NITRATE UPTAKE BY VICIA-FABA L. PLANTS - A PHYSIOLOGICAL APPROACH

This study was conducted to obtain information on the properties of th e nitrate (NO3-) uptake system in Vicia faba. The results showed that in plants grown in the absence of NO3-, they cannot absorb it upon ini tial exposure to this anion. When this plant makes contact with enviro nmental NO3-, NO3- uptake by the roots is progressively induced. Howev er, in NO3--grown plants, the uptake system is yet induced so they had the ability to take up NO3- at high rates from the beginning. The Mic haelis' constant (K-M) and maximum uptake rate (V-max) were estimated from measurements of NO3- depletion in the uptake medium. The apparent K-M value for net NO3- uptake in induced V. faba plants was 60 mu M a nd the corresponding V-max 0.44 mu mol NO3- (g FW)(-1) h(-1). This las t value was low compared with that of other species, while K-M is in t he range for other higher plants. From the results of our experiments at low and high NO3- concentrations, we discuss the possibility of the existence of two NO3- uptake systems, one of a high affinity (functio ning at low NO3- concentrations) and another of low affinity (operativ e at high NO3- level in the medium).