Ionic and osmotic effects on nitrate reductase activity in tomato seedlings

To discriminate between ionic and osmotic effects on nitrate reductase activity, isoconic solutions of Cl-, SO42- and PEG were used in intact tomato seedlings. PEG was found to be more detrimental to seedling growth than isotonic Cl- or SO42- treatments. Net NO3- uptake was inhibited more by Cl- salts than by SO42- salts, and drastically decreased with PEG. In viva NRA in leaves was unaffected by the SO42- and PEG treatments, whereas it was reduced by Cl-. The leaf osmotic potential was similar under the 3 types or stresses (Cl-, SO42- and PEG), which indicates that an osmotic effect on NRA can be disregarded. Under our experimental conditions, the primary cause of a decrease of NRA in the leaves was a specific effect associated with the presence of Cl- salts in the external medium. This effect of Cl- seems to be due to a reduction in NO3- uptake and consequently a lower NO3- concentration in the leaves, although a direct effect of Cl- on the activity of the enzyme cannot be discarded.