We tested the involvement of ethylene in maize (Zea mays L.) root gravitropism by measuring the kinetics of curvature and lateral auxin movement in roots treated with ethylene, inhibitors of ethylene synthesis, or inhibitors of ethylene action. In the presence of ethylene the latent period of gravitropic curvature appeared to be increased somewhat. However, ethylene-treated roots continued to curve after control roots had reached their final angle of curvature. Consequently, maximum curvature in the presence of ethylene was much greater in ethylene-treated roots than in controls. Inhibitors of ethylene biosynthesis or action had effects on the kinetics of curvature opposite to that of ethylene, i.e. the latent period appeared to be shortened somewhat while total curvature was reduced relative to that of controls. Label from applied 3H-indole-3-acetic acid was preferentially transported toward the lower side of stimulated roots. In parallel with effects on curvature, ethylene treatment delayed the development of gravity-induced asymmetric auxin movement across the root but extended its duration once initiated. The auxin transport inhibitor, 1-N-naphthylphthalamic acid reduced both gravitropic curvature and the effect of ethylene on curvature. Since neither ethylene nor inhibitors of ethylene biosynthesis or action prevented curvature, we conclude that ethylene does not mediate the primary differential growth response causing curvature. Because ethylene affects curvature and auxin transport in parallel, we suggest that ethylene modifies curvature by affecting gravityinduced lateral transport of auxin, perhaps by interfering with adaptation of the auxin transport system to the gravistimulus.The role of ethylene in gravitropism is controversial. Burg and Burg (1) suggested that accumulation of auxin along the lower side of gravistimulated roots leads to enhancement ofethylene production there. They proposed that the elevation of ethylene leads to growth inhibition resulting in downward curvature. This interpretation now seems overly simplified since we now know that root gravitropism results from acceleration of growth along the upper side as well as inhibition along the lower side (9, 15). Also, there is evidence that the elevation of ethylene levels in auxin-treated roots may be insufficient to account for auxin-induced growth inhibition (3). Wheeler and Salisbury (19) reported that inhibitors of ethylene biosynthesis or action retard shoot gravitropism, and they suggested that ethylene mediates gravitropic curvature in these shoots. However, questions have been raised regarding both the statistical significance of the inhibitions observed and the specificity of the high concentrations of inhibitors used in this study (14). Harrison and Pickard (6) studied the role of ethylene in the negative gravitropic response oftomato hypocotyls. They found that gravitropism can occur without substantial changes in ethylene production and also that curvature is unaffected by raising or lowering ethylene in the hypocoty...