Phytoncide

(in Russian, fitontsid), a biologically active substance of plant origin that kills or inhibits the growth and development of bacteria, microscopic fungi, and protozoa. Phytoncides play an important role in plant immunity and in the interrelationships of organisms in biogeocoenoses (ecosystems). They were discovered by B. P. Tokin in 1928.

The ability to produce phytoncides is a quality common to all plants that was acquired in the process of their historical development. The secretion of phytoncides increases when the plant is injured. Phytoncides vary in chemical composition. They usually consist of groups of compounds such as glycosides, terpenoids, tanning materials, and other secondary metabolites not belonging to the major classes of natural compounds (proteins, carbohydrates, and fats). The main types are nonexcretory phytoncides, which are found in the protoplasma of cells, and volatile fractions of phytoncides, which are released into the atmosphere, soil, or water (by aquatic plants). Volatile phytoncides are capable of producing an effect at a distance, for example, those from leaves of oak, eucalyptus, pine, and many other trees.

The antimicrobial potency and range of phytoncides vary greatly among different species of plants. The phytoncides of garlic, onion, and horseradish kill many types of protozoa, bacteria, and microscopic fungi within minutes, and even seconds. The volatile phytoncides of the sweet grass Glyceria aquatica destroy such protozoa as infusorians in two hours and kill many insects within a few minutes. Phytoncides are involved in the natural immunity of plants (the means by which plants sterilize themselves with the products of their vital activity). As a result of the conjugate evolution of higher plants and microorganisms, only a few microorganisms became adapted as pathogens of any single plant species.

Phytoncides fulfill their role as plant protectors not only by destroying microorganisms, but also by hampering their reproduction, effecting the negative chemotaxis of mobile forms, and stimulating the vital activity of those that fight pathogenic forms; they also repel insects. However, phytoncides should not be regarded exclusively as protective substances. They may also contribute to thermoregulation and other processes in the vital activity of plants.

As one of the factors of plant immunity, phytoncides play an important role in the interrelations between the organisms comprising biogeocoenoses. For 1 hectare (ha) of pine forest approximately 5 kg of volatile phytoncides are released into the atmosphere in one day, while for 1 ha of juniper forest approximately 30 kg are released, reducing the number of microflora in the air. Therefore, in coniferous forests, particularly young pine forests, (regardless of latitude and the proximity of populated areas), the air is practically sterile, containing only about 200–300 bacterial cells per cu m; this fact should be of interest to hygienists, planners of health resorts, and directors of urban landscaping. It has been established that plants of one species may either inhibit or stimulate germination, growth, and development in plants of other species. For example, the phytoncides of wheatgrass and oats stimulate germination in alfalfa, while those of timothy suppress the process. These discoveries encouraged early research in allelopathy.

Because of the antimicrobial properties of phytoncides, considerable research has been done to investigate their use in medicine, veterinary medicine, plant protection, fruit and vegetable storage, and the food-processing industry.

Preparations of onion, garlic, horseradish, perforated St. John’s wort (imanin), and other plants containing phytoncides are used in medicine to treat purulent wounds, trophic ulcers, and trichomonal colpitis. The phytoncides of a number of plants stimulate the motor and secretory activity of the gastrointestinal tract, and they stimulate cardiac activity as well.