Carbon Metabolism, Nitrogen Assimilation, and Seed Yield of Cowpea (Vigna unquiculataL. Walp.) Grown in an Adverse Temperature Regime

Abstract

When grown in an environment known not to favour the production of large seed yields (warm days-cool nights; 33–19 °C), non-nodulated plants of cowpea cv. K 2809 supplied with abundant inorganic nitrogen not only assimilated N more rapidly but also produced larger total dry weights and seed yields than plants dependent on Rhizobium CB 756. Remobilization of nitrogen from vegetative organs started sooner in nitrate-dependent than in nodulated plants and contributed 69 and 47%, respectively, to the N content of mature fruits. Plants dependent on nodules relied more on current assimilation of nitrogen during the later stages of fruit growth than those given inorganic N; they also utilized a larger proportion of shoot-derived photosynthates in growth of organs below ground and in the respiratory activities of both nodules and supporting roots. Although nitrate-dependent plants developed larger shoot systems than those relying on nodules, the distribution of carbon and nitrogen to leaves decreased markedly as branches extended during early reproductive growth. The respiration of roots on nodulated plants became more efficient during the later stages of fruit growth whereas the population of secondary nodules present at this stage of development respired less efficiently (mg C consumed per mg N assimilated) than the primary nodules present earlier during development.