Salt tolerance of Casuarina equisetifolia and Frankia Ceq1 strain isolated from the root nodules of C. equisetifolia

Effects of NaCl on the seed germination and growth of Casuarina equisetifolia seedlings and multiplication of the Frankia Ceq1 strain isolated from the root nodules of C. equisetifolia were examined. The germination rate of the seeds markedly decreased as the NaCl concentration increased and germination did not occur at 300 mM NaCl. The fresh weight of both shoots and roots of the seedlings treated with NaCl for 6 weeks apparently decreased as the NaCl concentration increased. However, root nodules were formed by inoculation with the Frankia Ceq1 strain in some seedlings treated with 300 mM NaCl and the viability of the seedlings at 500 mM NaCl was almost the same as that of the seedlings not subjected to the NaCl treatment. The Na⁺ concentration in the shoots sharply increased with the elevation of the NaCl concentration in the ambient solution, but the level was approximately 300 mM even in the seedlings treated with 500 mM NaCl for 6 weeks. On the other hand, the increase of the Na⁺ concentration in the roots by the NaCl treatment was much smaller than that in the shoots and the level was less than 150 mM. The growth of the free-living Frankia Ceq1 strain was approximately linearly suppressed as the NaCl concentration in the medium increased and the hyphae became somewhat thicker and shorter or disintegrated in the medium containing NaCl at a concentration above 150 mM. The Na⁺ concentration in the cells increased as the NaCl concentration in the medium increased, but the level was maintained at less than 30 mM even in the medium containing 500 mM NaCl. The cells whose growth was suppressed by the NaCI treatment grew actively again at almost the same rate as the control cells (not subjected to the NaCl treatment) when they were transferred to NaCl-free medium. These results strongly suggested that both C. equisetifolia seedlings and Frankia Ceq1 strain are highly tolerant to salt and this symbiotic system is useful for the recovery of the vegetation in areas with severe salt accumulation.