Chemotaxonomy: Introduction and Primary Metabolites

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Starch grains

Plant Chemotaxonomy is one of the more fashionable and rapidly extending areas of plant taxonomy that seeks to utilize chemical information to improve classification of plants.It has many diverse and indeed ancient origins.

1. The foremost comes the search by Herbalists, and latterly by pharmacists, for drugs, which has involved the accumulation of information on the chemical content of a very wide ranges of plants.

2. A second major ancient origin of chemotaxonomy has been through the fields of morphology and anatomy. For example, colour can be regarded as either morphological or chemical, and different forms of crystals as either anatomical or chemical.

Chemotaxonomic characters, like any others, are useful at all levels of the taxonomic hierarchy.

Historical Perspective

The potential importance of chemical evidence in taxonomy was suggested by a number of early taxonomists, i.e. Decandolle, Hoffman, Hallier and Molisch .

Abbott (1886) first attempted to correlate chemistry with the phylogenetic level of development and concluded that all saponin-containing groups are closely allied and possess, in addition some other structural similarities.

Greshoff (1909) suggested that chemical characters should be included in a natural classification and proposed that every description of a new genus or species should include a short chemical description of that plant taxon.

Baker and Smith (1920) first successfully explored the essential oils in Eucalyptus and combined chemical and morphological evidence in that genus. Based on morphological and chemical data they divided Eucalyptus into three groups differing in both morphological and chemical constituents.

McNair (1935) postulated that more closely related taxa produce more similar chemical products, and that more highly evolved members produce larger molecules 9alkaloids, volatile oils and glycerides)

MeNair (1945) attempted to show that chemical ontogeny can be evidence for chemical phylogeny.

Chemical characters used in Taxonomy

The chemical characteristics of plants can be categorized as:

  1. Starch, raphides, cystoliths, silica etc
  2. Secondary metabolites, such as alkaloids, flavonoids, terpenoids etc
  3. Protein analyses

Starch

Starch are the commonest chemical substances which can be easily identified. Starch granules are pigmented in iodine solution and based on their size, shape and types starch can be easily identified. They may be classified as simple or compound and may be acentric or metacentric.

For examples:

  • Acentric starch = Solanaceae (Solanum tuberosum)
  • Metacentric (simple) starch = Fabaceae (Cicer arietinum, Pisum sativum)
  • Metacentric (compound) starch – Poaceae (Oryza sativa, Triticum aestivum)

Raphides

They are calcium oxalate. The granules of calcium oxalates may occur singly or in groups. After Sectioning they can be observed under microscope. Raphides are not common in all families, rather in a limited number, say for examples, they are found around 35 angiosperm families, including Araceae, Palmae, Orchidaceae, Typhaceae, Rubiaceae, Vitaceae. No raphide is found in the primitive families of monocots.

Cystolith

These are the granules of Calcium carbonate which are found in leaf below the epidermis. Some cells of hypodermis become larger where the granules of CaCO3 are found (look like a bunch of grapes). Cystoliths are found in the families Acanthaceae, Moraceae etc. The genus Ficus under Moraceae is very distinct by presence of Cystolith.

Silica

They are produced through different metabolic processes. They are somehow significant to characterize different families, for example, Conical shapes of cystolith is a characteristic of the family Cyperaceae, while dumb-bell or saddle shapes silica are found in Bambusoideae subfamily of Poaceae. Different palms can be distinguished based on different shapes of silica.

COMPOUNDS USEFUL IN PLANT TAXONOMY

Although in theory all the chemical constituents of a plant are potentially valuable to a taxonomists, in practice, some sorts of molecules are far more valuable than others.

Apart from inorganic compounds (which are of little use relatively), three broad categories of compounds are recognized.

These are:

  • Primary metabolites
  • Secondary metabolites
  • Semantides

Primary metabolites

Primary metabolites are parts of vital metabolic pathways, and most of them are universal occurrence, or at least occur in a very wide range of plants.

Aconitic acids (first isolated from Aconitum) and Citric acid (from Curs) are present in all aerobic organisms. Presence or absence of such compounds have not much taxonomic value. However, in some cases the quantities of such metabolites vary considerably between taxa, can be taxonomically useful.


(This article is completely based on lecture by Dr. Md Oliur Rahman, Professor, Department of Botany, University of Dhaka.
Some pictures and info have been added by the author. Any mistake, error, misinformation and other related things found in this article are only author’s to blame)