The distribution of IAA-2-14C or IAA-5-3H applied to the apex in the upper and lower (with respect to gravity) halves of geotropically stimulated stems of cucumber and pea was examined and the results obtained are as follows: 1. A larger amount of IAA-2-14C or IAA-5-3H was detected in the lower than upper half of cucumber hypocotyls with 3-hr geostimulation. 2. A larger amount of IAA-2-14C was distributed in the lower than upper epidermis of pea epicotyls with 1-hr geostimulation. 3. Freezing autoradiography revealed that IAA-2-14C was concentrated in the vascular bundles and epidermis of cucumber hypocotyls, the distribution being affected by geostimulation only in the latter. 4. Application of 1% TIBA in lanolin inhibited the distribution of IAA-2-14C in the lower epidermis of pea epicotyls, causing suppression of geotropic curvature. 5. From these results, we concluded that IAA which accumulated in the lower epidermis of the stem upon geostimulation caused the negative geotropic curvature of the stem.
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Iwami et al. (1976) studied this question.