Nitrogen Nutrition of Nodules in Relation to `N-Hunger' in Cowpea (Vigna unguiculata L. Walp)

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Abstract
Early growth, nodule development, and nitrogen fixation by two cultivars of cowpea (Vigna unguiculata L. Walp), one large-seeded (Vita 3; 146.0 ± 0.9 milligrams seed dry weight, 4.1 ± 0.2 milligrams seed N), the other small-seeded (Caloona; 57.5 ± 2.5 milligrams seed dry weight, 1.8 ± 0.1 milligrams seed N), were compared under conditions of sand culture with nutrient solution free of combined N. The seed stocks used had been obtained from plants uniformly labeled with 15N, thus enabling changes with time in distribution of cotyledon and fixed N among plant parts to be measured by isotope dilution. Caloona, but not Vita 3, showed physiological symptoms of `N hunger,' i.e. transient loss of chlorophyll (visible yellowing) and N from the first-formed unifoliolate leaves at or around the onset of symbiotic functioning and N2 fixation. The smaller-seeded Caloona showed higher early nitrogenase activity than the larger-seeded Vita 3 and by 28 days had fixed 6.6 milligrams of N per milligram of seed N [mg N · (mg seed N)−1] versus only 3.5 mg N · (mg seed N)−1 in Vita 3. Both cultivars lost around 30% of their initial seed N at germination, mostly as fallen cotyledons. Abscised cotyledons of Caloona contained 1.21 ± 0.17% N; those of Vita 3 contained 2.61 ± 0.37% N. When compared on the basis of cotyledon N available for seedling growth, Caloona was shown to have fixed 10.6 mg N · (mg seed N)−1 and Vita 3 only 5.3 mg N · (mg seed N)−1. Most of the cotyledon N withdrawn from the unifoliolate leaf pair of Caloona during `N-hunger' was committed to early nodule growth and, in total, 20 to 25% of the cotyledon N resource of this cultivar was ultimately invested in establishment of symbiosis compared with only 7% in Vita 3.