Diurnal changes in nitrogen assimilation of tobacco roots

To gain an insight into the diurnal changes of nitrogen assimilation in roo ts the in vitro activities of cytosolic and plasma membrane-bound nitrate r eductase (EC 1.6.6.1), nitrite reductase (EC 1.7.7.1) and cytosolic and pla stidic glutamine synthetase (EC 6.3.1.2) were studied. Simultaneously, chan ges in the contents of total protein, nitrate, nitrite, and ammonium were f ollowed. Roots of intact tobacco plants (Nicotiana tabacum cv, Samsun) were extracted every 3 h during a diurnal cycle. Nitrate reductase, nitrite red uctase and glutamine synthetase were active throughout the day-night cycle. Two temporarily distinct peaks of nitrate reductase were detected: during the day a peak of soluble nitrate reductase in the cytosol, in the dark pha se a peak of plasma membrane-bound nitrate reductase in the apoplast, The t otal activities of nitrate reduction were similar by day and night. High ac tivities of nitrite reductase prevented the accumulation of toxic amounts o f nitrite throughout the entire diurnal cycle. The resulting ammonium was a ssimilated by cytosolic glutamine synthetase whose two activity peaks, one in the light period and one in the dark, closely followed those of nitrate reductase. The contribution of plastidic glutamine synthetase was negligibl e. These results strongly indicate that nitrate assimilation in roots takes place at similar rates day and night and is thus differently regulated fro m that in leaves.