Influence of Nitrogen Fertiliser Rates on the Amino Acids Composition in African Nightshade (Solanum scabrum) Young Leaves

Abstract

African leafy vegetables are important sources of dietary nutrients, including amino acids, but agronomic factors can influence their nutritional composition. Such nutrients include amino acids (AAs), which play important roles in human health, and some of their by-products are implicated in the management of lifestyle diseases. For example, gamma-aminobutyric acid (GABA) from glutamic acid (GA) is implicated in the management of both hypertension and diabetes. African leafy vegetables (ALVs) are consumed within poor communities where the disease burden is grave. The potential use of ALVs to manage lifestyle diseases is not acknowledged. African nightshade (Solanum scabrum) is among the most consumed ALVs in western Kenya. However, agronomic methods for enhancing AA levels in African nightshade are not well documented. Nitrogen fertilisers have been documented to enhance AA levels in some plants. The influence of nitrogen rates on the AA composition of African nightshade was investigated. The trial was laid out in a randomised complete design with three replications, with nitrogen (urea) rates (0, 50, 100 and 150 Kg N/ha) as treatments. The young leaves were harvested after six weeks, processed and analysed for AAs using LC-MS. A total of fourteen AAs were detected, with leucine > GA > isoleucine as the main AAs. Levels of all AAs increased (p ≤ 0.05) with increasing nitrogen rates. The presence of high levels of GA in African nightshade suggests its potential use in the production of GABA-rich vegetables in the human diet. GA is readily decarboxylated to GABA when the vegetable is subjected to milk treatment or kept under anaerobic conditions. The use of high rates of nitrogenous fertiliser is recommended as an agronomic technology to enhance levels of beneficial AAs in African nightshade.