NITROGEN ASSIMILATION AND DISTRIBUTION IN FIELD-GROWN COWPEA AT VARIOUS GROWTH-STAGES

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 N-15 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 aboveground 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.