It was shown that the plastome mutation induced by nitrosomethylurea and resulting in the formation of salt tolerant mustard plants of the “SR3” line improved the germination of the mutant seeds and seedlings growth both under water conditions and NaCl salinisation compared to the parent variety “Donskaya-5”. At low and medium salt concentrations (20, 50 and 100 mM) stimulation of seedling growth was noted in both forms of mustard, more pronounced in mutant plants which also contaned a lower amount of reactive oxygen species compared to “Donskaya-5”. In this conditions in both forms of plants it was detected a significant decrease of proline content in comparison with seedlings grown on water with the most difference between control and experimental plants of the line SR3. It was noted a negative effect of higher salt concentrations (150, 200, and 250 mM) on the seedlings growth of both mustard forms with a lesser negative effect of 150 mM salt on the growth of mutant plants. An increase in the salt concentration to 150, 200 and 250 mM NaCl led also to a gradual increase in the proline content, which occurs at a faster rate in the plants of “SR3” line, and ultimately led to a maximum proline content in this variant (718 µg/g of fresh weight) and 404 µg/g of fresh weight in plants of the cultivar “Donskaya-5” at 250 mM NaCl. Under salinization with 100 and 250 mM salt, the ability of SR3 mustard plants to accumulate ALA was significantly higher than in “Donskaya-5” plants by 2.6 and 1.5 times respectively. A high ability of the salt tolerant mutant plants to accumulate ALA was also noted when seedlings were grown under normal conditions in the absence of NaCl (3.8 times higher than in the case of the “Donskaya-5” variety). This supports our earlier assumption that high ability of plants to produce ALA under normal conditions may serve as an indicator of their high stress resistance when exposed to stress.