Milk thistle (Silybum marianum) is used as a medicinal plant in the treatment of liver diseases. In order to investigate germination and seedling growth in S. marianum subjected to NaCl, a three replicated experiment was carried out in a randomized complete block design in the laboratory of Gorgan University of Agricultural Sciences and Natural Resources. Treatments included seven salinity levels (50, 75, 100, 125, 150, 175 and 200 mM) and a control (distilled water). Germination percentage (GP), mean germination time (MGT), seed-ling vigour index (SVI), shoot, root and seedling weight were measured. The effect of salinity levels was significant on GP, SVI, MGT and seedling weight and length (p ≤ 0.01). However, the NaCl concentration effect was not significant on shoot : root length ratio and or shoot : root weight ratios. Results showed that germination decreased when salinity increased, while MGT increased. MGT was 1.75 times higher than in the control at the highest salt concentration. MGT difference was not significant between 75 mM NaCl and con-trol, while it dramatically increased by increasing the NaCl concentration from 150 to 200 mM NaCl. Reduc-tion slope and salt tolerance index (STI) were estimated for germination (0.54 and 231.9) and seedling stage (0.24 and 237.4). According to the results, milk thistle could be considered as a valuable medicinal plant in fairly salinized areas.
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