Abstract
An in vivo assay for nitrate reductase (NR) activity was adapted to measure total NR activity in soybean [Glycine max (L.) Merr.] plants grown for a 29-day period indoors. Disappearance of nitrate from the nutrient solution, plant nitrate and total plant nitrogen (N) also were measured. Under the conditions of this experiment, nitrate reduction estimated from NR activities agreed closely with actual nitrate reduction. The same assay was used to measure leaf NR activities of field-grown soybeans throughout the 1971 growing season. Leaf NR activities accounted for 77 and 72% of the total N uptake in plants receiving 0 and 280 kg N as NH4NO3/ha, respectively. Measurements of nitrate and ammonium losses from soil under soybeans and under adjacent bare soil at three stages of plant development suggested that in plots receiving no fertilizer N, 86% of N uptake from the soil was in the form of nitrate. The NR activity of field-grown plants agreed well with total plant N derived from soil nitrates. Results indicated that leaf NR activities were proportional to nitrate uptake and might be used to determine amounts and seasonal patterns of nitrate uptake by soybean plants.
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