Phytoncide

Phytoncides are antimicrobial allelochemic  volatile organic compounds derived from plants. The word, which means "exterminated by the plant" (from the Greek φυτόν "plant" and the Latin caedere "to kill"), was coined in 1928 by Boris P. Tokin, a Soviet biochemist then studying at Moscow State University. He found that some plants give off very active substances that help to prevent them from rotting or from being eaten by some insects and animals.

Phytoncides are a biologically active substance of plant origin that kills or inhibits growth and development of bacteria, microscopic fungi, and protozoa. Phytoncides play an important role in plant immunity and in the relationships between organisms within an ecosystem.

The ability to produce phytoncides is a quality common among plants. The release of phytoncides increase when a plant is injured. Phytoncide compound compositions vary depending on whether the compound is considered a glycoside, terpenoid, or other secondary metabolites.

Categories
There are two categories of phytoncides: 1) nonexcretory phytoncides (found in the protoplasma of cells) and 2) volatile phytoncides (released into the atmosphere, soil and water). Examples of plants with nonexcretory phytoncides are onion, garlic, and horseradish. Examples of plants with volatile phytoncides are pine, oak, eucalyptus, and members of the Sophora genus.

Some phytoncides affect only insects feeding on the plant, acting on the insect's autonomic nervous system. Other phytoncides target mainly microbes. The antimicrobial potency and range of phytoncides vary greatly among species. Some can kill many types of protozoa, bacteria, fungi, and insects within minutes or seconds, while others may take hours or only harm the pest. In addition to acting as a “plant protector”, phytoncides can also impede the reproduction of pests.

General effect on environment
Regarding how phytoncides affect a plant's immunity, for example, 1 hectare of pine forest will release approximately 5 kg of volatile phytoncides into the atmosphere in one day, reducing the amount of microflora in the air and essentially sterilizing the atmosphere among the forest, containing only about 200-300 bacterial cells/m3. This effect is found more commonly in coniferous forests as opposed to deciduous.

General uses
Because of the antimicrobial properties of phytoncides, research has been done to investigate their potential use in medicine,   as a plant protector in greenhouses, and in the shipping and storing of perishables such as fruits and vegetables.

One volatile phytoncide, sophoraflavanone G, is of particular interest due to its potential for use against methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococcus bacteria.

Occurrence and function
Cedar, garlic, locust, oak, onion, pine, tea tree, many spices, and many other plants give off phytoncides. Garlic contains allicin and diallyl disulfide. Pine contains alpha-pinene, carene, myrcene, and other terpenes. Sophora flavescens contains sophoraflavanone G.

More than 5,000 volatile substances defend plants that protect them from bacteria, fungi, and insects. Phytoncides work by inhibiting or preventing the growth of the attacking organism.

Use
Phytoncides are used in Russian, Ukrainian, Korean, Chinese, and Japanese traditional medicine, as well as in alternative medicine, aromatherapy, and veterinary medicine.