The "acid growth effect" and geotropism.

The curvature developed by segments of sunflower hypocotyl exposed to gravitational stimulus was enhanced in buffer solutions between pH 3.4 and 4.0 in the absence of added auxin. This effect was observed both when the segments were submerged during the stimulus and when they floated near the surface of the solution. 5-10 min in a horizontal position was sufficient to induce subsequent curvature.Straight growth of the segments was also promoted in buffers of this pH range.The acid effect on curvature was insensitive to KAsO2, HgCl2 and cycloheximide, inhibitors which drastically reduced auxin-induced curvature. Furthermore, acid buffer, but not auxin, restored the ability of segments taken from etiolated and "starved" plants to respond to gravity. These results suggest that the polarisation following gravistimulus may not be resticted to the asymmetric distribution of auxin and auxin co-factors but may involve a general physiological asymmetry.