Modelling nitrate influx in young tomato (Lycopersicon esculentum Mill.) plants

The effects of light and NO3- nutrition on (NO3-)-N-15 influx in roots were investigated in young, 19-d-old, induced tomato plants grown at a constant air and solution temperature of 20 degrees C. Nitrate influx was measured by N-15 accumulation for 5 min, on plants exposed to a wide range of exogenous concentrations, from 10x10(-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 nonadditive 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 NO3- the asymptotic maximum influx rate (I-max) Of each phase declined during the night until 24 h darkness. At the end of the day period, about a P-fold enhancement of I-max was observed when plants were pretreated for 3 d with 0.2 instead of 3.0 mol m(-3) NO3-. The influx rates measured at any given NO3- concentration and the I-max 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 NO3- concentrations.