The possible use of in vitro shoot morphogenesis and shoot apex culture to evaluate salt tolerance in cultivated tomato (Lycopersicon esculentum Mill.) has been analyzed, using two cultivars with similar salt tolerance, Pera and Hellfrucht frühstamm (HF). The effect of salt on shoot regeneration was studied by culturing leaf explants on media supplemented with 0, 43, 86, 129 and 172 mM NaCl. The presence of NaCl in the regeneration media at 86 mM strongly inhibited shoot regeneration in the cultivar HF, but not in Pera. However, the substitution of NaCl by mannitol, maintaining the same water potential in the culture media, decreased the regeneration percentage in Pera but did not affect HF. Shoot apices of both cultivars were also subcultured at 6-week intervals, for 4 subcultures, at the same NaCl concentrations as used in the previous experiment, and the shoot growth, leaf and root number, rooted shoot and shoot necrosis were recorded at the end of each subculture. Root formation was the parameter most affected by salt in both cultivars, Pera being more sensitive than HF. The substitution of NaCl by mannitol significantly increased the percentage of rooted shoots in Pera after four subcultures, and slightly decreased this percentage in HF. Shoot necrosis was only observed in the last subculture at NaCl higher than 86 mM, the percentage of necrotic shoots being higher in Pera than in HF (75% and 45%, respectively). The lack of agreement between the results obtained with the in vitro tests, e.g., adventitious shoot formation and growth of apical stem sections, suggests that this approach may not be a reliable tool to evaluate salt tolerance in cultivated tomato.
