Utilization of nitrite and nitrate by dwarf bean

The uptake and conversion of NO₂^-and the effect of NO₂^-on the uptake and reduction of NO₃^-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 NO₂^-concentration (0–0.7 mmol dm^-3). In this concentration range root oxygen consumption was unaffected by NO₂^-, indicating that the decrease of NO₂^-uptake was not related to respiration. After 6 h NO₂^-supply, about one-third of the absorbed NO₂^-had accumulated, mainly in the root system. Oxidation of NO₂^-to NO₃^-was not observed. The apparent induction period for NO₃^-uptake was about 6 h in control plants and 3.5 h in plants that were pretreated for 18 h with NO₂^-. In contrast, the time course of NO₂^-uptake was unaffected by pretreatment with NO₃^-. Steadystate NO₃^-uptake was less affected by NO₂^-than was steady-state NO₂^-uptake by NO₃^-. Nitrate reductase activity (NRA) in leaves and roots was induced by both NO₃^-and NO₂^-. In roots, induction with NO₂^-was faster than with NO₃^-, but there was no difference in NRA after 5 h. Nitrite inhibited NRA in the roots of NO₃^--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 NO₂^-and NO₃^-seems unlikely. Expression of the NO₂^-effect on the induction of NO₃^-uptake required more time than the induction itself. We therefore conclude that NO₂^-is not the physiological inducer of NO₃^-uptake.Abbreviations NR(A) nitrate reductase (activity) - BM basal medium