Nectar is a sweet, aqueous solution produced by plants to engage in mutually beneficial relationships with animals. This sugar-rich liquid serves as a caloric reward, attracting diverse consumers from insects to vertebrates. Plants secrete this reward to encourage animals to perform a service, such as moving pollen between flowers or defending the plant from threats.
Chemical Makeup
Nectar is fundamentally an aqueous solution where sugars constitute the largest proportion of dissolved solids. The three main carbohydrates present are the disaccharide sucrose and its component monosaccharides, glucose and fructose. The relative concentrations of these three sugars vary widely across plant species, often corresponding to the plant’s intended consumer. For instance, moths, hummingbirds, and certain long-tongued bees prefer sucrose-rich nectar. Conversely, nectars dominated by hexoses, such as glucose and fructose, are associated with plants pollinated by bats, flies, or short-tongued bees.
Beyond sugars, nectar contains smaller quantities of other compounds that contribute to its nutritional profile and defensive properties. Amino acids are present in lower concentrations than sugars, but they provide a source of nitrogen valuable to consumers, especially insects. Nectar also contains trace amounts of minerals, organic acids, and proteins. Certain plants incorporate secondary metabolites, such as alkaloids and phenolics, which can influence the nectar’s taste, viscosity, and potential toxicity to non-mutualistic visitors.
The Production Glands
Nectar is produced and secreted by specialized plant structures called nectaries, which can be found in various locations across the plant body. These secretory glands are composed of modified tissue, often located at the base of petals or stamens within a flower. The process begins when the vascular tissue, specifically the phloem, delivers sucrose from photosynthesizing parts of the plant to the nectary cells. These metabolically active cells process the incoming sugars before release.
Nectar secretion involves the active transport of sugars from the nectary cells into the extracellular space. This energy-intensive process, known as eccrine secretion, utilizes specific transport proteins like SWEET9 to move sugar molecules against a concentration gradient. The resulting fluid accumulates in the apoplast before being exuded onto the nectary surface. Final release occurs through modified stomata or a permeable cuticle layer on the gland’s surface.
Nectar’s Ecological Role
Nectar serves two distinct ecological roles in the overall ecology of the plant, depending on its location. Floral nectar, contained within the flower, is the primary attractant and reward for animal pollinators. This forms a mutualistic relationship where the animal receives a food source, and the plant achieves reproductive success through pollen transfer. The chemical composition, volume, and concentration of this floral nectar are finely tuned to the physiological needs and preferences of specific pollinator groups.
The second function is achieved by extrafloral nectar (EFN), secreted by nectaries located on vegetative parts of the plant, such as leaves, stems, or leaf stalks. EFN is not involved in pollination but acts as a reward to attract predatory arthropods, most commonly ants and wasps. These defensive insects patrol the plant while consuming the sugary fluid, protecting the plant by attacking or deterring herbivores. This indirect defense mechanism recruits an animal bodyguard service for the plant.
The ecological success of a plant is closely tied to its nectar production, as it facilitates these beneficial animal interactions. By offering a consistent, high-quality reward through floral nectar, plants promote pollinator fidelity, which increases the likelihood of successful cross-pollination. The presence of extrafloral nectar can also significantly reduce damage from herbivores, increasing the plant’s overall survival and reproductive output.