Nectar is a sugar-rich liquid produced by plants in glands called nectaries or nectarines, 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. Nectar is primarily an aqueous solution of sugars (fructose, glucose, and sucrose), but it also contains trace amounts of proteins, salts, acids, and essential oils. The sugar content varies from 3 to 80%, depending on factors such as plant species and soil and air conditions. The production of nectar as a food reward for animals is a classic example of co-evolution.
Pollinators reduce its volume, often stimulating additional secretions in the process, and in the process often contaminate it with microbes. The most numerous nectar consumers are found in three of the four largest orders of insects: Diptera, Lepidoptera, and Hymenoptera. But nectar-eating birds and bats also provide a reliable pollination service in warmer areas of the planet.
Nectar secretion is difficult to study because it is a dynamic process that includes the activation of several tissues simultaneously. Considering the rate of nectar secretion, that is, the amount of nectar secreted in a unit of time (usually one hour), Cruden et al. (1983) recognized three classes of nectar producers:
- Slow producers secrete 5-10% of their maximum accumulation per hour.
- Fast producers secrete 22-68% of their maximum per hour.
- Excellent producers secrete two to three times as much as fast producers.
Most nectar is produced when flower buds are present. Slow producers are generally early starters and their nectar is commonly protected by a thick corolla or calyx to prevent predation or evaporation. In contrast, fast producers start late and offer less protection to the nectar.
Sugars in nectar may be derived directly from photosynthesis or from stored material. In species whose pollinators require rapid production of large amounts of nectar, starch storage provides the most efficient means of accumulating the constituents needed to produce nectar. With these energy reserves, plants can produce nectar at any time of day or night.
The origin of nectar components, especially sugars, are closely related to the rate of nectar production: a high rate of production requires the storage of reserve material in the cells of nectars. The rate of nectar production and its total quantity is in turn related to the type of animal that forages it, its behavior and its nutritional needs. It is not surprising then that nectar consumers are mainly those animals that have evolved by developing the ability to fly such as insects, birds and bats.
In nectar production in Mediterranean environment for example, where summer drought is quite common, flowers tend to produce small quantities of concentrated nectar: here bees are the dominant pollinators.
The presence of a reward in the flowers of plants pollinated by animals (in this case nectar), is believed to have played a role in the evolution of plants with rewardless flowers that resemble (or smell like) other plants that offer rewards. These flowers are pollinated through deception. On the other hand, it happens that nectar, especially in the more open and accessible flowers, is also consumed by “nectar thieves,” who bring no benefit to the plant.
The composition of nectar varies both quantitatively and qualitatively, most likely precisely because its production is intended to reward different types of specific animals. In addition to rewarding animals with water, carbohydrates, amino acids and low molecular weight proteins, nectar also contains odorous compounds that serve to attract its consumers. It also contains enzymes and antioxidants that serve to maintain homeostasis (a state of physical chemical stability). Nectar also may contain toxic compounds to deter unwanted visitors.
- Water: the amount of water in nectar depends on the activity of nectarers, removal by foragers, and is influenced by the balance with ambient humidity. The water within the nectar is also highly dependent on the microclimate relative to the flower, and can be largely dependent on evaporation in exposed flowers. The concentration of nectar determines its viscosity and consequently its palatability to different pollinators. Water within nectar can also be vital to pollinators when this resource in the area is scarce.
- Carbohydrates: the main carbohydrates contained in nectar are the sugars sucrose, glucose and fructose; their total concentration varies from 7 to 70%. Invertase activity in nectars determines the proportion of sucrose to glucose and fructose. Other monosaccharides and disaccharides may be present in smaller amounts, as well as for oligosaccharides such as stachyose, and polyols such as sorbitol. However, oligosaccharides are present more in honeydew than in nectar. Sometimes polysaccharides can impart a gelatinous consistency to nectar. Carbohydrates present within nectar come from the sap, nectars, stored starch, or degeneration of certain parts of the nectars. The sugars in nectar are a primary source of energy for its consumers, in fact there are clear correlations between the content of sugars in flowers and the energy requirements of the animals that take care of their pollination.
- Amino acids and proteins: amino acids in nectar include a wide range of both essential and non-essential amino acids, as well as some non-protein amino acids. The proteins we find in nectar include enzymes and preservatives. These amino acids can influence taste preferences on the part of insects, and they also influence their nutritional level. They also appear to promote nectar homeostasis.
- Ions: as with proteins, the nutritional benefits of these molecules depend on the plant of origin. High concentrations of potassium in nectar, such as the nectar of onion flowers, has a deterrent effect towards bees.
- Antioxidants: antioxidants commonly found in nectar, such as ascorbic acid, are involved in nectar homeostasis.
- Lipids: they are a great source of energy, but we usually find only traces of them in nectar. In some particular flowers, however, these oils are offered to pollinators instead of nectar.
- Terpenoids: volatile terpenoids are key components of flower odor and can accumulate in nectar.
- Secondary compounds: toxic compounds such as phenols and alkaloids can create a selective effect on pollinators, discouraging some and attracting others. They are associated with plant resistance to herbivores and are present in some types of nectar.
Secretion, cessation and reabsorption
The temporal patterns of nectar secretion, its cessation and reabsorption (when it occurs), define the dynamics of nectar production. This parameter is usually related to the foraging behavior of animals, whose activity, together with constantly varying environmental parameters, is responsible for the amount of nectar we can find in the flower at any given time.
Defoliation experiments conducted with Impatiens glandulifera (Balsaminaceae) have shown that only a small part of the day’s nectar secretion depends on the photosynthesis that occurred that day, so the remaining nectar is mobilized from stored starch. Some rosaceae that flower before leaf emission use these stored substances for nectar production.
In most cases, flowers begin secreting nectar before pollinators begin their harvesting activities, and in some cases even before the flowers open. Nectar secretion may be continuous throughout the life of the flower until senescence or may cease at certain times. The cessation of nectar secretion can happen in two different moments:
- when the maximum amount that can be produced is reached.
- during periods of inactivity of pollinators.
In the first case, the removal of nectar by pollinators may or may not induce the resumption of secretion. For example, secretion does not resume after nectar removal in species where a single visit by pollinators is sufficient to maximize seed production. Nectar secretion proceeds concurrently with nectar reabsorption, and sometimes reabsorption continues after secretion has ended. The main purpose of this mechanism is to recover the resources invested in nectar production, an operation that requires a considerable expenditure of energy for the plant.
Removal of nectar from the flowers of Blandfordia nobilis (Blandfordiaceae) specimens increased net nectar production but reduced the plant’s ability to produce seeds, which may result in reduced plant growth and reproduction during the following season.
Nectar reabsorption may also reduce the negative effects of post-pollination visits, which can in some cases damage flowers that have already been pollinated. Resource recovery is therefore a stratagem by which plants attempt to reuse this unharvested carbohydrate source from pollinators.
Nectar formation and secretion require high energy requirements. Southwick (1984) and Pyke (1991) quantified the energy required to produce nectar and showed that over 30% of the daily product of photosynthesis is used by nectar-producing plants to produce nectar.
Using radiolabeled sucrose, Shuel (1961) showed that some of the stigma exudate comes from reabsorbed nectar and that sugar reabsorbed from the stigma exudate can appear in nectar. This suggests that there is a general recycling of substances within the flower. Reabsorbed sugars may also be stored temporarily by the plant in the form of starch granules. In alfalfa (Medicago sativa), reabsorbed sugars are found in the roots and leaves.