Thickening Growth Inhibition and Re-thickening Growth of Tuberous Roots of Sweet Potato Plants (Ipomoea batatas Poiret)

Abstract

The morphological and physiological features of tuberous roots in the process of thickening growth inhibition and re-thickening growth were tested by means of the light exposure treatment for growing tuberous roots. Cultivar (Okinawa, No.1OO) was used as sample and the light exposure treatment for the growing tuberous roots were conducted in the field. The tuberous roots were outcropped on the ground by washing away the soil covering the hills. Those outcropped tuberous roots were exposed to the sun light for about thirty to seventy days. The light exposure treatment was stopped by covering the outcropped tuberous roots with soil. The results are as followed: (1) The thickening growth of tuberous roots was inhibited by exposing them to the light. On the contrary, the translocation of photosynthates to tuberous roots had continued and the thickening growth of underground part of the treated tuberous roots (tuberous roots exposed to the light) was apparently promoted. (2) The starch content in the treated tuberous roots decreased in the extreme and the accumulation of starch granules decreased starting from the cortex to the central cylinder with the lapse of time after the treatment and the size of starch granules diminished also. Such changes were brought about in parallel with the decrease in the starch content. (3) In case of the treated tuberous roots, the sclerenchymalization of parenchymatous cells near the primary cambium was found apparently. It showed an increase also in the number of vessels near the primary cambium. But, the number of parenchymatous cells in the central cylinder decreased due to the treatment. (4) The re-thickening of tuberous roots of the plants was found due to the stop of treatment. The starch content had increased with the re-thickening of tuberous roots and the histochemical features were similar to those of the non-treatment tuberous roots. (5) From the above-mentioned facts, it may be considered that the thickening of tuberous roots of sweet potato plants is related to the development of starch storage function and that the starch storage furnction is concerned with the activity of starch biosynthesis, starch accumulation, and the increase in parenchymatous cells. Viewed from the result of tuberous roots to the light as well as of the re-thickening growth of tuberous roots due to the stop of the treatment, it may be suggested that the starch storage function of tuberous roots is apt to adapt itself to the environmental conditions