Effects of the Direction of Gravistimulation on Tuber Formation and Amyloplast Distribution in Tuber Tips of Chinese Yam

摘要

The effects of gravistimulation direction on tuber formation and amyloplast distribution in the tuber tips of Chinese yam were investigated. Tubers placed vertically for either 2 or 14 d elongated in the direction of gravity vector. The subapical parts of tubers that were positioned horizontally for 2 d curved slightly toward the gravity direction, and such tubers elongated downward after 14 d. In tubers that were inversed for 14 d, the tips bent and eventually elongated in the direction of the gravity vector. These results indicated that the subapical part of the tuber was the initial site of curvature in response to gravistimulation. Microscopy and measurement of amyloplast sedimentation ratios showed that many amyloplasts sedimented downward in each tip cell of tubers that were placed vertically, inversely or horizontally. The results indicated that amyloplast sedimentation direction corresponded with the eventual direction of tuber elongation under different gravistimulation orientations. Tubers rotated at 90o intervals every day for 14 d did not have an elongating shape and grew multiple tips at different angles. In the rotated tubers, amyloplasts dispersed within the tip cells. These results showed that proper sedimentation of amyloplasts would be essential for the normal elongation and formation of tubers. The results of this study supported the possibility that sedimentable amyloplasts in the tuber tip were involved in gravity sensing and tuber formation.