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

The kinetic parameters K-m and V-max for the uptake of NO3- and NH4+ w ere 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 disappear ance of NO3- and NH4+ from solutions. Within the range of concentratio n of nitrogen employed (0 to 1 mM), only one absorption system was det ected 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 re duced with NH4+. The rates of NO3- and NH4+ net uptake followed Michae lis-Menten kinetics. A depressive effect of Cl- on NO3- uptake was det ected, although no proof for competition was obtained. When both nitro gen sources were present in the nutrient solution, NH4+ was taken up p referentially 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 conditi ons, the addition of both nitrogen forms may be beneficial because hig her nitrogen uptake rates lead to better growth and development of the plant.