Inhibition of Gravitropism in Etiolated Radish Hypocotyls by Chloramphenicol

The hypocotyls of radish seedlings can grow at an almost normalrate 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, thehypocotyls of those growing in water bend completely uprightwithin 3 h, whereas those growing in CAP react slowly or notat all. The elongation rate of these gravitropically stimulatedhypocotyls is about 0.87 and 0.72 mm h^–1 in water andin CAP, respectively, and both these values are higher thanthose of unstimulated seedlings, where a rate of 0.29 mm h^–1occurred in both water and CAP treatments. The effect of CAPon gravitropism occurs only when CAP is applied to seedlingsin the first 24 h after sowing. At the cell level the main characteristicof the hypocotyls of CAP-treated seedlings not showing geocurvatureis the absence of starch in the cell layer surrounding the vasculartissues. From a comparison between the presence of starch andgravitropic reaction it is suggested that amyloplasts containedin endodermal cells are the receptors of the gravitational stimulus. Raphanus sativus, radish, gravitropism, amyloplasts, chloramphenicol