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The effects of various concentrations of sorbitol (100, 200 and 360 mM) and NaCl (100, 200 and 300 mM) on root meristem cells of in vitro-cultured Allium cepa L. were analyzed after 10 and 20 days. Both root meristem cell cross-section area and nuclear volume decreased under osmotic and salt stress. The osmotic component of applied stresses had a greater impact on cell shrinkage, while ionic stress perturbed cell functioning, resulting in cell cycle arrest and various aberrations, affecting nucleus integrity. A concentration of 300 mM of NaCl in the culture medium caused complete inhibition of mitotic activity in onion root tip cells after 20 days of exposure. Analysis of the action of iso-osmotic concentrations of NaCl (200 mM) and sorbitol (360 mM) showed stronger mitodepressive effects of salt stress in comparison to osmotic stress.