Thigmotropism and the modulation of tropistic curvature by mechanical perturbation in cucumber hypocotyls

Mechanical perturbation (MP) applied unilaterally to cucumber (Cucumis sativus L.) hypocotyls induced thigmotropic curvature toward the stimulus. Gravitropic or phototropic curvature of the hypocotyl was inhibited by symmetrical application of MP to both sides of the hypocotyl. When both MP and IAA were unilaterally applied simultaneously to the same side, the hypocotyls always bent toward the MP stimulus, as in thigmotropism alone. Thus, the exogenous IAA did not control the direction of curvature. Aminoethoxyvinyl glycine (AVG) blocked thigmotropism as well as gravitropism and phototropism, but promoted IAA‐induced curvature. MP‐stimulated ethylene evolution peaked about 4 h after MP, followed by a peak of thigmotropic curvature. For all tropisms more ethylene evolved from the stimulated side than from the other side of the hypocotyls. MP‐induced ethylene acting as a growth inhibitor, auxin‐transport inhibitor, and/or modulator of tissue sensitivity to auxin, may be involved in thigmotropism and MP‐induced inhibition of various tropisms. Ethylene produced as a result of MP was not affected by the removal of cotyledons. This MP‐induced ethylene was additive to that of phototropically or gravitropically stimulated ethylene.