 100 to 1,000 times lower than carbohydrates. These compounds are important for insects like butterflies, which do not consume pollen but require nitrogen for reproduction and maintenance.
All 20 protein-forming amino acids can be found in nectar, along with non-protein amino acids like gamma-aminobutyric acid (GABA) and \(\beta\)-alanine. Specific amino acids, such as phenylalanine, can act as a feeding stimulant for certain bee species. Nectar also contains trace elements like inorganic ions, including sodium and potassium, which help pollinators maintain their salt balance. Secondary metabolites, such as alkaloids and phenolics, are sometimes present, deterring non-pollinators or serving as an anti-parasitic defense for the insects that consume them.
Where and How Nectar is Produced
Nectar production occurs in specialized secretory glands called nectaries. These are typically located within the flower (floral nectaries) and are strategically positioned to ensure a visiting animal contacts the reproductive parts of the flower while accessing the reward. Some plants also possess extrafloral nectaries, found on leaves, stems, or petioles, which secrete nectar to attract predatory insects like ants to defend the plant against herbivores.
Secretion begins with the transport of sucrose from the plant’s vascular system, the phloem, into the nectary tissue. Specific sugar transporter proteins, such as SWEET9, facilitate the movement of sugar into the nectary’s extracellular space. Once secreted, an enzyme called invertase may break down the sucrose into the simpler sugars, glucose and fructose. This enzymatic step determines the final sugar ratio of the nectar.