Salt-induced NO sub(3)

Salinity remarkably inhibits NO sub(3) super(-) uptake but the mechanisms are not well understood. This study was addressed to elucidate the role of ionic and osmotic components of salinity on NO sub(3) super(-) 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 Na sub(2)SO sub(4) at 100 mM concentrations, in both LATS and HATS, Michaelis constant (K sub(m)) was similar for the three salts and maximum rate (V sub(max)) decreased as follows: KCl > NaCl > Na sub(2)SO sub(4), compared to control indicating a non-competitive interaction with NO sub(3) super(-). Unexpectedly, iso-osmotic solutions (osmotic potential psi sub( pi ) = -0.450) of polyethylene glycol (PEG, 17.84 %, v/v) and mannitol (100 mM) remarkably increased K sub(m) in both the LATS and the HATS, but V sub(max) did not change indicating a competitive inhibition. Under the PEG and mannitol treatments, K sub(m) and V sub(max) were higher than under the salt treatments. The salts increased slightly NO sub(3) super(-) efflux in the following order KCl > NaCl > Na sub(2)SO sub(4). In contrast, mannitol strongly stimulated and the PEG inhibited NO sub(3) super(-) efflux. The obtained data reveal that salinity effects were not dependent on the anion type (Cl super(-) versus SO sub(4) super(2-)) indicating a non-competitive inhibition mechanism between Cl super(-) and NO sub(3) super(-). In contrast, the cation types (K super(+) versus Na super(+)) had a pronounced effect. The osmotic component is important to net NO sub(3) super(-) uptake affecting remarkably the influx in both LATS and HATS components of cowpea roots.