KINETICS OF NO3- AND NH4+ UPTAKE BY WHEAT SEEDLINGS - EFFECT OF SALINITY AND NITROGEN-SOURCE

The kinetic parameters K-m and V-max for the uptake of NO3- and NH4+ were determined in wheat plants grown in nutrient solutions containing NO3-, NH4+ or NO3- + NH4+, with 1 mM (control) and 60 mM (saline) NaCl each. The net uptake rates were calculated by measuring the disappearance of NO3- and NH4+ from solutions. Within the range of concentration of nitrogen employed (0 to 1 mM), only one absorption system was detected for both NO3- and NH4+. The presence of NaCl increased K-m, but not V-max when nitrogen was added as NO3-, whereas K-m was slightly reduced with NH4+. The rates of NO3- and NH4+ net uptake followed Michaelis-Menten kinetics. A depressive effect of Cl- on NO3- uptake was detected, although no proof for competition was obtained. When both nitrogen sources were present in the nutrient solution, NH4+ was taken up preferentially to NO3-, and higher rates of total nitrogen uptake were obtained than when each source was alone. The uptake of each nitrogen source was affected by the presence of the other. Under saline conditions, the addition of both nitrogen forms may be beneficial because higher nitrogen uptake rates lead to better growth and development of the plant.