UPTAKE OF NITRATE BY MUTANTS OF ARABIDOPSIS-THALIANA, DISTURBED IN UPTAKE OR REDUCTION OF NITRATE .3. REGULATION

Wildtype and nitrate reductase-less mutant B 25 of A. thaliana (L.) Heynh. were grown on a nutrient solution with NH4NO3 as the N source. Uptake of nitrate was determined with a continuous method in which the decrease of the absorbance at 210 nm of the ambient solution was measured. The uptake rate, plotted against concentration, deviated in the low concentration range from the standard Michaelis-Menten curve and appeared to follow the 1st tract of a sigmoid curve, indicating positive cooperativity kinetics. Starvation of the plants for nitrate during a few days resulted in an increased capacity (Vmax) and affinity (decreased Km) of nitrate uptake. After prolonged starvation the uptake capacity decreased again. The highest rates of uptake were reached after 2-3 days'' starvation for the wildtype and after 1-2 days'' starvation for mutant B 25. Soluble carbohydrates were not correlated with nitrate uptake rates. Malate accumulated during nitrate starvation, resulting in an inverse relation between malate and nitrate content of the plants. Certain amino acids, like L-serine and L-arginine inhibited nitrate uptake. Based on these, and previous results, a model was proposed for the regulation of nitrate uptake, which includes allosteric regulation of the nitrate carrier by nitrate and amino acids.