Sodium chloride stimulates growth and alleviates sorbitol-induced osmotic stress in sugar beet seedlings

Sugar beet (Beta vulgaris L.) is a natrophilic species well adapted to salinity and water stress. To investigate the physiological impact of salt on osmotic stress response, in the present study, 3-week-old seedlings were treated with a series of additional external NaCl concentrations (5-150 mM). Seedlings were also subjected to 0 or 240 mM sorbitol in the presence or absence of 50 mM NaCl. It is clear that the addition of NaCl at a concentration of 50 mM in the medium was optimal for growth of sugar beet plants. Sorbitol-induced osmotic stress strongly reduced its growth and led to a significant decrease in shoot osmotic potential (Psi(s)), water content and K+ concentrations, whereas Na+ concentrations remained unchanged. The addition of 50 mM NaCl also mitigated the deleterious impact of osmotic stress on growth and reduced the shoot Psi(s). Presence of NaCl in the medium, together with sorbitol, significantly increased the levels of Na+ and proline in both shoot and root, enhanced Na+ net uptake rate, and resulted in a decrease of K+ accumulation. These results suggest that the ability of NaCl to improve plant performance under osmotic stress may be due to its effect on osmotic adjustment through Na+ and proline accumulation.