ECOLOGICAL BIOCHEMISTRY - Study Guide - V. M. Isaenko 2005

Chapter 3. ALLELOPATHIC INTERACTIONS IN HIGHER PLANTS

In the Struggle for Existence, plants compete with one another within ecosystems for nutrients, moisture, and light, while employing various defense mechanisms against each other. The biochemically mediated interaction between plants, driven by chemical substances produced and released by them, is referred to as allelopathy (from Greek *allelon* meaning "one another, mutually" and *pathos* meaning "suffering, harm, disease").

The term allelopathy generally implies interactions between higher plants. However, one must take into account that the efficacy of chemical (allelopathic) compounds produced by one higher plant to affect another depends on the ability of soil microorganisms to metabolize, inhibit, or enhance the action of these ROOT-exuded substances. Furthermore, allelopathic interactions also occur among microorganisms, as well as between microorganisms and higher plants. In a broader sense, the term "allelopathy" is used to denote the direct or indirect NEGATIVE IMPACT OF certain plants and soil microorganisms on others through chemical substances produced and released into the environment. In this sense, The phenomenon of allelopathy differs from competition, which involves the partial or complete depletion from the environment of a specific factor (such as Water, mineral and other nutrients, or light) required by another plant.

It is worth noting that alongside allelopathic interactions among higher plants, other biochemically mediated interactions also exist, including mutually beneficial ones. For instance, certain plants release substances that stimulate the root growth of other plants. An example of this is the release of capillarol from the leaves of *Artemisia capillaris*, which stimulates rice root growth. Another example of reciprocal interaction is plant-to-plant communication. For example, a plant attacked by insects releases volatile Organic compounds that signal neighboring plants to induce the synthesis of anti-herbivore defense compounds in their leaves.

Among higher plants, There are also species with parasitic and hemiparasitic modes of Nutrition. These plants have completely or partially lost the capacity for Photosynthesis. For example, approximately 200 species of broomrape (*Orobanche*) completely lack chlorophyll. *Orobanche cumana* parasitizes sunflower, tobacco, and tomato plants, whereas *Orobanche ramosa* parasitizes cabbage, potato, and other crops. They lack true roots, and their leaves are reduced to colorless scales. Broomrape is a perennial root parasite. Its seeds germinate into a thin seedling that penetrates the soil and, upon encountering the roots of a host plant, attaches to it, penetrates down to the xylem, and subsequently develops a bud from which an aerial stem grows, bearing small, chlorophyll-free scale leaves. As it grows, this parasitic plant absorbs nitrogenous compounds, CARBOHYDRATES, and mineral nutrients from The Root System of the host plants, which in turn exude compounds that stimulate broomrape germination.

Mistletoe (genus *Viscum*) is a hemiparasite. It is a predominantly evergreen plant that forms specialized absorbing structures known as haustoria instead of roots, developing vascular connections that integrate with The Vascular System of the host plant.