Proline accumulates in Casuarina equisetifolia seedlings under salt stress

The fresh weight of Casuarina equisetifolia seedlings decreased slightly with elevation of NaCl concentration in the culture solution. Na(+) concentration in the shoots gradually increased with increasing NaCl concentration in the culture solution and with increasing duration of the treatment. The Na(+) level reached 150 mmol L(-1) in seedlings treated with 500 mrnol L(-1) NaCl for 6 weeks. The proline content of the shoots increased with increasing NaCl concentration in the culture solution and reached 13.5 mu mol g(-1) fresh weight in the seedlings treated with 500 mrnol L(-1) NaCl for 6 weeks. The Na(+) concentration and proline content in the roots also increased with NaCl treatment, but the level was considerably lower than that recorded in the shoots. A highly positive correlation between Na(+) concentration and proline content was observed in the shoots. These results strongly suggest that C. equisetifolia plants, which are highly tolerant to salt stress, primarily synthesize proline as a major compatible solute to adjust the osmotic pressure when Na(+) accumulates in the cells, and maintain cell homeostasis under salt-stress conditions.