Toxic Plants - Series 4 Article 2 - Lets Talk About Toxins

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.

Toxin types

The general definition of a toxin is that it is a poisonous substance, which is a very broad definition. For our purposes in the rest of the articles in this series, it is a substance that is produced naturally by a plant that has a deleterious (and potentially fatal) effect if it is consumed by animals and/or people.

Toxins can vary in their chemical make-up. They may impart a particular color or flavor to the plant and they may be ingested and/or absorbed through the skin. Their mechanisms of action may be to initiate an inflammatory or allergic response, to cause nausea/vomiting or colic, to cause birth defects in livestock, to cause excessive salivation, breathing difficulties, neurological difficulties, sunlight sensitivity, anemia, cardiac arrythmias, and even death. The six major classes of plant toxins are proteins, alkaloids, terpenes, glycosides, organic acids, and resins.

Proteins that have a sugar molecule attached in order to make them more soluble are called glycoproteins. Lectins are one group of plant toxins, the best-known of which may be ricin, produced by the castor bean plant (Ricinus communis). In addition, many plants in the bean family (soybeans, kidney beans) also contain lectins and, as such, can have varying levels of toxicity, depending on how they are consumed. Lectin toxicity can cause damage to the liver, clumping of red blood cells, and/or bleeding in tissues of the gastrointestinal tract.

Plants also make other proteins to help them combat attack by microorganisms. Some of these (protease inhibitors) prevent the breakdown of the plant’s structural or cellular proteins while others (pore-forming toxins) attack the cell walls or membranes of bacteria. 

Alkaloids are organic compounds containing nitrogen and derived from amino acids, the building blocks of proteins. Examples include colchicine (from autumn crocus), nicotine (from tobacco), aconitine (from monkshood), taxine (from yew), atropine (from nightshade), solanine (from greening or sprouting on potatoes), and vicine (from fava beans). They tend to be bitter, water-soluble, and affect neuromuscular functions in some way: dilation of pupils, central nervous system stimulation, narcotic analgesia, and paralyzing agents (e.g., curarine).  

Terpenes are organic compounds that tend to be highly aromatic. They are produced by many plants and are utilized in flavorings, extracts, and essential oils. Conifers produce terpene resins that form the basis for turpentine. Cannabis  plants are said to have varying “flavors” that can be attributed to their terpenes. Some plant terpenes seem to serve to attract insects for pollination while others seem to play a role in defense against insect attack.

Glycosides have sugar molecules attached to a (usually) non-sugar organic compound.

Anthocyanins are glycosides that give fruits, vegetables, and flowers their red, blue, or purple coloration. They are also used in foods as natural colorants.

There are several other types of glycosides produced by plants that are toxins:

Saponins are glycocides that cause foaming and are named for the soapwort plant (Saponaria). Some pasture weeds contain saponins: corn cockle (Agrostemma githago, soapwort/bouncing bet (Saponaria officinalis), cow cockle (Saponaria vaccaria), and broomweed (Gutierrezia sarothrae). These can cause serious toxicity problems for grazing livestock. Alfalfa also contains saponins, levels of which vary from season to season, which is why it must be limited in feed. Christmas rose (Helleborus niger), horse chestnut trees, asparagus fern, and English daisies (Bellis perennis) also contain saponins.

Coumerin glycocides inhibit blood clotting. The sweet clovers (Melilotus alba and M. officinalis) contain a non-toxic precursor that, when contaminated with certain Penicillium or Aspergillus molds, is converted to the active anticoagulant dicoumarol. 

This has led to cows bleeding into joints, loss of calves, and even death.

Furanocoumarins are formed when certain chemicals in mature pasture weeds or seeds) are converted via common biochemical pathways. Bishop's weed (Ammi majus), spring parsley (Cymopterus watsonii), and dutchman's breeches (Thamnosma texana and T. montana) are known to cause this problem. Rather than causing bleeding in grazing livestock, they interact with UV light to form blisters on the animals’ skin or eyes, clouding of the cornea, or blistering of the udder and teats of lactating ewes and cows. Similarly, parsley and parsnips can undergo the same chemical reactions if contaminated with pink rot fungus (Sclerotinia sclerotiorum). Farm workers harvesting and handling parsley that has been contaminated have had problems with blistering from these chemicals as well. In addition to these examples, there are many other plants that can form furanocoumarins and we will examine these in later sections.

Cyanogenic glycosides are found in cherries, apricots, plums, peaches, almonds, and apples. The compounds are concentrated in the seeds or leaves, and eating/digesting them releases cyanide. Dose-dependent symptoms range from mild (headache, chest or throat tightness) to severe (neurological impairment, cyanosis/oxygen deprivation, convulsions, coma, and death).

Cardiac glycosides affect the function of cardiac muscle. The cardiac drug Digoxin (from Digitalis purpurea) increases the force of contraction in heart muscle. This is a good thing when prescribed for a failing heart, but potentially fatal if consumed by accident. Other cardiac glycosides (e.g., ouabain) have been used to poison arrows in ancient cultures. Lily of the valley, oleander, and milkweeds also contain cardiac glycosides.

Organic Acids include malic acid from apples, as well as citric acid and ascorbic acid from citrus fruits. All of these are non-toxic. Oxalic acid and its salts (potassium oxalate and sodium oxalate) can be quite toxic, however. Oxalates are deterrents to grazing by animals or to consumption by insects. Oxalates also serve to detoxify high aluminum levels in soils, thus conferring a growth advantage on the plants (such as tea and buckwheat) that are able to leach it from their roots into the surrounding soil. Some plants produce tiny oxalate needles that pierce the exoskeletons of insects.

Oxalates are found in brassicas (broccoli, cauliflower and cabbage), sorrel, parsley, and spinach. Rhubarb leaves contain oxalic acid (as do Virginia creeper berries and sap), while jack-in-the-pulpit contains calcium oxalate.

Citric acid structure. Photo from the U.S. National Library of Medicine

In plants that contain calcium oxalate (e.g., Dieffenbachia, Caladium, Calla, elephant ear, peace lily, mother-in-law’s tongue, etc.) the crystalline oxalate needles cause burning pain and potentially life-threatening swelling of oral tissues in children or pets that make the mistake of “tasting” them.

Resins or Oils with toxic properties include tetrahydrocannabinol (THC), urushiol, and hypericin. THC is the psycho-active constituent of Cannabis sativa. Urushiol is the topically-irritating oil found in Poison Ivy, Poison Oak, Poison Sumac, and the other plants in the Rhus (sumac) genus. Interestingly, it is also found in Mango sap, so those with poison ivy or oak sensitivity may need to be careful when handling the skin of the fruit. Hypericin is in St. John’s Wort (Hypericum) and can cause skin photo-sensitivity, with associated redness, itching, and rash.

In summing up this second article on Plant Toxicity, we see that plants produce many chemicals that serve to protect them from being eaten by animals or infected by microorganisms. These chemicals have enabled longevity and reproductive success. The following articles in our series will discuss specific plants that are toxic in some manner to animals and/or to people.