Toxic Plants - Series 4 Article 1 - An Introduction to Plant Warfare

By Merry Bogert 

Caveat and general statements regarding toxicity:

1)     Not every toxic plant is discussed in these articles and just because a plant is not discussed in the article does not mean it is not toxic.

2)     Some plants are species-specific with regards to toxicity – in other words, not all plants are toxic in the same way with all animals or people

3)     The “poisonous plant triangle” applies to any potentially toxic plant. This term means that three things are required for a particular plant to be toxic:    

 (a) the presence of some particular chemical in the plant, AND

 (b) a susceptibility or sensitivity to that chemical by the animal or person   

       ingesting or contacting the plant, AND

 (c) consumption/contact with sufficient quantity to cause a reaction.

All three must be present in order for a plant to be toxic to an individual animal or person at one particular point in time.

4)     Toxicity can range from dermatitis (a skin reaction, such as is seen with poison ivy) to gastro-intestinal issues and even death.

5)     Most accidental ingestions are unintentional and often involve children attracted to brightly colored berries, flowers, etc.

An Introduction to Plant Warfare

Most gardeners know about a few of the plants that are “heavy hitters” from the perspective of their toxicity to people. Foxglove (Digitalis spp.), Daffodils (Narcissus spp.), Water Hemlock (Cicuta davidii), Poison Hemlock (Conium maculatum), Deadly Nightshade (Atropa belladonna), and Oleander (Nerium oleander) are some that are likely to spring to mind. But what they don’t know is that there are many plants that are toxic at some level or other to plants or to animals.

So-called toxic plants have evolved various methods to protect themselves from being eaten or to allow themselves to grow without competition for water and nutrients – all so they can live long enough to reproduce. While the methods they use are varied, they all involve chemicals made by the plants and excreted or leached into the soil (by their roots or from decomposition), into their sap, or into the air via volatile compounds (in the case of some aromatic plants).

This phenomenon is known as allelopathy and the chemicals are known as allelopathic agents. The term allelopathy is derived from the Greek words allelo (mutual/one another) and pathos (harm or suffering). It was coined in 1937 by the Austrian botanist Hans Molisch in his book The Effect of Plants on Each Other – Allelopathy, but the negative effect of one plant species on another was observed as early as 300 BC by Theophrastus, observing the inhibition of the growth of alfalfa by pigweed.

Theoretically, allelopathic agents may act in a positive or negative fashion. Any beneficial effect of an allelopathic chemical on another plant species could occur only at very low concentrations of the chemical. At low concentrations, the chemical might stimulate another plant’s defense mechanisms, while at higher concentrations it might be inhibitory or toxic. Generally, the term refers to a plant’s negative effects on others.

The chemicals are highly varied in nature: they may be alkaloids, amino acids, sterols, carbohydrates, phenols, or a combination thereof. They can affect seed germination, growth vigor, cell division, pollen germination, nutrient uptake, photosynthesis, or specific enzyme functions of nearby, potentially competing, species.

Much research is being done on the application of allelopathy in agricultural crop production. Some well-known field research has been on the allelopathic effects of various rice cultivars on one another.  Additional areas of interest include the effects of weeds on crops and vice versa, with an eye to development of natural growth regulators or herbicides. One example is Callisto® (a Syngenta product), a herbicide developed to control broadleaf weeds in field corn, seed corn, yellow popcorn, sweet corn and other labeled crops. Its active ingredient is mesotrione, a more bioactive analog of leptospermone, the allelopathic agent produced by the lemon bottlebrush, Callistemon citrinis.

It is interesting to learn that invasive plants often employ allelopathic agents to out-compete natives. B-M Chen et al. published the paper “Role of allelopathy in plant invasion and control of invasive plants”, Allelopathy Journal 41(2):155-166, July 2017.  The tree of heaven (Ailanthus altissima) produces allelochemicals in its roots that inhibit the growth of many plants and contribute to its establishment in dense groves. Recently, this tree has become particularly important in the monitoring of the invasive Spotted Lanternfly (Lycorma delicatula) planthopper in the Eastern United States because it is a preferred host plant for these destructive insects.

Allelopathy has also been shown to play an important role in influencing the composition of the vegetation in forests, providing insight into the patterns of forest regeneration. One fairly well-known example of this is the black walnut (Juglans nigra), which produces the allelochemical juglone. Juglone has an inhibitory or toxic effect on some plant species. For a concise overview, please see the following link: https://www.extension.iastate.edu/news/2005/jul/070701.htm

To sum up this first article in the series on Plant Toxicity, we have seen that plants have the ability to control their environments (and have been doing so for millennia) by manufacturing biochemically-active compounds that inhibit or kill other plants that could be competing for resources essential for their longevity and successful reproduction. The next articles will address specific types of toxins and their effects on animals and/or humans.