N Form and Concentration: Effects on N Absorption, Growth, and Total N Accumulation with Southernpeas1

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Abstract
Nitrate and NH4+ absorption by southernpeas (Vigna unguiculata (L.) Walp.) during the growth cycle was determined in solution culture at 7-day intervals. Dry weight and total mg of N for each plant part were determined at bloom initiation, pod initiation, and seed maturity. When the N concentration was deficient virtually all of the N3− was absorbed by southernpea plants from the 3rd week until harvest. At sufficient N concentrations a single peak N3− uptake period occurred in the growth cycle prior to bloom initiation. Nitrate absorption predominated when NH4+ constituted 50% or more of the N form at both N concentrations. Ammonium had either 1, 2 or 3 absorption peaks depending upon its relative concentration with NO3−. The first NH4+ absorption peak was the largest and coincided with the growth cycle just prior to bloom initiation. Dry weight and N accumulation were greatest prior to bloom initiation. Subsequent accumulation occurred primarily in the seed. A reduction in N content and dry weight in the vegetative portions, particularly the leaves, coincided with the accumulation of N and dry weight in the seeds. Use of total mg of N in the vegetative tissue as a means of indicating the N status of the plant and subsequent yield was not accurate.

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