Abstract
Salinity is a serious threat to agriculture in arid and semi-arid regions including Egypt. There is currently a need to select highly salt-tolerant plants to improve recycling of agricultural drainage waters. Due to the importance of Casuarina in soil reclamation and rehabilitation, our study was carried out to evaluate the survival and growth of inoculated Casuarina glauca in hydroponic N-free medium supplemented with different NaCl concentrations (50, 100, 200, 300, 400 and 500 mM). Salt tolerant Frankia strains CcI156 and CgIM4, isolated from root nodules of Casuarina trees grown in loamy sand soils in Egypt, were used for seedling inoculation. Our results showed that inoculated Casuarina seedlings were able to withstand up to 200 mM NaCl. At higher NaCl concentrations (300 to 500 mM), the seedling died gradually three days after exposure to salt stress. However, at lower NaCl (50 mM) concentration, inoculated C. glauca seedlings showed a higher growth rate and higher percentages of nodulation. Nitrogenase activity and the total nitrogen content of nodulated seedlings were also influenced by elevated NaCl concentrations and recorded a reduction at high concentration (200 mM). Salt stress had a strong effect on biosynthesis of osmoprotectants molecules like L-Proline and carbohydrates, which strongly correlated (r = 0.98 and 0.87, respectively) with increasing salt concentrations.