Medicinal properties and growth of Merwilla natalensis.
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Date
2003
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Abstract
Merwilla natalensis (Planchon) Speta is ranked as one of the most commonly sold
medicinal plants at most of the informal medicinal plant markets found throughout
South Africa. The increasing demand for medicinal plants has resulted in over-exploitation
of many of the wild populations. Overharvesting has resulted in M. natalensis being declared vulnerable. Although this species is so popular, and reports
state that the bulbs are used for a variety of ailments, very little is known about its
pharmacological activity or phytochemical composition.
Extracts were made from mature M. natalensis bulbs using hexane,
dichloromethane, methanol and water. These extracts were screened for
antibacterial, anticancer, anti-inflammatory, antischistosomal and anthelmintic
activity. Antibacterial activity was evaluated using the minimal inhibitory concentration
(MIC) assay. Methanol extracts displayed good antibacterial activity against both
Gram-positive (Bacillus subtilis and Staphylococcus aureus) and Gram-negative
(Escherichia coli and Klebsiella pneumoniae) bacteria. Anti-inflammatory activity was
evaluated using the COX-1 and COX-2 bioassays. Dichloromethane extracts
displayed the highest inhibitory activity against both COX-1 and -2 enzymes. (80%
and 91% inhibition respectively) Very good activity was displayed against the free-living
nematode Caenorhabditis elegans and the schistosomula worms of
Schistosoma haematobium using microdilution techniques. Anticancer activity was
evaluated using the biochemical induction assay (BIA) in which DNA-damaging
properties are tested for. No activity was found using this assay, however, these
results do not prove that M. natalensis does not have other anticancer properties.
The phytochemical investigation of mature M. natalensis plants showed the
bulbs to contain both saponins and bufadienolides. One of the bufadienolides had the
same Rf value as proscillaridin A. Cytotoxicity tests reveal M. natalensis to be
extremely cytotoxic, yet the bulbs are commonly sold at traditional medicine markets
around South Africa. This cytotoxicity may be accredited to the presence of saponins
within the bulbs. No alkaloids or tannins were detected in the bulbs.
With the growing population in South Africa, there is an increasing demand for
traditional medicines. This increasing demand is placing tremendous strain on natural
populations growing in the wild. However, as the demand cannot continue to be met
other sources are needed. Tissue cultured plants have been grown at two different
regions of South Africa. These plants have been grown under different conditions to
determine the optimal ones needed to grow M. natalensis as a commercial crop on
small-scale farms.
Plantlets taken directly from tissue culture were acclimatized successfully for
cultivation by means of simple and cost effective methods. Cultivated plants were
harvested on a six-monthly basis for a period of two years. Field cultivation produced
bulbs of almost marketable size (±300g fresh weight) after 24 months. Bulb size was
not dependent on additional fertilizer or irrigation. No significant differences (p<_0.05)
were shown in the average dry weights of bulbs grown under different treatments
(control, fertilizer without irrigation, fertilizer with irrigation). Leaf senescence and
dormancy of young plants were prevented with irrigation. Flowering occurred after 24
months, with the irrigation and fertilizer plot having the most flowering plants. TLC
fingerprinting revealed differences in the chemical composition of the bulbs harvested
at different stages of growth. Noticeable differences were found between bulbs
cultivated at the different growing sites.
Pharmacological screenings were done of the harvested bulbs to investigate
the effect of age (time of harvest) and growing conditions on antibacterial, anti-inflammatory
and anthelmintic activity. Methanol extracts were screened against
Gram-positive (Bacillus subtilis and Staphylococcus aureus) and Gram-negative
(Escherichia coli and Klebsiella pneumoniae) bacteria. Variations in activity were
found. The time of harvest had a significant effect (p<_0.05) on biological activity, with
the younger plants being more active. Antibacterial activity decreased with an
increase in plants age.
Methanol extracts were also screened for anthelmintic activity against
Caenorhabditis elegans. Activity was found to increase with plant maturity. Irrigation
was found to increase activity at the low rainfall (Fort Hare) site. Bulbs harvested
from the irrigation treatment had significantly higher anthelmintic activity (p<_0.05)
than bulbs harvested from treatments without irrigation. Dichloromethane extracts
from bulbs grown at both sites had high anti-inflammatory activity. There were no
significant differences (p<_0.05) in the activity of bulbs harvested from the different
treatment plots. The time of harvest had an effect on the inhibition of prostaglandin
synthesis by COX-1 enzymes.
This study provides not only scientific verification for the use of M. natalensis
to some extent as a medicinal plant, but also important data needed to successfully
cultivate this species as a crop for small-scale farming.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 2003.
Keywords
Merwilla Natalensis., Traditional medicine--South Africa., Medicinal plants--South Africa., Theses--Botany.