There are 2 kinds of chemicals that are useful in systematics i.
e primary metabolites and
secondary metabolites. these chemicals are normally produced in plants as pollinator attractors as
well as for protection against herbivores.
Primary metabolites.
Are formed from vital metabolic cycles i.e anabolic and catabolic metabolism e.g assimilation,
respiration, transpiration e.t.c.
Without them, organisms will not survive. They occur in cells of most plants particularly those
that are aerobic i.e DNA, RNA, Chlorophyll, amino acids, starch, proteins etc.
If found in all organisms universally they are of taxonomic value particularly by looking at the
quantities produced in different organisms i.e citric acid is produced in great quanties by citric
plants.
Secondary metabolites are formed from non-vital pathways. These chemicals are not universal.
They tend to be limited to certain organisms or certain plants.
If you study the chemical many would be end products or side reactions before conclusion of the
cycle.
Quite a number are offshoots from the metabolic cycle. The more complex a molecule is, the
more the steps required for its formation and hence it will have a narrower distribution.
There are some compounds that are big enough and can be attributed to particular plants because
they are unique to those plants.
When using a compound for systematic/taxonomic judgment, you must ensure that they are
produced by a similar metabolic cycle before concluding that the plants are similar.
**in taxonomy chemical data have inherent appeal by offering a look at relationship of plants via
internal structures and characters. It is extended to the ultimate core that chemical data should be
able to go beyond cytology, genetics to direct sequences of DNA sequences.
Flavonoids
They are phenolic compounds which occur generally in a 3-ring system divided through cycling
of an intermediate from cinnamic acid and the derivative of co-enzyme a molecule. These
molecules are normally restricted in distribution hence important in taxonomy (not found in all
plants).
Flavonoids are useful secondary metabolites in assessing the relationship among closely related
species or in studies of infraspecifc variation, and they are also occasionally useful in assessing
phylogenetic relationships at higher levels.
3C 2 + C6–C3
They are important in taxonomy because they are
easy to separate and identify,
easy to isolate
they are stable and some have been found in herbarium.
Terpenoids
They are an important class of secondary plant metabolites used in taxonomic studies. They are
derived from geranylpyrophosphate. They include monoterpenes, diterpenes and triterpenes.
Monoterpenes are called essential oils and are found mainly in the family Labiatae/Lamiaceae.
They are low molecular weight compounds. They are extremely volatile and hence can be
detected in oduors. They can be analysed by gas chromatography.
Monoterpenoid cyclopentanoid lactones represent a separate class of compounds of taxonomic
significance called Iridoids. Some aspects of their taxonomic value are;
because of the presence of an iridoid compound, aucubin, genus Buddleia was transferred
from loganiaceae to Buddleiaceae, near scrophulariaceae.
The iridoid compound asperuloside, is the characteristic Rubiaceae where as aucubin is
found in orobanchaceae; scrophulariaceae and cornaceae.
On the basis of the presence of iridoids, the interfamilial relationships were clarified
between cornales, scrophulariales, caprifoliales and rubiales.
Several iridoid-families (Rubiaceae, Scrophulariaceae, cornaceae, Dipsacaceae and
caprifoliaceae) are considered putative progenitor of compositae.
Alkaloids
They are vegetable bases containing nitrogen and they are generally thought to be decomposition
products of proteins.
They are secreted in special cells/tubes and occur in many different families. Three groups are
recognized on the basis of their biosynthesis.
True-alkaloids and proto-alkaloids are derived from amino acids while pseudo-alkaloids are
derived from other compounds other than amino acids e.g purines, terpenes but not an amino
acid.
The difference between true and proto-alkaloids is that, in true-alkaloids nitrogen is in a
heterocyclic ring but in proto alkaloids, nitrogen is not in a heterocyclic ring i.e
True alkaloid
N Proto alkaloid
CH3O CH2CH2NH2
CH3O
Mescaline
O
Pseudo alkaloids also have nitrogen in the heterocyclic ring. They are structurally a diverse
group of compounds based on one or more nitrogen containing rings.
They have a dramatic effect on human beings. Studies show that there is a correlation between
morphology and the alkaloid member of the plant family e.g quinine atropine derived from
Atropa belladonna Nicotine (from Nicotiana tabacum) Connine (from Conium maculatum) etc.
Because of the accumulation of isoquinoline alkaloids containing protopine, in both Fumariaceae
and Papaveraceae the two are considered closely related.
Lupin alkaloids are the characteristic of the tribes siphoreae, genisteae and podalyrieae of the
lotoideae of leguminosae.
Betalains
They occur conspicuously only in the order caryophyllales they are both betaxanthins and
betacyanine also found in some Basidiomycota (Mushrooms). Betacyanine are the reddidh to
violet colour while betaxanthines are the yellow to orange colour.
Among the flowering plant order Caryophyllales, most members produce betalains and lack
anthocyanins. Of all the familes in the caryophyllales, only the Caryophyllaceae (Carnation
family) and Molluginaceae produce anthocyanins instead of betalains.
Glucosinolates
These are sulphur containing substances when broken down or hydrolysed by enzyme myrsinase,
they give glucose and isothiosynate
Glucosinolates is a group of about 85 thioglucosides which on hydrolysis form glucose and a
corresponding isothiocyanate. They are called mustard oil glucosides because they are confined
to the taxa of the mustard family (Cruciferae) and some related families of Capparales.
Formely Cruciferae, capparaceae, Fumariaceae and papaveraceae were all include under one
order Rhoedales but the recent findings support the view that glucosinolate-producing families
(Cruciferae and Capparaceae) should be placed separately under Capparales, and the alkaloid-
containing families (Fumariaceae and papaveraceae) under papaverales.
Glucosinolate patterns are also used to document patterns of hybridization in some genera of
Cruciferae e.g Cakile.
Steroids
True steroids posses two methyl groups and are mostly alcohols or esters. They are located in
plant cutins and perform the function of water proofing. Their distribution proved helpful in the
taxonomy of the genera of tribe Veratreae of family liliaceae. These genera contain the steroid
veratum.
Micro-molecules
Typical ones include glucoparin (R=CH3) sinapine (R=P-OH-C6H4) sinogrin (R=CH2=CH-CH2)
glucoberin (R=MeS (CH2) 3)
Macromolecules
These are proteins and nucleic acids. They have been used in solving taxonomic problems in
higher levels of the hierarchy i.e division.
Physiological data
Under this we look at biosynthesis pathways in the manufacture of plant foods;
Types of synthesis are; C3 Pathway, C4 pathway, and CAM pathway (Crassulacean acid
Metabolism)
C3 Pathway
Here carbon dioxide from the atmosphere is fixed to the C 3 receptor molecule called ribulose 1,5
bisphosphate yielding two C3 molecules of phosphoglyceric acid
C4 pathway
Here carbondioxide is fixed to phosphoenol pyruvate to form malic acid and aspartic acids both
of which are C4 acids. Further the fixed carbondioxide is transferred to the bundle sheath cells of
leaves rather than stored in the mesophyll. Therefore the C 4 plants have a Kranz anatomy
syndrome.
CAM pathway
In these plants the malic and aspartic acids are from co2 in the form of C 4 however they are
produced at night rather than the day.
