Nitrate uptake by intact, non-nodulated bean plants: effect of ammonium and of light

Dual-phase kinetics were found for nitrate uptake at pH 5.5 by intact, non-nodulated bean plants. Apparent Km values for phase I (higher affinity for nitrate) were in the range between 0.2 and 0.4 mM and depended on the nitrogen nutrition of the plants prior to the experiment. Ammonium added to the medium at the same concentration as nitrate had no effect on nitrate uptake rates from nutrient Solutions containing 0.1-1.5 mM nitrate. Uptake rates for nitrate were considerably lower in plants preincubated on higher nitrate or ammonium concentrations than used for the experiment, compared to those preincubated on the same medium as used during the experiment. No effect of light on nitrate uptake from Solutions containing 0.1-1.5 mM nitrate was observed, although the transpiration rates were about 70% lower in darkness than in light. Nitrate uptake rates from Solutions containing more than 2 mM nitrate were slightly lower in darkness than in light.