Ionic and Osmotic Effects on Nitrate Reductase Activity in Tomato Seedlings

To discriminate between ionic and osmotic effects on nitrate reductase activity, isotonic 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 NO32− uptake was inhibited more by Cl2− salts than by SO42− salts, and drastically decreased with PEG. In vivo NRA in leaves was unaffected by the SO42− and PEG treatments, whereas it was reduced by C1−. The leaf osmotic potential was similar under the 3 types of stresses (CI-, sol- 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 NO32− uptake and consequently a lower NO32− concentration in the leaves, although a direct effect of Cl− on the activity of the enzyme cannot be discarded.