The effects of light and NO−3 nutrition on 15NO−3 influx in roots were investigated in young, 19-d-old, induced tomato plants grown at a constant air and solution temperature of 20 °C. Nitrate influx was measured by 15N accumulation for 5 min, on plants exposed to a wide range of exogenous concentrations, from 10 × 10−3 to 30 mol m−3. Influx kinetics, fitted to the data following a non-linear procedure, showed multiphasic patterns. The best fits were obtained when three pure and non-additive Michaelis-Menten kinetics were applied, with phase transitions at approximately 0.8 and 4 mol m−3. In plants grown at 3.0 mol m−3 NO−3, the asymptotic maximum influx rate (Imax) of each phase declined during the night until 24 h darkness. At the end of the day period, about a 2-fold enhancement of was Imax observed when plants were pretreated for 3 d with 0.2 instead of 3.0 mol m−3 NO−3. The influx rates measured at any given NO−3 concentration and the Imax for any phase showed a negative non-linear correlation with plant nitrate concentration. Furthermore, the results suggest the existence of a set point, approximately 66 mol m−3 plant nitrate, for which influx is null at any given solution nitrate concentration. A model using modified Michaelis-Menten kinetics is proposed to predict the influx rate as a function of both solution and plant NO−3 concentrations.
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Cárdenas‐Navarro et al. (1999) studied this question.