SUMMARY
The aim of this study was to investigate the effects of exogenous Melatonin (100, 300 and 500 µM) and L-tryptophan (125, 250, 375 ppm) applications on some growth parameters of lettuce plants grown under salt stress. The study was carried out under semi-controlled greenhouse conditions in spring (March/April) season. The exogenous applications to lettuce plants were carried out two times as foliar spraying. Salt stress was generated by adding NaCl (0 mM, 100 mM, 200 mM) to irrigation water. The complete randomized design was used with three replications in this experiment. At the end of the study, it was found that the highest doses of exogenous applications had the highest effect on the parameters of the number of leaves, salinity necrosis, fresh leaf weight, fresh root weight, and total surface area of lettuce plants under 200 mM salinity condition. When the effects of the subtract on these values were compared, the effect of melatonin was found to be more pronounced. Leaf width, leaf length, and leaf surface temperature values were not affected by the external application. These values only changed depending on salt concentration. As a result of the study, it was concluded that the application of 500 µM melatonin significantly increased salt tolerance in lettuce plants. However, in order to reach a more general conclusion, the dose ranges and genotype/variety numbers should be increased.