The 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 hydropon ically for 14-19 d without addition of NaCl were used for describing the ef fects of salt application on net nitrate uptake and for the calculation of kinetic parameters, The addition of NaCl, KCl, CaCl2, and Na2SO4 to the upt ake solution in the experiments led to similar inhibition of nitrate uptake , only at low and very high salt concentrations were ion-specific effects f ound. The same decrease in nitrate uptake can also be achieved by sorbitol or betaine at corresponding osmolalities. Thus, it was concluded that the i nhibition of uptake was caused mainly by the osmotic effects of salts. Diff erences in the mechanisms of inhibition were detected between the two syste ms 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(-3) was calculated using a Dixon-plot. Fitting an equation assuming a non-competitively inhibited HATS by computer program to the raw data res ulted in an apparent K-i of about 37 mol m(-3), In contrast, the LATS was a ffected in a complex way: up to 60 mol m(-3) NaCl the affinity was increase d, which led to a stimulation of nitrate uptake at low nitrate concentratio ns (<2 mol m(-3)), An inhibition of the LATS became obvious at concentratio ns 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 t he data pointed to a competitive effect.