Gravitropism and Phototropism of Maize Coleoptiles: Evaluation of the Cholodny-Went Theory Through Effects of Auxin Application and Decapitation

It was investigated whether or not gravitropism and phototropism of maize (Zea mays L.) coleoptiles behave as predicted by the Cholodny-Went theory in response to auxin application, decapitation and combinations of these treatments. Gravitropism was induced at an angle of 30° from the vertical, and phototropism, by a pulse of unilateral blue light. Either tropism of the coleoptile was inhibited by IAA, applied as a ring of IAA-lanolin paste to its sub-apical part, and by decapitation. The dose-response curves for the effects of applied IAA on tropisms and growth of intact coleoptiles as well as the time courses of tropisms induced in decapitated coleoptiles could be explained by the three conclusions in the literature: (1) the tip of the coleoptile is the site of auxin production, (2) lateral translocation of auxin in gravitropism occurs along the length of the coleoptile, and (3) lateral translocation of auxin in phototropism occurs in the coleoptile tip. By examining the effects of decapitation made at different distances from the top and of IAA applied to the cut surface of decapitated coleoptiles, it was indicated that auxin is produced in the apical 1 mm zone of an intact coleoptile and that lateral auxin translocation for phototropism takes place in an apical part that somewhat exceeds the zone of auxin production.