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Decreased Rates of Nitrate Uptake During Pod Fill by Cowpea, Green Gram, and Soybean

Dep. of Genetics, Iowa State Univ., Ames, IA 50011
Dep. of Agriculture, Univ. of Queensland, St. Lucia, Queensland, Australia 4067

^* Corresponding author.

Grain legumes derive their seed-N from soil-N uptake, remobilization of plant-N, and N₂ fixation. To establish the profile of nitrate uptake and the contribution made by soil-N uptake during pod fill, non-nodulated plants were grown in solution culture, and net rates of nitrate-N uptake were measured throughout the plant growth cycle. Green gram [Vigna radiata (L.) Wilzek cv. Berken], cowpea [Vigna unguiculata (L.) Walp. cv. Ife Brown], and soybean [Glycine max (L.) Merr. cvs. Bragg and Harosoy] were utilized in these studies. At physiological maturity, plants were harvested and dry weight and N content were measured. For the determinate legumes, green gram and Bragg soybean, seeds represented approximately 25% of the total plant dry weight and contained approximately 45% of the total plant-N. For the indeterminate cowpea and Harosoy soybean, seeds accounted for approximately 35 to 40% of the total plant dry matter and contained approximately 50 and 70%, respectively, of the total plant-N. The fraction of nitrate-N taken up after the onset of pod fill ranged from approximately 20% for the determinate to 30% for the indeterminate cultivars. Thus, it can be calculated that a maximum of approximately 50% of the total seed-N for each of the four cultivars was derived directly from nitrate-N uptake during pod fill, and the remaining 50% came from mobilization of N in vegetative tissue.

Received for publication October 8, 1987.

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