The kinetics of nitrate uptake from flowing solutions by rice: Influence of pretreatment and light

The kinetics of nitrate (NO−3) uptake by intact 23 day old rice plants was studied by measuring the depletion of NO−3 in solutions flowing over the plant roots. A Michaelis-Menten kinetic model was applied, allowing the uptake kinetics to be characterized by two parameters: the apparent half-velocity constant, Km, and the apparent maximum uptake rate, Vmax. A propagation of uncertainty calculation revealed that the kinetic parameters could be determined with a high degree of accuracy; the standard deviation in Km was typically 15% of the Km value; the standard deviation in Vmax was typically 7% of the Vmax value. The plants were exposed to full nutrient solutions containing NO−3 at 50, 200, 500 and 800 μ for 24 h prior to kinetic testing, and both Km and Vmax were found to vary with pretreatment NO−3 concentration, [NO−3]; plants pretreated at high [NO−3] had lower Vmax and higher Km values than plants pretreated at lower [NO−3]. However, the variations in Vmax were more consistent than those in Km. These changes in the kinetic parameters reflect an uptake system which is capable of compensating for changes in the external [NO−3] to maintain a virtually constant NO−3 uptake rate in the range studied. Changes in Km and Vmax begin within 4 h of a change in [NO−3]. Light deprivation during pretreatment in 200 μ NO−3 resulted in a complete cessation of NO−3 uptake; 4–8 h of illumination were required before the uptake resumed, and uptake rates had not yet reached normal levels 8 h after the resumption of illumination.