Pesticidal and germinating enhancing relevance of anthraquinones from cassia tora (LINN) seeds

ABSTRACT Vigna unguiculata Lam (Cowpea) seeds is the major essential leguminous crop cultivated in sub-Saharan Africa and occupies a greater area of land than any other legume. The leaves, green pods, green peas and dry grains of cowpea are eaten all over the continent in different dishes for being nutritious to both humans and animals. Callosobruchus maculatus (Cowpea weevils) are known to destroy up to about 70 % of the seeds in storage. Pre-treatment of the seeds before storage includes the use of neem oil and cinnamaldehyde but both have been observed on the field to be toxic to the seeds and also human health. Seeds treated in this way have shown very low germinability. Thus, alternative safe and ecofriendly pesticides are needed to replace these standard agents. Cassia tora is grown on farmlands alongside food crops in the South-West Region of Cameroon as a protectant to seeds/grains against insect pests. Therefore the present study sought to investigate its edible seeds for ecologically friendly pesticides against Callosobruchus maculatus (cowpea weevils) and its potential as enhancers of cowpea seed-germination and seedling-vigor as folklore suggest. The study also explores the most active C. tora seed extract and its constituents for larvicidal activity against Anopheles gambaei third-fourth instar larvae. The petroleum ether (pet-ether), ethyl acetate and methanol extracts of C. tora were tested for insecticidal, oviposition deterrent, feeding inhibition, germination and seedling vigor enhancement effect on cowpea seeds using neem oil and cinnamaldehyde reference compounds. Pet ether extract of C. tora seeds showed the highest insecticidal activity (LC50= 8.33±0.6 μg/mL) against adult weevils, followed by the ethyl acetate and methanol extracts respectively. The ethyl acetate showed the highest germination and seedling vigor enhancement effect. The ethyl acetate extract was selected and assessed for mosquito larvicidal activity against the larvae of the malaria vector Anopheles gambiae. It exhibited larvicidal activity that was comparable to the positive control azadirachtin. It was then subjected to column chromatographic purification to study its bioactive compounds. The anthraquinones aurantio-obtusin, obtusin and a novel compound cassiatorin were isolated and assessed for various pesticidal activities. Aurantio-obtusin and cassiatorin showed comparable antifeedant activity as the reference compounds. Aurantio-obtusin and cassiatorin were the most effective enhancers of germination and vigor of the cowpea seeds and seedlings respectively when compared with the negative control. Cassia tora seed extracts and its anthraquinones aurantio – obtusin and obtusin possess larvicidal activity against Anopheles gambiae third-fourth instar larvae, and could serve as eco-friendly larvicides for control or management of mosquito population. Thus the present study gives credence to the folkloric use of C. tora seeds for crop/grain-protection from insect pest infestations as well as crop yield-enhancement. Unlike the reference drugs, C. tora seed extracts and isolated compounds were not toxic to the seeds and showed over 70% germination and seedling vigor enhancement activities. These activities are being reported in the plant (C. tora) for the first time. Even though the anthraquinones aurantio-obtusin and obtusin have been isolated from C. tora and other cassia species, this is the first report of their insecticidal, oviposition deterrent, antifeedant, germination and seedling vigor enhancement activities. A novel compound, 1, 3, 8-trihydroxy-2, 7-dimethoxy-6-methyl-9, 10-anthraquinone is being reported for the first time and is given the trivial name ‘cassiatorin’ in conformity with the naming of anthraquinones from Cassia species using their botanical origin.

Source: KNUST