| Summary: | In vitro experiments were conducted to study the responses of potato (<i>Solanum tuberosum</i> L.) genotypes to osmotic stress. In vitro shoot cultures of 27 breeding lines and their drought-tolerant parents (referent lines: C103 and C107) were tested under osmotic stress induced by addition of PEG 6000 (Mw = 6000; 5.0, 7.5, 10.0%, <i>w</i>/<i>v</i>), D-mannitol (0.1, 0.2, 0.3 M) and PEG 600 (Mw = 600; 2.5, 5.0, 7.5%, <i>w</i>/<i>v</i>) to the Murashige-Skoog medium. Stress index (SI) was calculated from shoot length (SL) and root length (RL), root numbers (RN) and the rate of surviving shoots (SR) (SI<sub>SL;RL;RN;SR</sub> = Parameter<sub>SL;RL;RN;SR</sub> of treated shoots/Parameter<sub>SL;RL;RN;SR</sub> of control shoots × 100) to compare genotypes. In the average of each breeding line and concentration, the osmotic agents resulted in SI values of 40.1, 60.8, 82.6 and 76.0 for SI<sub>SL</sub>, SI<sub>RL</sub>, SI<sub>RN</sub> and SI<sub>SR</sub>, respectively. In general, all SI values of C103 and SI<sub>RL,RN</sub> of C107 were significantly higher than those of the breeding lines. Nine breeding lines were found to be promising based on their final ranking. According to the results, 7.5% and 10% PEG 6000 or 0.2 M and 0.3 M D-mannitol treatments proved to be suitable for the selection of osmotic stress-tolerant genotypes.
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