Effects of phosphorus deficiency on secondary metabolites and distribution of African nightshade in Siaya and Kisii counties, Kenya

African Indigenous Vegetables form an integral part of the Kenyan diets, among the most commonly consumed being the African nightshade. These vegetables contain important phenolics that have medicinal values and good health attributes. The abundance of these phenolic substances has strongly been associated with phosphorus use efficiency. In order to investigate the effect of phosphorus stress on African nightshade distribution, a purposive research was done from February 2014 on 70 random farmers growing African nightshade using semi-structured questionnaires in Siaya and Kisii Counties, Kenya. The coordinates, plant and soil samples for the surveyed regions were taken and later on used to map the vegetables according to plant and soil phosphorus, total phenolic content (TPC), total antioxidant activities (TAA) and the two dominant varieties. Solanum scabrum had 69% distribution whereas Solanum villosum had 21% hence chosen for greenhouse and field experiments. The two varieties were planted in greenhouse and two field cropping seasons (long and short rain seasons) at Kenyatta University farm as split plot arrangements with two varieties being the main plot and phosphorus levels (0, 20, 40 and 60 kg/ha) constituting the subplot in Randomized Complete Block Design. The treatments were replicated four times. Data on plant height, fresh weight, number of secondary buds, leaf and root area was recorded and later the effects resulting from these treatments analysed using ANOVA. In greenhouse and field experiments, plant height, fresh weight, number of secondary buds, leaf and root area, TPC and TAA were significantly (p≤0.05) affected by the level of phosphorus. TPC and TAA decreased with increase in P, whereas all the growth parameters increased with increase of Phosphorus levels apart from height and weight of plants treated with 60kgP/ha that showed either stagnation or a slight decline. The decline was attributed to zinc and copper deficiency. The highest fresh weight was recorded at 40 kgP/ha, where Solanum villosum yielded 6.1t/ha and Solanum scabrum yielded 6.35t/ha. Solanum villosum had higher TPC and TAA (6.09mg/g and 38.58%) respectively as compared to Solanum scabrum that had 5.49mg/g (TPC) and 35.92% (TAA). Solanum villosum had more secondary metabolites in the shoots as compared to the roots, the converse was reported for Solanum scabrum. Results obtained will be used to educate farmers and extension personnel on ways of improving productivity of African nightshade and will also help develop product labels highlighting their phenolic and antioxidant contents. Farmers are therefore advised to apply phosphorus at the rate 40 kg/ha as it had the highest biomass production. Solanum villosum is recommended for soils with limited phosphorus. Further research to be done on other important nutrients for instance anthraquinones and how different levels of macronutrients affect their production.