Pollination is an important step in the reproduction of seed plants: the transfer of pollen grains (male gametes) to the plant carpel, the structure that contains the ovule (female gamete). The receptive part of the carpel is called a stigma in the flowers of angiosperms and a micropyle in gymnosperms. The study of pollination brings together many disciplines, such as botany, horticulture, entomology, and ecology. Pollination is important in horticulture because most plant fruits will not develop if the ovules are not fertilized.
The process of pollination requires pollinators as agents that carry or move the pollen grains from the anther to the receptive part of the carpel. Methods of pollination, categorized by pollinator type, are:
- Entomophily: pollination by insects
- Zoophily: pollination by animals such as birds or bats
- Anemophily: pollination by wind
Hydrophily: pollination by water
- Mesophytes like Ribbonweed
Pollination management is a branch of horticulture that seeks to protect and enhance present pollinators, and often involves the culture and addition of pollinators in monoculture situations, such as commercial fruit orchards. The largest managed pollination event in the world is in California almonds, where nearly half (about one million hives) of the US honeybees are trucked to the almond orchards each spring. New York's apple crop requires about 30,000 hives; Maine's blueberry crop uses about 50,000 hives each year. Bees are also brought to commercial plantings of cucumbers, squash, melons, strawberries, and many other crops. Honeybees are not the only managed pollinators. Other kinds of bees are also cultivated as pollinators. The alfalfa leafcutter bee is an important pollinator for alfalfa seed in western United States and Canada. Bumblebees are increasingly cultured and used extensively for greenhouse tomatoes and other crops.
Pollination also requires consideration of pollenizers. (Pollinator and pollenizer are often confused: a pollinator is the agent that moves the pollen, whether it be wind, bees, bats, moths, or birds; a pollenizer is the plant that provides the pollen.) Some plants are self fertile or self compatible, and can pollenize themselves. Other plants have chemical or physical barriers to self pollenization, and need to be cross pollinated. With self infertile plants, not only pollinators must be considered but pollenizers as well. In pollination management, a good pollenizer is a plant that provides compatible, viable and plentiful pollen, and blooms at the same time as the plant that is pollenized.
Pollination can be cross-pollination with a pollinator and an external pollenizer, self-pollination without any pollinator, or self pollenization with a pollinator:
- Cross-pollination (syngamy): pollen is delivered to a flower of a different plant
- Self-pollenization (autogamy): pollen moves to the female part of the same flower, or to another flower on the same individual plant. This is sometimes referred to as self pollination, but this is not synonymous with autogamy. Clarity requires that the term self pollination be restricted to those plants that accomplish pollination without an external pollinator (example: the stamens actually grow into contact with the pistil to transfer the pollen). Most peach varieties are autogamous, but not truly self pollinated, as it is generally an insect pollinator that moves the pollen from anther to stigma.
- Cleistogamy, pollination that occurs before the flower opens is always self-pollination. Some cleistogamous flowers never open, in contrast to chasmogamous flowers that open and then are pollinated.
Cliestogamous flowers must of necessity be self compatible or self fertile plants. Other plants are self incompatible. These are end points on a continuum, not absolute points.
Peaches are considered self fertile because a commercial crop can be produced without cross pollination, though cross pollination usually gives a better crop. Apples are considered self incompatible, because a commercial crop must be cross pollinated. Remember that most fruits are grafted clones, genetically identical. An orchard block of apples of one variety is in effect all one plant. Growers now consider this a mistake. One means of correcting this mistake is to graft a limb of an appropriate pollenizer (generally a variety of crabapple) every six trees or so.
To attract pollinators, some flowers, such as sunflower, when viewed under ultraviolet light as seen by honeybees, have a darker centre, where the pollen is located. There may also be patterns upon the petals. These are called nectar guides.
Pollination of food crops has become an environmental issue, due to two cross trends. The trend to monoculture means that greater concentrations of pollinators are needed at bloom time than ever before, yet the area is forage poor or even deadly to bees for the rest of the season. The other trend is the decline of pollinator populations, due to pesticide misuse and overuse, new diseases and parasites of bees, clearcut logging, decline of beekeeping, suburban development, removal of hedges and other habitat from farms, and public paranoia about bees. Widespread aerial spraying for mosquitoes due to West Nile fears is causing an acceleration of the loss of pollinators.
It is estimated that one hive per acre (4,000 m² per hive) will sufficiently pollinate watermelons. In the 1950s when the woods were full of wild bee trees, and beehives were normally kept on most South Carolina farms, a farmer who grew ten acres (40,000 m²) of watermelons would be a large grower, and probably had all the pollination needed. But today's grower may grow 200 acres (800,000 m²), and, if lucky, there might be one bee tree left within range. The only option in the current economy is to bring beehives to the field during blossom time.
The US solution to the pollinator shortage, so far, has been for commercial beekeepers to become pollination contractors and to migrate. Just as the combines follow the wheat harvest from Texas to Manitoba, beekeepers follow the bloom from South to North, to provide pollination for many different crops.
Bees travel from flower to flower, collecting nectar (converted to honey later), and in the process pick up pollen grains. The bee collects the pollen by rubbing against the anthers. The pollen collects on the hind legs, in dense hairs referred to as a pollen basket. As the bee flies from flower to flower, the pollen grains are transferred onto the stigma of the female flower part.
Nectar provides the energy for bee nutrition, pollen provides the protein. When bees are rearing large quantities of brood (beekeepers say hives are building), bees will deliberately gather pollen to meet the nutritional needs of the brood. A honeybee that is deliberately gathering pollen is up to ten times more efficient as a pollinator than one that is primarily gathering nectar and is only unintentionally gathering pollen.
Good pollination management seeks to have bees in a building state during the bloom period of the crop, thus requiring them to gather pollen, and making them more efficient pollinators. Thus the management techniques of a beekeeper providing pollination service are different and somewhat incompatible, compared to a beekeeper who is trying to produce honey.
Some flowers are pollinated using buzz pollination. In some plants, the pollen in the anther is less accessible. The anther is typically tubular, with only an opening at one end. In order to release the pollen, the bee grabs onto the flower and moves its flight muscles, causing the flower and anther to vibrate, dislodging pollen. This is called buzz pollination. The honeybee does not perform buzz pollination. About 8% of the flowers of the world are pollinated using buzz pollination. The following plants are pollinated by buzz pollination:
- all Dodecatheon - shooting stars
- many members of the Solanaceae family
- Some members of the genus Vaccinium
- Arctostaphylos - manzanita
See also: Fruit tree pollination
- Insect Pollination Of Cultivated Crop Plants by S. E. McGregor USDA 1972 (needs updating but still valuable)
- The Pollination Home page ]
- Pulse of the Planet description of buzz pollination