Gravitropism of Primary Roots of Zea mays L. at Different Displacement Angles

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Abstract
Primary roots of Zea mays were oriented at various angles from the vertical ranging from 99° to 1° and their subsequent bending analysed from filmed records. The maximum rate of bending and the time before bending commenced both varied two-fold, but showed no correlation with the initial angle of tip displacement. Roots orientated to small initial angles (< 40°) often overshot the vertical and proceeded to oscillate around this orientation, whereas roots oriented to large initial angles (> 60°) often failed to achieve the vertical. Roots in this latter group resumed bending after an indeterminate time, or did so immediately after a second displacement of their tip, showing that they were not intrinsically unable to bend. The apparently spontaneous resumption of bending after a temporary plagiogravitropic phase is suggested as being due to noise in the graviperception system in the root cap. The tips of roots growing vertically downwards showed oscillatory bending movements up to 10° either side of vertical. This angle corresponds to the minimum angle of displacement which induces gravitropic bending. Only when roots were oriented 10-20° from vertical did they begin unequivocally to show a gravitropism since at such angles the deflection of their tips exceeded that due to their natural oscillation.Copyright 1993, 1999 Academic Press

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