THE EFFECT OF DISPLACEMENT ANGLE ON THE GRAVITROPIC AND AUTOTROPIC GROWTH-RESPONSES OF SUNFLOWER HYPOCOTYLS

Sunflower hypocotyls were gravistimulated at angles of 22.5 degrees, 4 5 degrees, 90 degrees, 135 degrees and 157.5 degrees from the vertical and the gravitropically-induced growth rate changes of the upper and lower surfaces measured.The lag period before the initiation of differ ent growth and also the magnitude of differential growth were similar at all displacement angles. The fact that very small or Very large ang les of displacement produce a maximal response needs to be addressed b y models of gravitropism. The autotropic straightening phase (the redu ction of the gravitropically-induced curvature) began first in the tre atments with the smaller displacement angle, consequently the initiati on of autotropism was independent of the radius of curvature at the ti me autotropism was induced. The fact that the lag period before the on set of autotropism is not fixed illustrates the need to take into acco unt autotropism when conducting studies of organs subjected to prolong ed gravistimulation.