Effect of difference in nitrogen media on salt-stress response and contents of nitrogen compounds in rice seedlings

Effect of the difference in nitrogen media on the salt stress response and contents of nitrogen compounds (amino acids, polyamines, and protein) was investigated in rice seedlings (Oryza sativa L. cv. Nipponbare) at the 2nd leaf stage. Plant growth and chlorophyll fluorescence (PhiII), reflecting the efficiency of photosynthetic energy conversion, markedly decreased during the 3-d period of the 100 mm NaCl treatment. Response to salt stress, however, varied with the difference in the nitrogen media. Rice seedlings grown in a low ammonium medium, containing a sub-optimal concentration of ammonium for seedling growth, showed the lowest sensitivity to the salt stress treatment. In these seedlings, free amino acids, especially amide glutamine and arginine, strikingly accumulated in the 2nd leaf blades with the NaCl treatment. Additionally, the leaf polyamine contents in the seedlings grown in a low ammonium medium increased under salt stress, while the leaf polyamine contents in the seedlings grown in high ammonium and nitrate media considerably decreased. Under salt stress conditions, a positive correlation was observed between the total polyamine content and PhiII in the 2nd leaf blades. These results suggest that the salt stress tolerance of rice seedlings is associated with effective assimilation of excess ammonia, induced by salt stress, by nitrogen compounds such as arginine, glutamine, and polyamines.