Distribution of fixed-N and nitrate-N in cowpea during pod development

If the quality and quantity of yields from cowpea (Vigna unguiculata L.] Walp.) are to be maximised, a complete understanding of the N nutrition of the plant must be achieved. The N requirement for developing pods of this species may come from mobilization of N in vegetative tissue, biological N fixation and uptake of N from soil. In this study, the fate of a pulse of fixed ¹⁵N₂ or of ¹⁵NO₃-given to different cowpea plants during pod development was determined. The plants were grown in vermiculite in plastic pots that were able to be sealed with silicone adhesive and equipped with a rubber septum so that ¹⁵N₂ gas could be injected into the air space above the vermiculite, and gas losses would be eliminated. Nineteen days after injection of ¹⁵N₂ the pods, leaves, nodules and roots contained 65%, 15%, 9%, and 4%, respectively of the quantity of ¹⁵N₂ fixed. When ¹⁵NO₃-¹⁵N was taken up by other plants during this period, these plant parts contained 40%, 26%, 3% and 19%, respectively, of the total plant ¹⁵N. The percentage ¹⁵N in roots was greater, and that of ¹⁵N in nodules was lower, when ¹⁵NO₃-¹⁵N was applied than when ¹⁵N₂ was utilised by plants. These results indicate that, while a high percentage of fixed-N or NO₃-N given to cowpea plants moved to the developing pods, other sinks were competing for this newly-aquired N.