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Longitudinal and lateral transport of [3, 4‐3h‐] gibberelling a1 and 3‐indolyl (acetic acid‐2‐14c) in upright and geo‐tropically responding green internode segments from helanthus annuus

The new phytologist 1976 v.76 no.1 pp. 1-9
Helianthus annuus, Zea mays, agar, carbon, coleoptiles, etiolation, gibberellic acid, gravity, hormones, indole acetic acid, internodes, radionuclides, seedlings, tissues, tritium
Distribution of radioactivity from [³H] GA₁ and [¹⁴C] IAA within the tissues of isolated 2.5 or 3 cm long segments cut from the first internodes of sunflower seedlings was unaffected by displacement from a vertical to horizontal position. This was true whether the agar donor blocks were applied uniformly across the apical cut end of the segments, or where asymmetric donors were used. Neither downward nor upward lateral transport of gibberellin or auxin was influenced. Similarly, use of a lateral donor technique failed to reveal any effect of gravity upon transport across the stem segment to a lateral agar receiver block on the opposite flank. Experiments on etiolated Zea mays coleoptile segments confirmed the existence in this organ of gravity‐induced lateral transport of [¹⁴C] IAA. No growth curvatures occurred in upright sunflower stem segments incubated with GA₁ or GA₃ apical donor blocks applied uniformly or asymmetrically, although elongation growth was enhanced. Horizontally positioned segments exhibited only very weak negative geotropic curvatures when in contact with plain agar blocks, but [³H] GA₁ (2 × 10‐⁶M) donors induced marked curvatures when applied either uniformly or asymmetrically to the apical ends. Asymmetric donor blocks containing IAA induced growth curvatures in both upright and horizontal stem segments. It is concluded that the mechanism of geotropic response in sunflower stems is likely to involve the participation of both auxin and gibberellin, but that these experiments do not suggest that lateral displacement of these hormones is part of such a mechanism. Differences in levels of endogenous gibberellins that are correlated with direction of geotropic curvature may be a consequence of preferential hormone synthesis or transport in the lower or upper sides of horizontally positioned orthogeotropic organs.