The effects of nitrogen rates on yield, quality and nodulation, and storage temperatures on water and ascorbic acid changes in cowpea (vigna unguiculata (L) walp) leaf vegetable

Abstract

Two experiments were carried out between October and December 1988, and repeated between January and May 1989 at the Field Station, Faculty of Agriculture, Kabete \campus, University of Nairobi, Kenya, to investigate the effect of four nitrogen rates (0, 60, 80 and 100 kg N ha") on cowpea leaf yield, dry matter, nutrient composition, nitrate accumulation and root nodulation using two varieties ( Vita-3 and K-80 ). Leaves from Vita-3 were also used for in storage experiments in which water and ascorbic acid changes were determined at room (21±7°C) and refrigeration (4±1°C ) temperatures. Results showed that application of N had no significant effect on fresh leaf yield, leaf dry matter, crude fibre, beta carotene, ascorbic acid, minerals and nitrate accumulation. The two varieties too, did not show significant differences in leaf nutrient composition. Nitrogen application reduced root nodule weight per plant significantly but had no effect on nodule numbers per plant. Both varieties differed significantly in nodule numbers and weights per plant. Leaf protein tended to increase with increased N-rates and was significant in one experiment in which it was 28.6% for cowpea that had received 100 kg N ha-'. Both leaf ether extract and ash increased significantly with applied N. A few minerals such as K+ and ca " tended to increase with applied nitrogen though not significantly. Laboratory experiments showed that cowpea leaves lost fresh weight, significantly at room and refrigeration storage. Losses were,however,less drastic in refrigerated storage relative to room storage. Significant losses in ascorbic acid occured for cowpea leaves stored for three days at room temperature. When refrigerated, the vitamin, however,was not significantly altered