Action of Gravity on Basipetal Transport of Auxin.

On the basis of tests with over 20 species of erect plants, we reported in 1962 (9) that a strong growth curvature develops in the axis when an erect plant is rotated slowly on a horizontal clinostat. A differential transport of endogenous auxin was indicated by the production of a similar, predictable curvature after the application of triiodobenzoic acid (TIBA) to 1 side of the immature axial tissues. This treatment, known to block the transport (7, 13) of indoleacetic acid (IAA), allowed the axis to grow more rapidly oni the opposite side but only when the effects of gravity were eliminated by continuous rotation on a clinostat. A similar growth curvature in a rotated plant was obtained by removing the leaves from 1 side of the growth zone of the axis. Subsequent growth was slower on the defoliated side. However, in all cases of TIBA blocks and partial defoliation the apparent imbalance of growth regulator in the elongating internodes was prevented or, if curvature had just appeared, corrected when the plant was allowed to grow for a day or 2 in its normal, erect position. The same results from elimination of unidirectional gravitation action were observed in defoliated and disbudded axes when growth hormone was supplied in solutions or emulsions of IAA. Even with a symmetrical application of this auxin to the cut end of the axis, its elongation was asymmetrical if growth took place on a horizontal clinostat. Gravity appeared to be a factor in maintaining an even distribution of growth regulator within the growth zone of the plant's axis. The need for more information on the basipetal movement of auxin within the axial tissues has led us to examine the actual distribution of the hormone within the growing stems, with and without the action of gravitational force along the axis. After bioassays of residual, encdogenous auxin in growth curvatures failed to reveal an imbalance of growth regulator, a series of tests with radioactive IAA provided evidence for its uneven distribution through lack of the action of gravity oln auxin transport down the stem. Other tests have shown a deleterious effect on the rate and form of growth of very young plants if they grow in the horizontal position.