Why Do Plants Produce Nectar?

Graphic of plant nectaries Plants had to solve a problem: they needed to find ways to spread their genetic material. Flying pollinators, insects, birds, and bats — were nature's solution. Nectar is made as a reward for pollinators. They need the plants in order to survive because nectar is their food source. Not all plants produce nectar, only plants that are visited by animal-type pollinators. Plants that are wind pollinated, for example, will not produce nectar.

Most everyone is aware that flowers commonly produce nectar that is important in encouraging pollination as well as providing food for hummingbirds and insects. However, few people are aware of the extra-floral nectaries, nectar-producing glands physically apart from the flower, that have been identified in more than 2000 plant species in more than 64 families

Floral nectar is presented inside the flower close to the reproductive organs and rewards animals that perform pollination while visiting the flower. Extra-floral nectar is involved in so-called indirect defense by attracting animals (generally ants) that prey on herbivores, or by discouraging herbivores from feeding on the plant.

Plant–animal relationships involving the two types of nectar have therefore been used for a long time as text-book examples of symmetric mutualism: services provided by animals to plants in exchange or food provided by plants to animals.

What is Nectar?

Nectar is a sweet liquid secreted by plants, and especially by flowers. Nectars range in sweetness from as little as 8% to as high as 50%. The concentration of sugar in Coca-cola is only 10%, for comparison. Although nectar is known for its sweetness, it also includes additional compounds such as vitamins, oils, amino acids and others. Nectar is produced in the plant by glands called nectaries. Floral nectaries can be located on various parts of the flower, depending on the species.

Why Do Plants Produce Floral Nectar?

Flowers produce nectar as a reward for pollination, the process of transferring pollen from flower to flower. Many flowers need pollen to reproduce. However, because plants are immobile they need help with pollen transfer. An animal that transfers pollen from flower to flower is called a pollinator. By rewarding pollinators with nectar, the animals inadvertently help the plant with pollen transfer. This monarch, covered with sticky grains of pollen, is serving as a pollinator.

How Do Plants Produce Nectar?

Plants use the energy in sunlight to make sugar from carbon dioxide and water — the process that's called photosynthesis. Most of the sugar is made in the leaves, the plant organ that is specialized to gather sunlight. From the leaves this sugar travels through the plant's conducting tissues to the other parts of the plant, the roots, stems and flowers. These plant parts then remove the sugar from the conductive tissues and use it to fuel all their metabolic processes.

When Do Plants Product Nectar?

Each species of flower has its own phenology (timing of life cycle events). The amount of nectar in a flower depends on the species. Even within a species, the quality and quantity of nectar can vary according to the age of the flower, the length of its season, the amount of precipitation, the ambient temperatures, and even the time of day. For example, in a study of dandelions in Alberta, researchers discovered:

  Larger flowers produce more nectar.
  The quantity and concentration of nectar was higher in flowers 2 days old than in those 1 day old.
  Most flowers open in the morning and close in the afternoon so nectar was not available all day.
  Nectar-sugar concentration and sugar value h increasing temperature.
  High nectar-foraging activity by honeybees coincided with peak nectar-sugar production.

Floral Nectaries

Floral nectary A nectary is a nectar-secreting gland found in different locations in the flower. The different types of floral nectaries include 'septal nectaries' found on the sepal, 'petal nectaries', 'staminal nectaries' found on the stamen, and 'gynoecial nectaries' found on the ovary tissue.

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.

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

Extra-Floral Nectaries

Ant gathering extra-floral nectar

Extra-floral nectaries are nectar-producing glands physically apart from the flower located on leaf laminae, petioles, rachids, bracts, stipules, pedicels, fruit, etc. Their size, shape and secretions vary with plant species. Extra-floral nectar content differs from floral nectar and may or may not flow in a daily pattern. Two functions for the extra-floral nectar have been hypothesized: (1) as an excretory organ for the plant to rid itself of metabolic wastes or (2) to attract beneficial insects for plant defense.

The nectar attracts predatory insects that consume both the nectar and plant-eating arthropods, functioning as bodyguards. Nectar-seeking ants expel herbivores and enhance the reproductive success of plants with extra-floral nectaries. The greater the importance of extra-floral nectar to the ants, the better for the plants, as this increases the ants' aggressiveness toward herbivores.

How Do Bees Convert Nectar Into Honey?

The actual process of transforming the flower nectar into honey requires teamwork. First, older forager worker bees fly out from the hive in search of nectar-rich flowers. Using its straw-like proboscis, a forager bee drinks the liquid nectar from a flower and stores it in a special organ called the honey stomach. The bee continues to forage until its honey stomach is full, visiting 50 to 100 flowers per trip from the hive.

Bee vomitting nectar after returning to the hive At the moment the nectars reach the honey stomach, enzymes begin to break down the complex sugars of the nectar into simpler sugars that are less prone to crystallization. This process is called inversion.

With a full belly, the forager bee heads back to the hive and regurgitates the already modified nectar directly to a younger house bee. The house bee ingests the sugary offering from the forager bee, and its own enzymes further break down the sugars. Within the hive, house bees pass the nectar from individual to individual until the water content is reduced to about 20 percent. At this point, the last house bee regurgitates the fully inverted nectar into a cell of the honeycomb.

Next, the hive bees beat their wings furiously, fanning the nectar to evaporate its remaining water content; evaporation is also helped by the temperature inside a hive being a constant 93 to 95 F. As the water evaporates, the sugars thicken into a substance recognizable as honey.

How Is Nectar Connected To Monarch Migration?

Monarch butterflies need floral nectar in the springtime to fuel migration and reproduction. The rate at which spring-blooming flowers develop is largely temperature-dependent; flowers bloom earlier in a spring with warmer temperatures. Because adult monarchs are generalists, they are able to eat nectar from a wide variety of spring flowers. This fact gives them some flexibility. In contrast, monarch larvae are specialists; they can only eat milkweed. The need for milkweed may determine more strongly when and where monarchs travel.

Other Readings:

 Create a Native Bee Sanctuary
 Climate Change: Butterflies Are Disappearing
 Solitary Bee Nesting
 Where Do Native Bees Go In Winter?
 Bumble Bee Buzz Pollination