Effects of Ni Deficiency on Some Nitrogen Metabolites in Cowpeas (Vigna unguiculata L. Walp)

Cowpeas grown in nutrient solutions, from which Ni had been removed by a ligand exchange technique, accumulated urea in most tissues. Urea levels were highest (up to 3.1 percent dry weight) in necrotic leaf tips. Urea accumulation in Ni-deficient cowpea tissues amounted to about 1 percent of the total N. The accumulation of urea was presumably associated with the catabolism of N compounds in older tissues and the redistribution of N catabolites within the plant during the reproductive growth. The exclusion of N salts from the nutrient media at a late stage of growth, either with or without added Ni, led to a general amelioration of urea accumulation and a lower level of the related amino acid, arginine, in root and stem tissue. Plant leaves that contained toxic levels of urea and displayed necrotic symptoms had tissue Ni levels ranging from less than 0.01 to 0.15 mug Ni per gram dry weight. Nickel concentrations in tissue from plants not treated with Ni, were initially very low, but increased as the cowpeas matured. Apparently, there was a source of Ni contamination in the Ni-deficient growth media which provided a source of Ni for uptake by the plants during growth. Ureide levels were low and unaffected by Ni deprivation. No evidence for free purines or uric acid accumulation in plant tissues could be found. It is hypothesized that Ni (and urease) participates in the normal N metabolism of these plants during the reproductive phase of growth.