NaCl has been frequently used for applying salt stress to plants. However, in this study, we tested to use sea salt instead of NaCl for applying salt stress to tomato plants. Plants were grown under hydroponic conditions, mixing artificial seawater to nutrient solution (liquid fertilizer). To estimate strength of the stress treatment, we used EC (Electrical Conductivity) value to control it. Fifty-six seedings of two tomato varieties ('Momotaro York' and 'Saturn') were grown at deep flow technique in a greenhouse, and the stress treatments were applied 10 days after the first truss bloomed. In experiment 1, we designed 3 treatments (EC was set for 5.0, but the ratio of Liquid Fertilizer : Artificial Seawater were 5.0 : 0, 1.5 : 3.5 and 2.5 : 2.5, respectively). In experiment 2, we designed 3 treatments (EC for liquid fertilizer was set at 1.5, but the concentration of artificial seawater was different as Liquid Fertilizer : Artificial Seawater were 1.5 : 0, 1.5 : 3.0 and 1.5 : 6.0, respectively). Fruits were harvested at almost 90 days after blooming and fruit weights were measured. The contents of 5 cations and 3 anions in fruit were analyzed by high-performance liquid chromatography (HPLC), then we added all of the ions contents as total ion contents. As a result, fruit weight was not different significantly at experiment 1, but decreased when EC value was increased at experiment 2. In experiment 1, when the ratio for liquid fertilizer was increased, the total ion contents were increased, especially Na in treatment at (1.5 : 3.5) and Ca2 in treatment at (5.0 : 0) were increased. At experiment 2, total ion contents increased when the ratio of sea salt was increased. In conclusion, we estimated various strength of stress by EC value in this study. Then the different ratio of liquid fertilizer and artificial seawater caused different ion contents in tomato fruits. This result was similar for the two varieties.
