characterization of inhibition of net nitrate uptake by salt in salt-tolerant barley (Hordeum vulgare L. cv. California Mariout)

Barley seedlings (Hordeum vulgare L. cv. California Mariout) grown hydroponically for 14-19 d without addition of NaCl were used for describing the effects of salt application on net nitrate uptake and for the calculation of kinetic parameters. The addition of NaCl, a KCl, CaCl(2), and Na(2)SO(4) to the uptake solution in the it experiments led to similar inhibition of nitrate uptake, only at low and very high salt concentrations were ion-specific effects found. The same decrease in nitrate uptake can also be achieved by sorbitol or betaine at corresponding osmolalities. Thus, it was concluded that the inhibition of uptake was caused mainly by the osmotic effects of salts. Differences in the mechanisms of inhibition were detected between the two systems of nitrate uptake (high affinity system: HATS, and low affinity system: LATS). The HATS was inhibited non-competitively by NaCl, an apparent K(i) of 60 mol m(-3) was calculated using a Dixon-plot. Fitting an equation assuming a non-competitively inhibited HATS by computer program to the raw data resulted in an apparent K(i) of about 37 mol m(-3). In contrast, the LATS was affected in a complex way: up to 60 mol m(-3) NaCl the affinity was increased, which led to a stimulation of nitrate uptake at low nitrate concentrations (< 2 mol m(-3)). An inhibition of the LATS became obvious at concentrations above 3 mol m(-3) nitrate (for all applied salt concentrations) or with 100 mol m(-3) NaCl (throughout the whole nitrate range). Related plots of the data pointed to a competitive effect.