Uptake and Reduction of [15N]Nitrate by Intact Soybean Plants in the Dark

Experiments were conducted to determine if nitrate ((15)N-labeled) was taken up and assimilated by intact soybean (Glycine max [L.] Merr. cv Williams) plants during extended periods of dark. Nitrate was taken up by soybean roots throughout a 12-hour dark period. The (15)N-labeled nitrogen was also translocated to the plant shoots, but at a slower rate than the rate of accumulation in the roots. Much of the nitrogen ((15)N-labeled) was present in a nonreduced form, although considerable soluble-reduced nitrogen ((15)N-labeled) accumulated throughout the dark period. The (15)N-labeled, soluble-reduced nitrogen fraction accounted for nearly 30% of the total (15)N found in plant roots and more than 63% of the total (15)N found in plant tops after 12 hours of dark. This provided evidence that intact soybean plants take up and metabolize significant quantities of nitrate to reduced N forms in the dark.In addition to nitrate influx during the dark, it was shown that there was a concomitant loss of (15)N-labeled nitrogen compounds from previously (15)N-labeled plants to a natural abundance (15)N nutrient solution. Thus, evidence was obtained which indicated that light was not directly essential for flux and reduction of nitrate by intact soybean plants.