Seed germination of the halophyteSuaeda japonica under salt stress

Suaeda japonica Makino belonging to the family Chenopodiaceae, is a halophyte and grows at the shore of Ariake sea in Japan. This plant presumably possesses high salt resistant nature, thus, we examined the mechanisms of seed germination under salt stress. The seeds maintained 80% germination rates on the medium containing 0.7 M NaCl. Germination rates varied depending on salt type; the germination rates under NaCl or KCI exhibited relatively lower values than ones under sodium gluconate or potassium gluconate. This different responses for salts seemed to be as a result of the presence of Cl⁻ ions. Although very high levels of betaine (compatible solute), were kept in the seedlings grown under no salt stress, the contents gradually increased as concentration of NaCl increased. Betaine is a factor present in plants that works to alleviate the effects of excessive soil salts. It is synthesized in leaves from betaine aldehyde, and this process is catabolized by betaine aldehyde dehydrogenase (BADH). When the seedlings were cultivated on the medium without NaCl, relatively high level of BADH activity was found. The activity increased 5-fold in the seedlings grown under 0.5 M NaCl stress. Increases in betaine content and BADH activity were found during seed germination. InS. japonica, the salt stress promoted BADH activity, subsequently endogenous betaine contents were increased, and increased betaine seemed to secure seed germination under salt stress.