Crithmum maritimum (sea fennel) withstands high salinity, and to better understand how different protective mechanisms against salinity are activated, young seedlings were exposed to increasing concentrations of NaCl (0 to 512 mM) over six weeks. Plant survival and chlorophyll content were reduced at > 85 mM NaCl and growth was affected at ≥ 341 mM NaCl. Relative water content fell and Na + accumulated more in leaves than in roots. Induction of Na + /H + antiporter expression reached a maximum at 427 mM NaCl in both tissues. Salinity induced the accumulation of proline, soluble sugars and glycine betaine. All three accumulated to higher levels in leaves than roots and greatest accumulation was after 6 weeks and the highest salt concentrations. Hydrogen peroxide levels fell with increasing salinity in leaves, while ascorbic acid and catalase activity rose. Overall, the most dramatic changes occurred after six weeks of saline stress but different mechanisms were activated at different salinity thresholds and in the two tissues. Key salinity thresholds in the response of Crithmum maritimum to salinity stress are identified activating different mechanisms. At 85 mM NaCl roots reach osmotic adjustment, at 171 mM further osmolyte protection mechanisms are activated, at 256 mM NaCl leaves reach osmotic adjustment, at 341 mM plant growth is affected and at the highest salinity tested, 512 mM, protective mechanisms are affected in leaves but not in roots.