It is very important to control the water stress at the growth stage of Satsuma Mandarin for high sugar content. It is known that a shift of the red edge wavelength of spectral reflectance to shorter wavelengths is caused in water stressed leaves. The authors investigated the transition of the water potential and the red edge wavelength of spectral reflectance in the actual fruit cultivation. Its period was from July to November in 2003. Consequently, the transition of the red edge wavelength was the same as the transition of the water potential in filmmulching on the ground surface of the fruit trees. Tow main results were obtained. First, an adequate correlation between the averaged sugar content in harvest fruit and amount of change of the red edge wavelength at the growth stage of fruit was indicated from the results of measurements. Second, it is assumed that the individual differences of mandarin trees, such as the amount of fruit, volume of tree, and nutritive condition, influence the red edge wavelength.
