Variation in ion accumulation as a measure of salt tolerance in seedling and callus of Stylosanthes guianensis

Seedlings of Stylosanthes guianensis CIAT 184 (Stylo 184) were germinated under salt-stress conditions (0-170.9 mM NaCl), and germination and ion content (Cl-, Na+, Ca2+, Mg2+ and K+) were measured after seven days. The salt treatment had no effect on the germination percentage, but the seedling showed reduced growth and displayed variation in ion uptake, thus accumulating Na+ and Cl- in the roots. Callus produced from seedlings selected as salt sensitive (T1) or salt tolerant (T2, T3, T4 and T5) was examined for growth and ion content under the influence of NaCl (0-256.4 mM NaCl) after 15 days. The sensitive clone (T1) contained the lowest Na+ and Cl- content with the highest percentage of K+ contributing to osmolality, suggesting it possesses an ion regulation mechanism that is typical of glycophytes; i.e., osmotic potential was adjusted by accumulating K+. The other clones (T2, T3, T4 and T5) expressed different strategies (osmotic adjustment) to cope with salt stress. T5 showed the highest mean dry weight on salt medium and displayed more effective ion regulation by maintaining low Na+: K+ and Na+: Ca2+ ratios. T2 expressed high Na+ and Cl- with the highest percentage of Na+ contribution to osmolality and water content (succulence). T3 and T4 had lower growth rates but similar ion regulation in relation to T5. Results indicated that the ion content can be used to discriminate salt sensitive and salt tolerant clones of Stylo 184.