Allelopathy- Chemical Warfare of the Plant World
Allelopathy refers to the use of biochemicals to influence the reproduction, survival, and growth of other plants. Plants compete for essential sunlight, water, and nutrients, so many plants have evolved to gain an advantage by creating a disadvantage for their competition. In some instances phytochemicals from one plant can benefit another but most often their aim is to suppress competitors.
The phenomenon of allelopathy was first mentioned in the 4th century BC botanical notes of Theophrastus, a Greek student and successor to Aristotle. He described how the chickpea plant “exhasuts” the soil and inhibits weeds. In the first century AD Pliny the Elder, the Roman scholar and naturalist, who wrote a famous eye-witness account of the eruption of Mt. Vesuvius that buried Pompeii and Herculaneum in 79 AD, noted how chick pea and barley "scorch up" wheat fields.
Pliny also mentioned that walnut trees are toxic to other plants. In the 1880s scientists identified a juglone, a compound that is produced in the fruit, leaves and branches of walnut trees and that can be excreted from the root system into the soil. All species of the walnut family (Juglandaceae) black walnut, butternut, the hickories and pecan produce juglone. However, black walnuts have the highest concentration. Injury and sometimes death results when the chemical comes in contact with a susceptible plant.
Target species are affected by these toxins in many different ways. The toxic chemicals may inhibit shoot/root growth, they may inhibit nutrient uptake, or they may attack a naturally occurring symbiotic relationship thereby destroying the plant's usable source ofa nutrient.
It wasn’t until the 1990s that scientists really began unlocking the secrets of allelopathic chemicals. They still don’t know why juglone, for example, can cause severe damage and kill cabbage, solanaceous crops (tomatoes, potatoes, peppers and eggplant) lilacs, yew, rhodedendrons, lilies, peonies, rhubrarb, alfalfa and many other plants but not most trees, vines, shrubs, annuals, perennials, corn, beans, onions, beets and carrots. Ongoing research is aimed at developing safer herbicides from phytoxic chemicals.
Examples of allelopathy in plants that grow in Florida:
Lantana - roots and shoots reduce germination and growth of milkweed vine
Mango – In experiments dried mango leaf powder kept purple nutsedge tubers from sprouting. Let’s hope a product will soon come to market for use on this intractable weed.
Broccoli – Broccoli residue interferes with growth of other cruciferous crops that follow.