Characteristics of nitrate uptake by plants under salinity

Under salt stress conditions, the uptake of nitrogen (N) by plants is generally suppressed. The objective of this study was to clarify the mechanism of inhibition of NO3 absorption under highly saline media prepared from NaCl or Na2SO4. Tomato (Lycopersicon esculentum Mill cv. Saturn) and rice (Oryza sativa L. cv. Koshihikari) were subjected to three N levels of 0.7 mmol L(-1) (LN), 7 mmol L(-1) (MN), and 14 mmol L(-1) (HN) under the same concentration (100 mM) of NaCl or Na2SO4 salinity. The N level of non-saline control was 7 mM. For both species the shoot dry weights (DW) of salt-treated plants were significantly lower than those grown in non-saline (CO) conditions. The application of N significantly enhanced shoot DW of tomato under SO4 salinity, while it had no effect on tomato growth under chloride (Cl) salinity. In rice, the LN level resulted in better growth than MN and HN levels. Nitrate-N concentration in both species was significantly increased by increasing N level in the solution. A large difference in NO3-N concentration between Cl and SO4 salinity was observed under MN and HN levels in tomato, and under LN and MN levels in rice, indicating an antagonism between Cl and NO3-N ions. Also, a close relationship between cumulative transpiration and NO3-N concentration in the shoots was observed for tomato. This indicates that NO3 absorption is related to water uptake in tomato. In contrast, not much difference was observed in cumulative transpiration among N levels in rice, which may indicate that transpiration was not related to NO3 uptake. However, the antagonism is considered not to be as strong as the relation between NO3 absorption and transpiration in tomato. It appeared that the inhibition of NO3-N absorption in tomato was more strongly related to reduced water uptake than to Cl antagonism from salt stress.