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 (Km) was similar for the three salts and maximum rate (Vmax) decreased as follows: KCl > NaCl > Na2SO4, compared to control indicating a non-competitive interaction with NO3 -. Unexpectedly, iso-osmotic solutions (osmotic potential Ψπ = -0.450) of polyethylene glycol (PEG, 17.84 %, v/v) and mannitol (100 mM) remarkably increased Km in both the LATS and the HATS, but Vmax did not change indicating a competitive inhibition. Under the PEG and mannitol treatments, Km and Vmax 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.