Studies on the Translocation of Photosynthates in Satsuma Mandarin

Diurnal fluctuations in diameter or satsuma mandarin fruit were precisely measured and recorded by means of a differential transformer. The fruit enlargement was found to be conspicuous from the middle of daytime to about 8:00 p.m. Thereafter a slight but steady expansion continued until the sunrise of the next morning. About an hour after sunrise, the fruit was gradually decreased in diameter. The rate of contraction and the time of expansion within a day depended mainly on the meteorological conditions. It was also clarified that on a fine day there observed short-period rhythmical fluctuations of expansion and contraction in diameter which were superposed on a general increasing trend of fruit growth. The period was about 20 to 30 minutes and the amplitude was about 40 to 60μ. However, when the whole tree was covered with black cheesecloth, these fluctuations disappeared and the fruit showed only a monotonous enlargement. When it was cloudy, there appeared no rhythmical fluctuations. Further, it was noticed that several minutes after the initiation of illumination the fruit responded to it quickly, i.e., when irradiation was decreased, the fruit immediately expanded and vice versa. Variations in leaf thickness also coincided with those in fruit diameter. The translocation of photosynthates was always decreased with low intensity of irradiation.From the results obtained, it may reasonably be concluded that the apparent enlargement of young satsuma mandarin fruit is closely associated with the moisture content within a tree. Thus, the magnitude of stomatal opening seems to be an important factor with respect to the induction of short-period rhythmic fluctuations of young fruits.