Reduction of Nitrate Ions by Morningglory Leaves

The reduction and assimilation of nitrate ions by 14-day old tall morningglory [Ipomoea purpurea (L.) Roth] seedlings were studied by monitoring the leaf nitrate, protein, and activity of the enzyme nitrate reductase (NADH: nitrate oxidoreductase, EC 1.6.6.1). The pH's of the grinding and assay mixtures yielding maximum nitrate reductase activity were determined to be 8.8 and 7.5 to 7.7, respectively. The maximum assay time for a linear reaction rate for nitrate reductase was estimated at 10 minutes. Decay rates of the enzyme preparations indicated a half-life of 30 min. Requirements for a cofactor to support the reaction were met with the addition of NADH (1 × 10−3 M final concentration) to the assay mixture. NADPH was not effective in supporting nitrate reductase activity. Inclusion of cysteine (1 × 10−2 M) in the grinding medium was necessary to insure maximum activity of the enzyme preparations. Both light and nitrate were required for nitrate reductase activity in morningglory leaves. Nitrate accumulated in leaf tissue in proportion to the concentration of nitrate in the nutrient medium. Above 7,530 lux luminescence, enzyme activity and tissue protein were related to the light intensity and the availability of nitrate in the nutrient medium on which the plants were grown.