Nitrogen Assimilation and Distribution in Field‐Grown Cowpea at Various Growth Stages

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Abstract
The sources of N and the periods during which they are assimilated can influence the redistribution of N within different organs and, consequently, the final grain yield and N content of legumes. The 15N isotope‐dilution approach was used to measure the amounts of N derived from atmospheric N2 (Ndfa), soil (Ndfs), and fertilizer (Ndff) in the aboveground portion of field‐grown cowpea [Vigna unguiculata (L.) Walp.] at the following developmental stages: late vegetative, early pod filling, mid pod filling, and physiological maturity. The soil was classified as an Oxic Paleudalf. The principal sources of cowpea's N changed markedly during development. While Ndfs supplied 80% of the total N assimilated in the aboveground plant parts during the first 42 d of growth, Ndfa furnished over 80% of the N assimilated during the remaining and most N‐demanding period of cowpea growth. At physiological maturity, the total above‐ground plant N was composed of 100, 50, and 5 kg N ha−1 as Ndfa, Ndfs, and Ndff, respectively. The preexisting N pools in vegetative parts of cowpea did not contribute much N towards pod fill, with the currently assimilated N (predominantly as Ndfa) contributing 87% of the N for pod fill. At physiological maturity, the N in pods was only slightly in excess of Ndfa in the aboveground plant parts. We conclude that Ndfa can contribute significantly towards the N nutrition of field‐grown cowpea, serving also as an important source of N for pod development.
Funding Information
  • IAEA Technical Assistance Project (NIR/5/013, RAF/5/010)
  • Inst. of Agricultural Research and Training

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