Abstract
Diurnal fluctuation patterns of in vivo and in vitro nitrate reductase activities in leaves and roots of cucumber (Cucumis sativus L.) were studied. Nitrate reductase activity in roots, both in vivo and in vitro, was nearly constant throughout the entire day/night cycle, whereas, the activity in leaves exhibited a distinct diurnal fluctuation in both assay methods. In vitro nitrate reductase activity changed diurnally in a parallel manner with light intensity during the daytime, and it remained constant at a much lower level during the night. In vivo activity, however, did not follow the same pattern with in vitro activity. It was lowest at noon, and highest at dusk or 3 hr after sunset, depending on the date of measurements. When nitrate was omitted in the assay medium, in vivo activity was negligible in the early afternoon. Leaf nitrate content in the metabolic pool decreased after day light; it became negligible in the early afternoon but was restored after dark to the pre-dawn level. When plants were grown under artificial light, leaf in vivo activity increased steadily during the day, and peaked at 2.5 hr after dark. The reasons for the inconsistency of in vivo and in vitro nitrate reductase activities with respect to their diurnal fluctuation patterns are discussed.
