John Wesonga

Indigenous leafy vegetables of Eastern Africa — A source of extraordinary secondary plant metabolites

Indigenous African leafy vegetables vary enormously in their secondary plant metabolites whereat genus and the species have a great impact. In African nightshade (Solanum scabrum), spiderplant (Cleome gynandra), amaranth (Amaranthus cruentus), cowpea (Vigna unguiculata), Ethiopian kale (Brassica carinata) and common kale (Brassica oleracea) the specific secondary metabolite profile was elucidated and gained detailed data about carotenoids, chlorophylls, glucosinolates and phenolic compounds all having an appropriate contribution to health beneficial properties of indigenous African leafy vegetables. Exemplarily, various quercetin glycosides such as quercetin-3-rutinoside occur in high concentrations in African nightshade, spiderplant, and amaranth between ~1400-3300μg/g DW. Additionally the extraordinary hydroxycinnamic acid derivatives such as glucaric isomers and isocitric acid isomers are found especially in amaranth (up to ~1250μg/g DW) and spiderplant (up to 120μg/g DW). Carotenoids concentrations are high in amaranth (up to101.7μg/g DW) and spiderplants (up to 64.7μg/g DW) showing high concentrations of β-carotene, the pro-vitamin A. In contrast to the ubiquitous occurring phenolics and carotenoids, glucosinolates are only present in the Brassicales species Ethiopian kale, common kale and spiderplant characterized by diverse glucosinolate profiles. Generally, the consumption of a variety of these indigenous African leafy vegetables can be recommended to contribute to different benefits such as antioxidant activity, increase pro-vitamin A and anticancerogenic compounds in a healthy diet.

Steroidal glycoalkaloids: chemical defence of edible African nightshades against the tomato red spider mite, Tetranychus evansi (Acari: Tetranychidae)

BACKGROUND Tetranychus evansi is an invasive pest of solanaceous crops in Africa, and in the field it differentially attacks edible African nightshades. The chemical basis for the differential attack on these plant species is largely unknown. Using bioassays and chemical analysis, we investigated the differential bioactivity of leaf extracts of three edible African nightshade species, Solanum sarrachoides, S. scabrum and S. villosum, on adult T. evansi females. RESULTS Only the bioactivity of the leaf extract of S. sarrachoides (LC50 7.44 mg mL(-1)) and that of its most polar fraction (LC50 5.44 mg mL(-1)) paralleled that of the positive control, neem oil (LC50 1.89 mg mL(-1)), across all doses tested. Liquid chromatography-quadruple time of flight-mass spectrometry identified a mixture of steroidal glycoalkaloids (SGAs), including α-solasonine, α-solamargine and derivatives of tomatine and demissine, which were neither detected in the crude extract nor in any of the fractions obtained from S. scabrum and S. villosum. CONCLUSION Our results suggest that the presence of SGAs may play a key role in the differential defence of edible African nightshades against attack by T. evansi. These findings may add to the plethora of environmentally friendly tools from natural plant products for management of T. evansi.

Steroidal glycoalkaloids: Edible African nightshades chemical defence against the tomato red spider mite, Tetranychus evansi (Acari: Tetranychidae)

BACKGROUND Tetranychus evansi is an invasive pest of solanaceous crops in Africa, and in the field it differentially attacks edible African nightshades. The chemical basis for the differential attack on these plant species is largely unknown. Using bioassays and chemical analysis, we investigated the differential bioactivity of leaf extracts of three edible African nightshade species, Solanum sarrachoides, S. scabrum and S. villosum, on adult T. evansi females. RESULTS Only the bioactivity of the leaf extract of S. sarrachoides (LC50 7.44 mg mL(-1)) and that of its most polar fraction (LC50 5.44 mg mL(-1)) paralleled that of the positive control, neem oil (LC50 1.89 mg mL(-1)), across all doses tested. Liquid chromatography-quadruple time of flight-mass spectrometry identified a mixture of steroidal glycoalkaloids (SGAs), including α-solasonine, α-solamargine and derivatives of tomatine and demissine, which were neither detected in the crude extract nor in any of the fractions obtained from S. scabrum and S. villosum. CONCLUSION Our results suggest that the presence of SGAs may play a key role in the differential defence of edible African nightshades against attack by T. evansi. These findings may add to the plethora of environmentally friendly tools from natural plant products for management of T. evansi.

Differential effects of various African nightshade species on the fecundity and movement of Tetranychus evansi (Acari: Tetranychidae)

The tomato red spider mite Tetranychus evansi Baker & Pritchard is a serious pest of solanaceous plants worldwide. Management of this oligophagous pest in African nightshades has been a challenge to smallholder African farmers due to its high reproductive rate and rapid development of resistance to synthetic pesticides. The aim of the present study was to determine the influence of leaf trichomes on T. evansi by comparing its fecundity and movement on the leaf surfaces of five African nightshade species, namely Solanum sarrachoides Sendter, S. villosum Miller, S. tarderemotum Bitter, S. americanum Miller and S. scabrum Miller. Data were recorded in the laboratory at 23 ± 1°C, 50–70% relative humidity and a 12 h light:12 h dark photoperiod for the effect of trichome type and density of the abaxial leaf surface on mite fecundity. Distances travelled by mites on the leaf surface from the edge of a thumbtack pin inserted on the leaf were also recorded. Different trichomes, glandular and non-glandular types, were identified. There was a significant negative correlation of fecundity and distance walked by mites with the density of glandular trichomes. Significantly fewer eggs were laid on S. sarrachoides in comparison with the other Solanum species. The distance walked by mites was also significantly shorter in this species, indicating that higher densities of glandular trichomes interfere with mite movements. These results suggest that African nightshade genotypes differ in their levels of resistance to T. evansi, which is partially associated with differences in trichome types and their densities.