Abstract
Plant of cowpea cv. K 2809 were nodulated with Rhizobium train CB 756 and grown in pot in a simulated tropical environment in growth cabinets. The nodulation, dinitrogen fixation, growth and seed yield of plants grown without inorganic nitrogen were compared with other plants given supplementary nitrogen (20 p.p.m. NO3-N in solution) either daily throughout growth or before or after the onset of flowering only. Providing inorganic nitrogen during the vegetative period stimulated nodulation and nitrogenase activity: plants were more leafy and produced a larger number of potential reproductive sites (nodes) and larger seed yields than those dependent on nodules before flowering. Applying inorganic nitrogen during the reproductive period impaired nitrogenase activity and restricted symbiotic longevity, but stimulated vegetative dry matter production and improved seed yield significantly compared with plants dependent on nodule throughout growth. Although all plants which received inorganic nitrogen produced larger economic yield than those which did not, irrespective of stage of growth or duration of supply, nodule-dependent plants were equally, if not better, able to distribute reduced nitrogen into seeds, at least as judged from the percentage values of fruit (seeds and pod walls). These data, together with many other on this symbiotic combination, are discussed in term of the physiological basis of 'successful symbiosis' and their implication for cowpea breeders who seek to produce effective symbiotic association for tropical, subsistence farming situations.
