Effects of nitrate level on nitrogen metabolism in winged bean and soya bean

The effects of high (15 mM) and low (0.75 mM) solution nitrate levels on nitrogen metabolism in three genotypes (IL 7A, IL 13 and IL 21) of winged beans [Psophocarpus tetragonolobus (L.) DC.] and one genotype (Williams) of soya bean [Glycine max (L.) Merrill] were investigated. Plants were grown for 42 days in a greenhouse in solution culture prior to sampling.The 15 mM nitrate treatment resulted in greater growth of all plant parts except roots. Growth of soya beans was more responsive to nitrate level than was growth of winged beans. The high nitrate level inhibited nodulation in all plants. The IL 13 and IL 21 winged bean genotypes had similar nitrogenase activity (acetylene reduction per plant) as the soya bean and IL 7A winged bean genotype had lower activity. However, the IL 13 winged bean genotype had higher nitrogenase activity (acetylene reduction per unit nodule mass) than the other three genotypes which all had similar activity. The 15 mM solution nitrate level stimulated leaf and root nitrate reductase (NR) activity for all plants. All winged bean genotypes had higher leaf NR activity and higher percentage reduced- and nitrate-nitrogen contents of leaves and stems compared with soya beans. However, total protein (reduced nitrogen) was greater in soya beans when sampled indicating that more nitrate had been metabolized by soya beans than by winged beans during the 42-day growth period.