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

The effects of light and NO3^- nutrition on¹⁵NO3^- influx in roots wereinvestigated in young, 19-d-old, induced tomato plants grown at a constantair and solution temperature of 20C. Nitrateinflux was measured by ¹⁵N accumulation for 5 min,on plants exposed to a wide range of exogenous concentrations, from 10 x10^-3 to 30 mol m^-3. Influxkinetics, fitted to the data following a non-linear procedure, showedmultiphasic patterns. The best fits were obtained when three pure andnon-additive Michaelis-Menten kinetics were applied, with phase transitionsat approximately 0.8 and 4 mol m^-3. In plants grownat 3.0 mol m^-3 NO3^-, theasymptotic maximum influx rate (Imax) of each phasedeclined during the night until 24 h darkness. At the end of the dayperiod, about a 2-fold enhancement of Imax wasobserved when plants were pretreated for 3 d with 0.2 instead of 3.0 molm^-3 NO3^-. The influx ratesmeasured at any given NO3^- concentration and theImax for any phase showed a negative non-linearcorrelation with plant nitrate concentration. Furthermore, the resultssuggest the existence of a set point, approximately 66 molm^-3 plant nitrate, for which influx is null at anygiven solution nitrate concentration. A model using modifiedMichaelis-Menten kinetics is proposed to predict the influx rate as afunction of both solution and plant NO3^-concentrations.