|
Abstract
Solanum nigrum
Complex is the name given to a group of closely related plants whose
taxonomic status is an International controversy among different
taxonomists. But no chemotaxonomic relationship has so far been
studied due to lack of comprehensive investigation of chemical
composition of the individual plant taxa.
The present chemotaxonomic studies, alongwith nutritional and
biological evaluation, of the five locally available plant taxa of
S. nigrum Complex viz.: S. americanum Mill., S. chenopodioides Lam.,
S. nigrum L., S. retroflexum Dunal and S. villosum Mill. were
carried out in order to resolve the controversy on their taxonomic
status and to reveal the medicinal importance of the individual taxa.
In these studies the morphologically different plant taxa were grown
under controlled conditions in the Botanic Garden, GC University
Lahore, Pakistan, and third accession of each taxon was taken for
the chemotaxonomic investigations. Comparative analyses of these
plant samples were undertaken with respect to the Alkaloids,
Flavonoids and Epicuticular wax as potential characters. The HPLC
and GC-MS analyses of these constituents had not been reported
previously. Also, with the exception of S. nigrum, literature is
silent on the detailed chemical analysis of the taxa under study.
Statistical analyses of results grouped taxa into different
clusters.
The comparison of alkaloidal profile of the five taxa was used to
establish the boundaries among close taxonomic groups. Yield of
total glycoalkaloids in the five taxa ranged from 68.9±0.6 to
25±0.8% as determined by Titrimetric method. Glycoalkaloids analysed
by HPLC demonstrated that the concentration of β-Solamargine was
much higher in S. villosum (9.8 mg/g) than other samples but that of
α-Solamargine was relatively higher in S. nigrum (5.03 mg/g). There
was a gradual change in Solasonine concentration ranging from 2.01
mg/g (S. villosum) to 5.8 mg/g (S. nigrum). α-Solanine concentration
was maximum in S. retroflexum. GC-MS of the aglycones depicted that
percentage of Solanidine in the samples varied from 8.85-20.31%
(being highest in S. retroflexum) while that of Solasodine from
66.99-85.67% (being highest in S. americanum). Significant distances
were shown between S. chenopodioides and S. villosum as well as in
americanum and S. nigrum in their respective clusters. However S.
retroflexum did not show such a marked difference with respect to S.
nigrum.
In order to further compare the differences among the five plant
taxa, their flavonoid compositions were investigated. Total
flavonoid contents in the taxa were calculated by two complementary
colorimetric methods and ranged from 0.883±0.020 to 2.116±0.032.
From HPLC, it was found that S. americanum had the highest
concentration of both Quercetin-3-glucoside (0.03520 mg/100mL) and
Quercetin-3-galactoside (0.00750 mg/100mL) as well as of Quercetin
aglycon (6.46±0.01 mg/100g) when determined by GC-MS. Percentage of
quercetin in the samples varied from 7.28±0.33 to 92.92±0.45%.
Statistical analyses of the results howed marked distances among S.
americanum, S. chenopodioides, S. nigrum and S. villosum but
indicated similarity between S. nigrum and S. retroflexum.
Epicuticular wax, a complex mixture of different constituents, is
considered another important parameter for chemotaxonomic studies.
The yields and physicochemical characters like Colour, Melting
point, Refractive index, Saponification value, Acid value and Ester
value of waxes extracted were compared. TLC indicated the presence
of different classes of compounds in the waxes. GC-MS analysis
showed the presence of Squalene, Phytol, Palmitic acid, Linolenic
acid, ester of Palmitoleic acid along with a variety of hydrocarbons
as the chemical constituents of these epicuticular waxes. The
hydrocarbons, alcohols, some of the esters, acids, aldehyde and
ketone identified had been reported for the first time in S. nigrum.
The cluster analysis indicated significant differences between S.
chenopodioides and S. villosum as well as in between S. americanum
and S. nigrum in their respective clusters. Again S. retroflexum
depicted great resemblance with S. nigrum in its epicuticular wax
composition.
The similarity index and the Euclidean distance among the clusters
formed by Multivariate cluster analysis of the above discussed
parameters helped drawing the conclusion that S. americanum, S.
chenopodioides, S. nigrum and S. villosum are distinct species of
genus Solanum but S. retroflexum might be regarded as a
variety/subspecies of S. nigrum.
The nutritional potentials of the plants were assessed through their
proximate and mineral analyses. The results of this research
indicated that the plants had nutritional qualities that could
provide the users with additional nutrients. Comparatively, because
of the relatively high contents of total protein, total ash and
crude fibres, the taxon S. nigrum could be good source of nutrients.
Antibacterial and antioxidant activities were carried out to
evaluate the medicinal value of the plants. The methanolic extracts
of the five taxa had shown significant antibacterial activity
against the Gram +ve and Gram –ve bacteria used. Infact, the
methanolic extracts of S. villosum showed a higher MIC value against
Proteus mirabilis compared to standard Benzyl penicillin. The
extracts of S. nigrum and S. retroflexum demonstrated matching
results which supported our conclusion that S. retroflexum may be
considered as a variety of S. nigrum. S. chenopodioides and S.
villosum also gave comparable but less coordinated results.
Antioxidant activity of the plant extracts was evaluated using six
different antioxidant assays. Results suggested that all taxa have
moderate effects on scavenging DPPH free radical. Total Phenolic
Contents of the five samples showed slight variations, ranging from
20.31-26.58 mg of GAE/100 g DW. In ABTS assay, S. retroflexum had
highest antioxidant capacity (33.88 mM/100 g DW). The effect of the
S. chenopodioides (70.37%) on metal chelation was found to be more
than all the other taxa. These activities were attributed to the
appreciable amounts of alkaloids, flavonoids and phenolic components
present in these plant samples.
The incredible morphological and chemical diversity, fundamental
economic importance and worldwide distribution make the Solanaceae
one of the most fascinating groups of flowering plants.
|
