Secondary Metabolites and Allelopathic Plant Interactions

This is one of my homework assignments for my permaculture course. It might be dry reading for some, but I am utterly fascinated by how plants communicate, and allelopathic interactions specifically. As an interesting aside, I learned that it is the secondary metabolites in plants that are used as medicine. Some of the sources I read stated that secondary metabolites basically serve no purpose and are simply byproducts of primary metabolism. Secondary metabolites are only just beginning to be understood, but I think it's very clear that they serve incredibly important functions in the ecosystem.

Update: allelopathy is now spelled correctly! :)
Black walnut tree protecting itself from grapevines that have completely overtaken the neighbouring tree.


'Every plant is a chemical factory for complex substances which exceeds any human capability. In their poisons, antibiotic agents, prickles and foul tastes, they developed defences against attack long before human stockades and pesticides.'
~Anthony Huxley, Green Inheritance, 1984

Plants secrete compounds to accomplish a number of ecological functions. For example, plant roots make nutrients in the soil available to the plant by secreting acids, which release cations bound by humus. Scent glands release volatile oils into the air sending out complex, chemical messages to many different species. Gibberellins are biochemical plant hormones that regulate developmental processes, such as germination, flowering and fruiting. These gibberellin hormones are some of the primary metabolites found in plants.

Primary metabolites are essential, organic compounds found in all plants, that are directly involved in growth, development, and reproduction. They are needed for basic cell metabolism. The main primary metabolites are carbohydrates, proteins, lipids and nucleic acids.

Less well understood are the secondary metabolites. Secondary metabolites are compounds that are not directly involved in plant growth and development. They are less widely distributed, more specialized and accumulate in smaller amounts than primary metabolites. They tend to be formed at different stages of growth, serving a wide range of ecological functions that are unique to plant families. The main secondary metabolites are alkaloids, terpenoids and phenols. These compounds play roles in attracting pollinators and beneficial insects, adapting to environmental stress and mounting defenses against pathogens, predators and competitors.

Secondary metabolites are also involved in plant to plant communication, within and between species. When the secondary metabolites of one plant has a chemical effect on another, this is referred to as allelopathy. Allelopathy can positively or negatively influence growth, behavior, reproduction, and survival of associated species, although many of the allelochemicals very often have negative effects on neighbouring plants, by secreting substances that are toxic to the competing plant. Signs that a plant is suffering from allelotoxins include leaf wilting and yellowing or death of part or all of the plant. Allelochemcials are found, variously in flowers, leaves, leaf litter and mulch, stems, bark, roots, and the soil surrounding the roots. These chemicals may biodegrade over time or persist in the soil.

The Tree-Of-Heaven, or Ailanthus (Ailanthus altissima) is known to exhibit allelotoxic effects, as well as hackberry, spotted knapweed, sorghum, chick peas, eucalyptus, sugar maple, southern waxmyrtle, American sycamore, cottonwood, black cherry, red oak, black locust, sassafrass, and American elm. Perhaps the best known allelopathic plant is the black walnut (Juglans nigra). All parts release the chemical juglone, which inhibits the growth of many species of plants, as well as its own offspring. In Arboretum America: A Philosophy of the Forest, Diana Beresford-Kroeger writes, “A number of daughter chemicals are produced throughout the plant in the roots, stems, nut, and leaves. These are fungicides; they inhibit the growth of root mycorrhiza of any seedling nut that dares to grow within the leaf canopy of the tree... In the suburban garden, black walnuts should be planted the greatest distance away from the vegetable garden to a maximum of 40 feet (12m). The roots, as they grow in size, produce greater amounts of juglone, which will affect the growth of all the Solanacea family... the Ericaceae, or the heath family, blackberries, and members of the red pine family.” (p. 85-86)

Knowledge and experience of the role that allelopathy plays in the growth and survival of certain plants can be of great benefit, particularly in the realm of permaculture. In a polyculture or guild system, knowing which plants release chemicals toxic to other plants can avert disaster. Alternatively, it may be possible to use allelopathy to one's advantage in the form of organic weed control. A deeper understanding of how plants use allelopathy can lead to some fascinating connections and opportunities.
Toby Hemenway discusses these possibilities in Gaia's Garden: A Guide to Home-Scale Permaculture:

“Since hackberries seem to thrive beneath walnut trees, the shrub's growth is evidently not squelched by juglone. Like walnuts, hackberries secrete a competition-suppressing substance; an intriguing harmony vibrates between these two allelopathic birds-of-a-feather. The toxins from the two species seem to complement each other. Juglone, though stunting the growth of many plants, doesn't have much effect on grass, whereas hackberry's toxins inhibit grasses and other shallow-rooted plants.
Here is a subtle interweaving that leaves a highly specialized niche. What rare combination of qualities results in plants that can grow amidst walnuts and hackberries, unscathed by this broad spectrum of toxicity?” p. 194-195

The study of allelopathy is still in its infancy. Identifying allelopathic activity is complex and requires experimentation and the close observation of nature, plant growth and interactions. However, understanding what allelopathy is, how it works and which plants exhibit it, can open up exciting opportunities to the organic gardener, permaculturist and plant enthusiast alike.



References:
The Global Forest, Diana Beresford-Kroeger
Arboretum America: A Philosophy of the Forest, Diana Beresford-Kroeger
Gaia's Garden: A Guide to Home-Scale Permaculture, Toby Hemenway
http://pubs.ext.vt.edu/430/430-021/430-021.html
http://www.open2.net/sciencetechnologynature/worldaroundus/chemicalplants.html
http://www.ibiblio.org/neoherbal/about.php
http://apsjournals.apsnet.org/doi/abs/10.1094/MPMI-22-5-0487
http://5e.plantphys.net/article.php?ch=&id=377
http://earthfriendlygardening.wordpress.com/2006/11/30/allelopathic-plants-natures-weedkillers/
http://csip.cornell.edu/Projects/CEIRP/AR/Allelopathy.htm