Nitrogen Partitioning in Cowpea as Influenced by Rhizobial Strain and Mineral Nitrogen

Partitioning of nitrogen into the pods of legumes is an important yield determinant. The ability of cowpea [Vigna unguiculata(L.) Walp.] ‘Bush Purple Hull’ to partition N from dinitrogen fixation and assimilated nitrate into pods was measured with the aid of 15N. Cowpea was inoculated with four strains of Bradyrhizobiumspecies and grown in a glasshouse to maturity. Two nitrate treatments were included; one provided an initial dose of starter N (40 mg plant−1) and the other provided split applications of N throughout the growth period (total of 240 mg plant−1). With starter N only, the results indicated that the strain of rhizobia did not influence partitioning of biologically fixed dinitrogen or nitrate N into pods. However when inoculated with either of two strains, cowpea partitioned approximately 63% of the total assimilated nitrate from the continuous N treatment into the pods, whereas another strain partitioned approximately 52% into pods. Of the total N from dinitrogen fixation, 53 and 58% was partitioned into the pods for the starter N and continuous N treatments respectively. The strains of rhizobia were similar in the quantity of nodule tissue formed on the roots, but the quantity of N contained in the nodule tissue varied from 40 to 20 mg N plant−1for the starter N treatment. It appears that strains of rhizobia should be considered when undertaking experiments involving N partitioning.