Effects of water stress on antioxidant activity and phenolic contents of African nightshades and their distribution in Kisii and Siaya counties of Kenya

Drought is a major abiotic factor limiting crop productivity in many regions of the world. It causes reduction in plant growth, dry matter accumulation and decline in plant water status of the plants and in certain cases interferes with biochemical processes within cultivated crops. The aim of the study was to map out the distribution of different African Nightshade species in Siaya and Kisii Counties of Kenya and to determine water stress effect on total antioxidant capacity and total phenolic content of two selected African nightshade namely, giant nightshade (Solanum scabrum) and black nightshade (Solanum villosum). Prior to selection of the two varieties, the study involved farmer field visits, mapping of nightshades present and administration of semi-structured questionnaires to farmers to determine the indigenous vegetables being grown, the nightshade species grown and factors affecting their production. Experiments were conducted both in the field and greenhouse conditions. Watering intervals were at 15 cbars, 50 cbars and 85 cbars. Data on number of secondary buds, leaf area, shoot height, shoot and root dry weights were gathered. The total antioxidant capacity was recorded using DPPH radical scavenging method and the total phenolic content using Folin-Ciolcalteu method. The data collected were subjected to ANOVA. In both counties production was 100% under small scale, with Solanum scabrum being the main variety grown is Siaya County (36%) while in Kisii the main variety was Solanum villosum (32%). There were significant differences (P≤0.05) among treatments in leaf area, plant height, shoot biomass, number of secondary buds, leaf and root total phenolic content and leaf and root antioxidant activity. Solanum scabrum had the tallest plants at all stress levels with a maximum height of 45.17cm at 15cbars, while Solanum villosum had the shortest plants at all stress levels, with the shortest one being recorded at 16.65 cm at 85 cbars. Solanum scabrum also had the highest root dry weight (7.78g), shoot dry weight (50.78g) and highest leaf area of (304.45cm2), however Solanum villosum had the highest number of secondary buds at all stress levels with the highest being 24 at 15 cbars. With respect to phytochemicals, Solanum villosum had a higher concentration of both the total phenolics and antioxidant activity in the shoots (46.41g GAE/Kg DM total phenolic content and 52.68% total antioxidant activity) while Solanum scabrum had higher concentration in the roots (25.06gGAE/Kg DM total phenolic content and 27.18% total antioxidant activity). Water stress led to a decline in all growth parameters but enhanced phytochemical accumulation in nightshade accessions grown. It is therefore recommended that for better yields irrigation should be done at every 15 cbars, however for adequate phytochemical accumulation, irrigation should be carried out at 50 cbars. Further research to explore and quantify other phytochemical components as affected by different watering regimes need to be undertaken.