Changes in leaf yield and nutritive quality of the black nightshade (Solanum nigrum) as influenced by nitrogen application
The effect of nitrogen application rates on the leaf yield and nutritive quality of the black nightshade (Solanum nigrum L.), an indigenous Kenyan leafy vegetable was investigated. Application of 5 g N per plant gave the highest fresh leaf yield of 19 and 40 tons/ha after 10 and 12 weeks of growth, respectively. The dry matter and crude fibre contents decreased with increasing nitrogen rates. With a supply of 5 to 15 g N per plant, the dry matter and crude fibre contents were on average 8.3% and 17.8 g/100 g dry matter. The crude fat and protein contents increased with nitrogen supply. Plants treated with N fertilizer yielded about 60% more proteins than the control plants grown with zero g N. The level of beta-carotene in the leaves increased by about 60% from 63 mg/100 g fresh weight with zero g N to 103 mg/100 g fresh weight with 5 g N per plant. Although, the ascorbic acid content increased by over 100% in plants grown with 5 g N per plant, thereafter, a declining trend was observed with higher rates of N supply. Nitrogen application had little effect on the levels of potassium, calcium and magnesium in the leaves. In addition, no major effect of nitrogen supply was observed on the levels of oxalates and phenolic compounds. However, the accumulation of nitrate-nitrogen (NO3-N) in the leaves was greatly enhanced by nitrogen application. In leaves of plants supplied with 5 g N, the amount of NO3-N was ten times more than in the control plants. Important aspects on the dietary contribution of S. nigrum leaves are also discussed.
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