Effects of water stress on phenolic content and antioxidant activity of African nightshades

Abstract

Abstract. Okello OP, Gweyi JPO, Nawiri Mp, Musila W. 2017. Effects of water stress on phenolic contents and antioxidant activity of African nightshades. Biofarmasi J Nat Prod Biochem 15: 74-90. This study aimed to map out the distribution of different African Nightshade species in Siaya and Kisii Counties of Kenya and determine the 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). Before selecting the two varieties, the study involved field visits, mapping of nightshades present, and administering semi-structured questionnaires to farmers to determine the indigenous vegetables being grown, the nightshade species grown, and factors affecting their production. The experiments were conducted both in the field and in greenhouse conditions. Watering intervals were at 15 cbars, 50 cbars, and 85 cbars. Data on the number of secondary buds, leaf area, shoot height, and shoot and root dry weights were gathered. The total antioxidant capacity and the total phenolic content were recorded using the DPPH radical scavenging method and the Folin-Ciolcalteu method, respectively. The data collected were subjected to ANOVA. In both counties where production was 100% under small scale, Solanum scabrum was the main variety grown in 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. At all stress levels, Solanum scabrum exhibited the tallest plant with a maximum height of 45.17cm at 15cbars. At the same time, Solanum villosum had the shortest plants at all stress levels, with the shortest one being recorded at 16.65 cm at 85 cbars. S. scabrum also had the highest root dry weight (7.78g), shoot dry weight (50.78g), and highest leaf area (304.45cm2). However, Solanum villosum had the highest number of secondary buds at all stress levels, with the highest being 24 at 15 cbars. Concerning 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). Meanwhile, Solanum scabrum had a higher concentration in the roots (25.06gGAE/Kg DM total phenolic content and 27.18% total antioxidant activity). Water stress causes a decline in all growth parameters but increases phytochemical accumulation in nightshade accessions grown. Therefore, it is suggested that for better yields, irrigation should be performed at every 15 cbars; however, for adequate phytochemical accumulation, the irrigation should be carried out at 50 cbars. Further research must explore and quantify other phytochemical components affected by different watering regimes.