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, 45 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 different growth and also the magnitude of differential growth were similar at all displacement angles. The fact that very small or Very large angles of displacement produce a maximal response needs to be addressed by models of gravitropism. The autotropic straightening phase (the reduction of the gravitropically-induced curvature) began first in the treatments with the smaller displacement angle, consequently the initiation of autotropism was independent of the radius of curvature at the time autotropism was induced. The fact that the lag period before the onset of autotropism is not fixed illustrates the need to take into account autotropism when conducting studies of organs subjected to prolonged gravistimulation.