Effects of NO 3 - -N on the growth and salinity tolerance of Tamarix laxa Willd

The influence of NO (3) (-) -N on growth and osmotic adjustment was studied in Tamarix laxa Willd., a halophyte with salt glands on its twigs. Seedlings of T. laxa Willd. were exposed to 1 mM (control) or 300 mM NaCl, with 0.05, 1 or 10 mM NO (3) (-) -N for 24 days. The relative growth rate of seedlings at 300 mM NaCl was lower than that of control plants at all NO (3) (-) -N levels, but the concentrations of organic N and total N in the twigs did not differ between the two NaCl treatments. Increasing NO (3) (-) supply under 300 mM NaCl improved the growth of T. laxa, indicating that NO (3) (-) played positive roles in improving salt resistance of the plant. The twigs of T. laxa Willd. accumulated mainly inorganic ions, especially Na+ and Cl-, to lower osmotic potential (Is): the contributions of Na+ and Cl- to Is were estimated at 31% and 27% respectively, at the highest levels of supply of both NaCl and NO (3) (-) -N. The estimated contribution of NO (3) (-) -N to Is was as high as 20% in the twigs in these conditions, indicating that NO (3) (-) was also involved in osmotic adjustment in the twigs. Furthermore, increases in tissue NO (3) (-) were accompanied by decreases in tissue Cl- and proline under 300 mM NaCl. The estimated contribution of proline to Is declined as with NO (3) (-) -N supply increased from 1 to 10 mM, while the contributions of nitrate to Is were enhanced under 300 mM NaCl. This suggested that higher accumulation of nitrate in the vacuole alleviated the effects of salinity stress on the plant by balancing the osmotic potential. In conclusion, NO (3) (-) -N played both nutritional and osmotic roles in T. laxa Willd. in saline conditions.