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Quality characteristics and yield of 'Maehyang' and 'Seolhyang' strawberry cultivars, which have been registered in Korea, as affected by water stress (WS) was investigated. The total contents of carbohydrates (TC; fructose, glucose, and sucrose), organic acid (TOA; citric, malic, and oxalic acids) and antioxidants such as anthocyanin (AC), ascorbic acid (AA), and ellagic acid (EA) in the fruits were analysed. Yield of 'Seolhyang' was significantly greater than that of 'Maehyang' both in WS and control treatments. The yield of both cultivars decreased in WS treatments by 13.1 and 13.6%, respectively, in comparison with control treatments. In control treatments, TC, AC, and AA were greater, while TOA was inferior in 'Maehyang' than 'Seolhyang' throughout the experimental period. Since TOA increased in 'Maehyang' but decreased in 'Seolhyang', while TC did not change both in both cultivars, carbohydrate-organic acid ratio decreased in 'Maehyang' but increased in 'Seolhyang' in WS treatments. On the other hand, significant differences in AC, AA, and EA as affected by WS treatment were not found in both cultivars except that AC in both cultivars increased at the early starts of harvest. Results indicate that WS treatment is not an appropriate method to improve the quality without decrease in yield for strawberry cultivation even though the contents of antioxidants increased in an early period after being water stressed.

The heat transfer characteristics of a hydroponic system were experimentally verified after theoretical establishment and the effect of nutrient solution cooling on the plant temperature was investigated. About 96 percent of the total heat flow transferred from culture bed to nutrient solution was the conductive heat through planting board and partitioning materials. The average and maximum temperatures of the leaf lettuce decreased 0.6 and 1.5 degrees C., respectively, with cooling of nutrient solution by 6 degrees C. A numerical model for prediction of cooling load of nutrient solution in a hydroponic greenhouse was developed, and the results from the simulation model showed a good agreement with those from experiments. A mechanical cooling system using the counter flow type with double pipes was developed for cooling the nutrient solution. Also the heat transfer characteristics of the system were analyzed experimentally and theoretically, and compared with the other existing cooling systems of nutrient solution. The cooling capacities of three different systems, which used polyethylene tube in solution tank, stainless tube in solution tank, and the counter flow type with double pipes, were comparatively evaluated.

The optimum management of nutrient solution in soilless culture needs the accurate control of nutrient solution, especially in recycled soilless culture system. To keep the electrical conductivity (EC) of nutrient solution within the adequate range after application of combined fertilizers, theoretically derived EC prediction methods are required. In this study, the experimental EC prediction equation, an extended form of the Robinson and Stroke's theoretical equation only available for a binary electrolyte, was developed for predicting the EC of the nutrient solution containing many kinds of inorganic compounds. And the multilayer perceptron consisting of three layers with the back propagation learning algorithm was developed for EC prediction. It consists of nine variables in the input layer for the concentrations of seven macro elements, Na+ and Cl, and one variable in the output layer for the EC of nutrient solution. The predicted ECs by experimental model as well as neural networks for the nutrient solution were compared to the measured ones and showed good agreements.