Effects of salinity and nitrogen source on growth and nitrogen fixation in alfalfa

The interaction between the effects of nitrate (NO3) and sodium chloride (NaCl) concentration on growth, water relations, nitrogen (N) contents and N fixation were investigated in alfalfa (Medicago sativa L. cv. Magali). The plants were grown hydroponically in a growth chamber, in the presence or absence of 3 mM potassium nitrate (KNO3), and exposed to vvarious concentrations of NaCl. Increased salinity resulted in a significant decrease in shoot and root biomass, relative water content and water potential. Shoot growth was more inhibited by NaCl than root biomass. The plants grown in the presence of NO3 Were slightly less affected by NaCl than the plants dependent on N fixation for their N nutrition. Nitrogenase activity measured by acetylene reduction activity was substantially inhibited by NaCl, and this inhibition was significantly correlated to the inhibition of shoot growth and total N contents. The comparison of the curves of ARA response to oxygen (O-2) partial pressure showed that the salt-induced inhibition of nitrogenase activity was associated with a significant increase in the critical O-2, pressure of the nodules exposed to NaCl. This result shows that NaCl decreases the nodule permeability to O-2, diffusion in undeterminate nodule of alfalfa, like previously shown with determinate nodules of soybean.