Nitrate uptake by Vicia faba L. Plants: A physiological approach 1

This study was conducted to obtain information on the properties of the nitrate (NO₃ ^‐) uptake system in Vicia faba. The results showed that in plants grown in the absence of NO₃ ^‐, they cannot absorb it upon initial exposure to this anion. When this plant makes contact with environmental NO₃ ^‐, NO₃ ^‐uptake by the roots is progressively induced. However, 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 Michaelis’ 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 μM and the corresponding V_max 0.44 umol NO₃ ^‐ (g FW)^‐1 h^‐1. This last value was low compared with that of other species, while K_M is in the 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 (functioning at low NO₃ ^‐concentrations) and another of low affinity (operative at high NO₃ ^‐ level in the medium).