Nitrate Reductase Activity in Soybeans (Glycine max [L.] Merr.)

Abstract

Growth chamber studies with soybeans (Glycine max [L.] Merr.) were designed to determine the relative limitations of NO₃⁻, NADH, and nitrate reductase (NR) per se on nitrate metabolism as affected by light and temperature. Three NR enzyme assays (+NO₃⁻in vivo, −NO₃⁻in vivo, and in vitro) were compared. NR activity decreased with all assays when plants were exposed to dark. Addition of NO₃⁻ to the in vivo NR assay medium increased activity (over that of the −NO₃⁻in vivo assay) at all sampling periods of a normal day-night sequence (14 hr-30 C day; 10 hr-20 C night), indicating that NO₃⁻ was rate-limiting. The stimulation of in vivo NR activity by NO₃⁻ was not seen in plants exposed to extended dark periods at elevated temperatures (16 hr-30 C), indicating that under those conditions, NO₃⁻ was not the limiting factor. Under the latter condition, in vitro NR activity was appreciable (19 μmol NO₂⁻ [g fresh weight, hr]⁻¹) suggesting that enzyme level per se was not the limiting factor and that reductant energy might be limiting.