Salt-induced NO3 - uptake inhibition in cowpea roots is dependent on the ionic composition of the salt and its osmotic effect

Salinity remarkably inhibits NO3 (-) uptake but the mechanisms are not well understood. This study was addressed to elucidate the role of ionic and osmotic components of salinity on NO3 (-) influx and efflux employing classic kinetics involving a low affinity transport system (LATS) and a high affinity transport system (HATS). In the presence of KCl, NaCl, and Na2SO4 at 100 mM concentrations, in both LATS and HATS, Michaelis constant (K-m) was similar for the three salts and maximum rate (V-max) decreased as follows: KCl > NaCl > Na2SO4, compared to control indicating a non-competitive interaction with NO3 (-). Unexpectedly, iso-osmotic solutions (osmotic potential I(pi) = -0.450) of polyethylene glycol (PEG, 17.84 %, v/v) and mannitol (100 mM) remarkably increased K-m in both the LATS and the HATS, but V-max did not change indicating a competitive inhibition. Under the PEG and mannitol treatments, K-m and V-max were higher than under the salt treatments. The salts increased slightly NO3 (-) efflux in the following order KCl > NaCl > Na2SO4. In contrast, mannitol strongly stimulated and the PEG inhibited NO3 (-) efflux. The obtained data reveal that salinity effects were not dependent on the anion type (Cl- versus SO4 (2-)) indicating a non-competitive inhibition mechanism between Cl- and NO3 (-). In contrast, the cation types (K+ versus Na+) had a pronounced effect. The osmotic component is important to net NO3 (-) uptake affecting remarkably the influx in both LATS and HATS components of cowpea roots.