In a previous study, we developed a method for measuring the stem water status (Rrev) using Time Domain Reflectometry (TDR) in the satsuma mandarin (Citrus unshiu Marcow.) tree. However, the relationship between fruit quality and Rrev under water stress has not been clarified. Thus, the purpose of this study was to identify this relationship in satsuma mandarin and. Simultaneously, we determined the effects of fruit quality under water stress according to different intensities or periods on the fruit development stage. The results showed that the water stress integral (Sθ) of Rrev was correlated with an increase in the sugar content and fruit enlargement during July-September in satsuma mandarin and highly correlated with the same parameters during August-September in 'Shiranui'. These results were similar to the correlation between fruit quality and the water stress integral (Sψ) of the maximum leaf water potential. Therefore, it was confirmed that Rrev is useful for water management. The effects of fruit quality under water stress in satsuma mandarin included the suppression of fruit enlargement and increase in the sugar content during the fruit developmental stage and suppression of acidity reduction in and after September. In 'Shiranui', it was determined that the water stress has a marked effect on the increasing sugar content in August-September, but it suppressed acidity reduction during autumn and fruit enlargement during the fruit enlargement stage. In conclusion, we propose that Rrev is a useful indicator for fruit quality control under water stress.Key Words：irrigation, leaf water potential, sheet mulching cultivation, water stress integral

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