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(Chemosystematics/chemical taxonomy/chemical plant taxonomy/Plant


Chemotaxonomy is the characterization and classification of plants on the basis of

their chemical constituents. The approach of taxonomy in which chemical nature of
plants is used in developing classification or in solving taxonomical problems is
called chemotaxonomy.

It gives the close relationship between chemical constituents of plants and their
taxonomical status as chemotaxonomy establishes relationship between the position
of the plant and exact understanding of biological evolution and natural relationship.

Chemotaxonomy, therefore based on chemical constituents, i.e. on “molecular

characteristics” which are controlled genetically and may be considered ‘closer’ to
gene than some or more visual characters of plant form and have the advantage
over morphological characteristics that they can be very exactly described in terms
of definite structural and configurational chemical formulae.

The method of “chemical taxonomy” is simple in principle and consists of the

investigation of the distribution of chemical compounds or groups of biosynthetically
related compounds in series of related or supposedly related plants

Purpose of Chemotaxonomy

Chemotaxonomy has been used in all levels of classification. Chemical evidences

have been used in all the groups of the plant kingdom, starting from simple organism

Main purposes of chemotaxonomy

1. To improve the existing system of plants differentiation.

2. To develop the present day knowledge of natural relationship of plants.

General principles in chemotaxonomy

1. Chemical classification is based on chemical constituents, i.e. on “molecular



2. Generally, the principle chemical constituents studied in relation to classification
are those secondary metabolites which have intermediate frequency of
occurrence in plant kingdom.

3. Plant constituents such as carbohydrate units of common polysaccharides,

protein forming amino acids which are almost universal occurrence are of little
interest for taxonomic study

4. Many complex chemical compounds formed by relatively simple biosynthetic

processes (simple building units) which have wide spread occurrence are of little
interest for taxonomic study than related compounds which have undergone
rearrangements or other secondary modifications with limited occurrence. (There
are relationships between the “biosynthetic complexity” of the substance and its taxonomic

Compounds of wide occurrence Modified structures of limited

1. Stearic acid Stearolic acid
2. Hexoses Deoxyhexoses eg. Digitoxose
3. Monoanthrones Dianthrones

5. Compounds found in a single species are also taxonomically useless if not

biosynthetically related to plant constituents of intermediate distribution.

6. During course of evolution different groups of plants may have undergone a

similar type of changes and produce identical compounds (Parallelism or
chemical convergence) and a group of related plants may develop
nonidentical/different compounds (Diversification or chemical divergence). Such
chemically abnormal plants cause great chemotaxonomic difficulties.

Eg. Parallelism – Sparteine alkaloid is produced in Cytisus scoparius

(Leguminosae) and in Chelidonium majus (Papaveraceae), Diversification –
Distribution of diverse chemical groups such as cyanogenic glycosides, alkaloids,
triterpenoids, flavonoids and seed protein indicate diversification.

7. So chemotaxonomic studies include the investigation of the pattern of

compounds occurring in the plant and preferentially from all various individual
parts of plant such as bark, wood, leaves, root, seed etc. such integrated
investigations are necessary in order to obtain really convincing evidence for the
relationship of plants.


8. Before any important conclusions are drawn about the presence or absence of a
particular compound, adequate sampling of a large number of members of
species at different stages of development and growing in different environment
should be made.

Stages of Chemotaxonomic investigation

It could be said that chemotaxonomic investigations are hybrids, involving both

chemical and taxonomic research. Therefore, both the contributory kind of
information, taxonomic and chemical need to be thoroughly understood and well-
planned before a productive chemotaxonomic exercise can be undertaken.

The important stages involved in such investigations are

1. Taxonomic survey
2. Chemical techniques and pilot surveys
3. Full analysis of all material
4. Interpretation and comparison of data
1. Taxonomic survey

It covers the choice of the taxonomic group to be explored, studying its variation and
its size including the sound method of sampling. An understanding of natural
variation and knowledge of previously suggested phylogenies are both vital in good

2. Chemical techniques and pilot surveys

The investigator has to choose which chemical constituents are likely to exhibit
useful variations at the taxonomic level of family, genus or species. Similarly suitable
techniques to study these chemical constituents have to be determined. A pilot
survey is generally helpful in investigating the concentration of compounds within
organs or tissues and also seasonal and environmental fluctuations of content. e.g.
Iodine values of fixed oils are quite variable with temperature.

3. Full analysis of all material

Once a technique is adopted, pilot projects indicate the course of main investigation.
All customary safeguards of scientific studies, e.g. controls, replications and
accurate records apply to chemotaxonomic work. Incomplete data may spoil and
hamper the whole investigation. It is not good to exhaust all plant materials in the
analysis. Some material should be retained for stock maintenance and voucher


purposes. In case of anomalous or strange results further checking may be carried
out using the material from the stock.

4. Interpretation and comparison of data

All the data obtained from chemistry, phylogeny and taxonomy is interpreted and
depending upon the evidence the classification of plant should be reconsidered. If
only a minor change occurs in the match of chemical and other variation then the
classification is essentially confirmed.

Applications of Chemotaxonomy
Chemotaxonomical work can be applicable for
1. Separation of higher systemic categories.
2. Identifying similar enzyme systems in related plants producing analogous
3. Identifying different pathways to similar compounds
4. Elucidating the structural complexity and identifying the compounds of restricted
Advantages of chemotaxonomy
1. ‘Molecular characters’ of chemotaxonomy have advantage over morphological
one that they can be very exactly described in terms of definite structural and
configurational chemical formulae.
2. Chemotaxonomy also able to assist the botanist in solving some of their
problems such as those due to convergence or divergence.
3. The greatest virtue of chemical method is that it is entirely independent of
classical biological methods.
4. It is possible that in future the enzymes will be found to be more important for
chemical classification of plants than the low molecular weight secondary
Limitations of chemotaxonomy
1. The isolation and elucidation of plant constituents is often difficult and time
2. A few thousand natural products have been chemically characterized but over
knowledge of their distribution throughout 25000 known angiosperm is limited.
Thus it is difficult to reach to certain conclusions on the basis of inadequate


3. The chemical constituents generally vary considerably from organ to organ.
Therefore, an integrated investigation is necessary in order to obtain really
convincing evidence for the relationship or nonrelationship of plants.
4. It is always dangerous to draw taxonomic conclusion from occurrence or non-
occurrence of single compound in a plant.
5. Botanists are able, on morphological grounds, to differentiate-more-or-less
successfully-between large taxonomic categories, such as divisions, class, and
orders. This is at present generally beyond the capacity of chemists.
6. Character weighing problem, as same from classical taxonomic method is also
persisting here.

Case study 1 (Magnoliidae and Hamamelidae)

The Magnoliidae do possess a conservative flavonol pattern but lacking the primitive
traits of ellagic acid and myricetin (both are considered as primitive characters).
Sufficient taxa in the Magnoliidae have also been found to possess more advanced
flavones and methylated flavones. On the basis of these and other flavonoids
characters, the Magnoliidae appears to be rather more advanced in biosynthetic
Hamamelidae is characterized by general presence of ellagic acid and/or myricetin
as primitive characters. Flavonol pattern and with some increased exploitation of the
intermediate step of C-glycosylation and some developments of flavones synthesis
are also emphasized. In this respect, the Hamamelidae more closely approximate to
a chemically primitive angiosperm group than do the Magnoliidae.

Case study 2 (monocotyledons)

Flavonoids of monocots have very small difference with that of dicot. Not only are
the same widespread classes of flavonoids generally in monocot and dicot, but also
many of the same compounds can be isolated equally from plants of both types.
There may be difference, however, in the frequencies of occurrence. Flavonoid
difference between the two major types of angiosperms thus appears to be rather
subtle and complex in nature.
Chalcones, for example, are known only in Cyperaceae, Lilliaceae and
Zingiberaceae. Dihydrochalcones are described from Lilliaceae and Zingiberaceae.
The aurones mainly found in Cyperaceae with wide spread occurrence.
Isoflavonoids are rare and are restricted to Irridaceae and Lilliaceae. Flavanones
and flavanonols have few more records. Biflavonoids, which are mainly considered


to be gymnosperm constituents but do have few dicot occurrence, have been
reported in monocot until recently.
The presence of O-mythylation, increasing number of sugar substituents, increasing
number of hydroxyl groups carrying sugars and presence of acylation may all be
considered advanced features. On this basis only few families ie. Araceae,
Lemnaceae, Restionaceae, might be considered to have primitive pattern. By
contrast, the Bromeliaceae and Commelinaceae with 3, 7, 3’- and 3, 7, 3’-
trisubstitution pattern would be highly advanced families. Families in which flavones,
especially tricin, 6-hydroxyflavonoids and sulphates are regularly found might be
considered more advanced. Thus the families of the Arales, Fluviales and
Zingiberales would make one group, with families of the Lilliales being intwermediate
and all other are more specialized. Insufficient flavonoids data are yet available on
many of these families.