The effects of light and NO3
- nutrition on
15NO3
- influx in roots were
investigated in young, 19-d-old, induced tomato plants grown at a constant
air and solution temperature of 20C. Nitrate
influx was measured by
15N accumulation for 5 min,
on plants exposed to a wide range of exogenous concentrations, from 10 x
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 NO3
-, 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
Imax 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
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 NO3
-
concentrations.
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