Inhibition of Gravitropism in Etiolated Radish Hypocotyls by Chloramphenicol

The hypocotyls of radish seedlings can grow at an almost normal rate when the roots are immersed in 10−4 M chloramphenicol (CAP) although there is a reduction in the length of the roots. However, when 4-d-old seedlings are placed horizontally, the hypocotyls of those growing in water bend completely upright within 3 h, whereas those growing in CAP react slowly or not at all. The elongation rate of these gravitropically stimulated hypocotyls is about 0.87 and 0.72 mm h−1 in water and in CAP, respectively, and both these values are higher than those of unstimulated seedlings, where a rate of 0.29 mm h−1 occurred in both water and CAP treatments. The effect of CAP on gravitropism occurs only when CAP is applied to seedlings in the first 24 h after sowing. At the cell level the main characteristic of the hypocotyls of CAP-treated seedlings not showing geocurvature is the absence of starch in the cell layer surrounding the vascular tissues. From a comparison between the presence of starch and gravitropic reaction it is suggested that amyloplasts contained in endodermal cells are the receptors of the gravitational stimulus.