Allelopathy and plant survival

A number of plants produce chemicals that influence the growth, survival or reproductive potential of other plants. A name given to describe this condition is allelopathy.

Allelopathy is taken from the Greek words "allele" and "pathy," which means harm or suffering. This production of biologic chemicals is an important factor in determining plant distribution and abundance in certain plant communities.

Plants produce these allelopathic substances from the roots, leaves, stems, bark and flowers; the chemicals may be in the form of a liquid or a gas. Many invasive exotic species produce these chemicals and this is one strategy these plants use to out-compete native plants and overtake an area. Invasives such as garlic mustard, nutsedge, Japanese knotweed and ailanthus have been shown to produce chemicals that can be harmful to native plants.

Garlic mustard produces a chemical that inhibits beneficial bacteria in the soil from making nutrients more available to neighboring plants. This degradation of the soil favors plants like garlic mustard that can grow in infertile soils. The tree of heaven or ailanthus tree produces the chemical allanthone that has properties similar to nonselective herbicides like glyposate (Roundup) and paraquat.

Many native plants produce chemicals that can harm neighboring plants as well. One of the most well-known is the black walnut, which produces the chemical juglone as a gas or liquid. Walnuts can greatly harm some plants but do not bother other plants at all.

These interactions determine what kinds of trees and plants can grow alongside walnuts in the forest. Some of the plants that are very sensitive to juglone and should not be planted near these trees include, corn, lettuce, tomatoes, azaleas, rhododendrons and mountain laurel.

Other native trees that produces allelopathic chemicals include hickory, pecan, red and sugar maple, swamp chestnut oak, eastern red cedar and some species of pine.

Allelopathic interactions have been well documented throughout history. An ancient Chinese text lists nearly 300 plants that have allelopathic properties. The Greek scholar Theophrastus tells of pigweeds inhibitory effects on alfalfa. A Swiss text from the 1800s lists many allelopathic interactions as well.

These allelopathic interactions between plants can be very intricate and complicated and many are still not well understood or documented. There are a number of ongoing agricultural and forestry studies that attempt to shed more light on these interactions. In addition, many allelopathic compounds are being isolated to develop organic and chemical herbicides.

Nature Notes for 8/15/2010