Utilization of nitrite and nitrate by dwarf bean

The uptake and conversion of NO 2 (-) and the effect of NO 2 (-) on the uptake and reduction of NO 3 (-) were examined in N-depleted Phaseolus vulgaris L. Nitrite uptake at 0.1 mmol dm(-3) was against an electrochemical gradient and became constant after one or two initial phases. Steadystate uptake declined with increasing ambient NO 2 (-) concentration (0-0.7 mmol dm(-3)). In this concentration range root oxygen consumption was unaffected by NO 2 (-) , indicating that the decrease of NO 2 (-) uptake was not related to respiration. After 6 h NO 2 (-) supply, about one-third of the absorbed NO 2 (-) had accumulated, mainly in the root system. Oxidation of NO 2 (-) to NO 3 (-) was not observed. The apparent induction period for NO 3 (-) uptake was about 6 h in control plants and 3.5 h in plants that were pretreated for 18 h with NO 2 (-) . In contrast, the time course of NO 2 (-) uptake was unaffected by pretreatment with NO 3 (-) . Steadystate NO 3 (-) uptake was less affected by NO 2 (-) than was steady-state NO 2 (-) uptake by NO 3 (-) . Nitrate reductase activity (NRA) in leaves and roots was induced by both NO 3 (-) and NO 2 (-) . In roots, induction with NO 2 (-) was faster than with NO 3 (-) , but there was no difference in NRA after 5 h. Nitrite inhibited NRA in the roots of NO 3 (-) -induced plants and thus seems to stimulate the induction, but not the activity of induced nitrate reductase. In view of the observed differences in time course and mutual competition, a common uptake mechanism for NO 2 (-) and NO 3 (-) seems unlikely. Expression of the NO 2 (-) effect on the induction of NO 3 (-) uptake required more time than the induction itself. We therefore conclude that NO 2 (-) is not the physiological inducer of NO 3 (-) uptake.