Plant self defense……part 1

5th March 2013

Over the last few months we have mentioned several plants which should be avoided because they are toxic, and at the moment I have noticed there are quite a few poisonous plants emerging which haven’t been mentioned yet so today and tomorrow we will look in a bit more depth at plant toxins.

In this country there are some plants (fortunately only a handful or so) which are so toxic that consuming just a small amount is potentially fatal e.g. Yew, Hemlock etc. Other plants have to be consumed in relatively large amounts over extended periods of time to cause toxic effects e.g. Bracken, Comfrey, Ragwort etc.  Plants that are poisonous do not necessarily have the poison distributed evenly throughout the plant. For example the flesh on the berries of Yew are safe to eat but seeds inside are extremely toxic is chewed. The effects of plant poisons will also vary from individual to individual.

All plants contain a host of compounds. Some of these are necessary for the metabolism, growth, reproduction etc. These are known as primary metabolites and include proteins, amino acids, nucleotides, sugars and lipids etc. These primary metabolites will be similar to compounds that occur in animals bodies and will therefore generally be non-toxic if consumed.

Virtually all plant toxins are secondary metabolites, that is they have no role in the day to day metabolism of the plant but are produced by the plant primarily for one reason and that is to stop people and other animals from eating them.

Many (but not all) of the commonly occuring plant toxins fall into four main chemical types, namely;- Alkaloids, Glycosides, Oxalates and Tannins.

Alkaloids

These are nitrogenous alkaline carbon compounds derived from amino acids of which there are thousands of different sorts, most are toxic to mammals to a greater or lesser extent. They include familiar compounds such as nicotine, caffeine and morphine and frequently have a bitter taste.   The following is a list of some of the native plants which contain these compounds together with the particular alkaloid they contain;-

Monkshood – aconitine,  Yew – taxine, Hemlock – coniine, Deadly Nightshade – atropine, Woody Nightshade – solanine, White Bryony – bryonicine.

Although these compounds are chemically similar their action on the body is not, each will produce it’s own unique symptoms if ingested.  Alkaloids are also produced by some fungi  for example muscarine which is found in some Inocybes.

Glycosides

These are carbon based compounds known as aglycones which have a sugar molecule attached via a glycosidic bond. When consumed enzymes break this glycosidic bond releasing the aglycone into the body and it is these which have a wide range of toxic effects including affecting the heart (cardiac), releases cyanide (cyanogenic) and soap like (saponic).

Here is a list of glycoside containing plants together with their main glycoside and its type;-

Foxglove – digoxin – cardiac, Buttercups – ranunculin – forms protoanemonin (an irritant), Horse Chesnut – aesculin – a saponin, Yellow Flag – iridin, Bluebell – scillaren- cardiac, Braken – prunasin – cyanogenic.

Many plants contain oxalate salts or oxalic acid including Fathen, Common and Sheeps Sorrel, Wood Sorrel as well as cultivated plants such as Rhubarb and Spinach. As long as these plants are consumed infrequently and in small amounts they are unlikely to cause any problem.  Excess amounts in the diet however can in certain people cause calcium oxalate to precipitate out in the kidneys causing kidney stones, too much in the diet can also bind calcium in the gut preventing its absorption.

In some plants however it is present in such large amounts that is forms needle like crystals in the plants cells which when consumed will puncture through delicate mucus membranes in the mouth and oesophagus causing intense burning.  Lords and Ladies and Black Bryony are two plants that fall in to this catergory.

Higher Unsaturated Alcohols

These particularly toxic compounds are fortunately not found in many native plants but the most notable is Hemlock Water Dropwort, arguably our most toxic plant which contains an unsaturated higher alcohol called oenanthotoxin another called cicutoxin is present in Cowbane.

Tomorrow we will look at some other commonly occurring but perhaps less poisonous plant toxins.

Kev Palmer