The Mechanism of Geoperception in Lentil Roots

Abstract

The sedimentation of the amyloplasts in the statocytes of lentil roots was analysed in two different samples of seedlings. The roots of the first sample (HP) were grown in a horizontal position, whereas those of the second sample (VP + 20) were placed first in a vertical position and then subjected to a geotropic stimulation equal to the presentation time (i.e. minimum horizontal exposure to give a response). It is demonstrated that the statoliths can almost enter into contact with the plasma membrane lining the longitudinal wall of the statocytes of the HP sample. However, these organelles in the VP + 20 sample are able to provoke a geotropic stimulation though they are sedimented at a distance of 0-1350-0-2600 //m from the plasmalemma. From these data it is concluded that the cytoplasmic structures which may play a role in the geotropic stimulation are: (1) the endoplasmic reticulum located along the longitudinal wall of the statocytes and (2) the microtubules or the plasmalemma if the action of the statoliths is transmitted by the parietal cytoplasm. The large aggregations of endoplasmic reticulum which are situated in the distal part of the central root cap cells would not have any role in the perception of gravity by roots. These results are discussed in the light of recent work showing the action of a growth inhibitor in the geotropic reaction of roots.