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

Abstract

Experiments were conducted to determine if nitrate ("5N-labeled) was taken up and assimilated by intact soybean (Glyciae 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 '5N-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 ("5N-labeled) was present in a nonreduced form, although considerable solublereduced nitrogen ('5N-labeled) accumulated throughout the dark period. The '5N-labeled, soluble-reduced nitrogen fraction accounted for nearly 30% of the total '5N found in plant roots and more than 63% of the total '5N 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 '5N-labeled nitrogen compounds from previously '5N-labeled plants to a natural abundance '5N nutrient solution. Thus, evidence was obtained which indicated that light was not directly essential for flux and reduction of nitrate by intact soybean plants.Initial investigations concerning diurnal NRA' in higher plants indicated that assimilation of NO3-and NO2-to reduced N compounds (amino acids) was highly dependent on light and was negligible in darkness (2). More recent studies (1,6,7,8,10,15,19) have reported substantial rates of "5NO3 and '5NO2 assimilation in the dark in a wide variety of plant species. Evidence for dark assimilation of N into reduced N compounds has been derived from NR assay methods (5,6,16,19), from xylem exudate analysis (10), and from analysis of modified plant parts such as excised leaves (8), or excised roots (10). Evidence that soybeans are capable ofNO3-metabolism in the dark comes from in vivo assays on leaf sections where '5NO3 was shown to be fully reduced to amino N (15). However, no data was found in the literature to indicate that intact soybean plants were capable of reducing NO3-to the amino level in the dark.In the present study, hydroponically grown soybean plants were subjected to changes in '5N enrichment immediately prior to a dark cycle. Whole plants, plant parts, and nutrient solutions were analyzed for '5N content to determine flux and translocation of NO3-in the dark, and to establish that NO3-was fully reduced by intact soybeans in the dark.

Influence of fruit load and environmental factors on nitrate reductase activity and on concentration of nitrate and carbohydrates in leaves of eggplant (Solanum melongena)

“…This view is further supported by the observation that nitrate reduction in shoot and roots occurs in both light and darkness (Aslam et ai. 1979, Rufty et al 1984, Nicholas and Harper 1985.…”

Nitrate and ammonium influxes in soybean (Glycine max) roots: direct comparison of 13N and 15N tracing

“…Our results throw some interesting light on the transport of N from root to shoot. Soybean is a species in which the majority of nitrate reduction occurs in the leaves (Touraine et al 1988), even though root nitrate reductase activity may be appr-eciable in some circumstances (Nicholas & Harper 1985). It is reasonable to assurrre, however, that much of the N in the xylem sap was nitrate (Touraine et al 1994).…”