. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). GEOTROPIC RESPONSES IN ROOTS. SOME THEORETICAL AND TECHNICAL PROBLEMS p. Larsen Botanical Laboratory, University of Bergen THE AUXIN THEORY OF GEOTROPISM The auxin theory of geotropism offers a logical explanation of the upward curvature of stems and coleoptiles. The theory has been extended to explain the downward curvature of main roots as well. At least in the textbooks, the existence of a supra-optimal auxi
. The chemistry and mode of action of plant growth substances; proceedings of a symposium held at Wye College, University of London, July 1955. Plant regulators; Auxin; Growth (Plants). GEOTROPIC RESPONSES IN ROOTS. SOME THEORETICAL AND TECHNICAL PROBLEMS p. Larsen Botanical Laboratory, University of Bergen THE AUXIN THEORY OF GEOTROPISM The auxin theory of geotropism offers a logical explanation of the upward curvature of stems and coleoptiles. The theory has been extended to explain the downward curvature of main roots as well. At least in the textbooks, the existence of a supra-optimal auxin content in roots is generally accepted. The well-known schematic curve {Figure 1), representing the reladonship between growth rate and internal auxin concentration in the elongation zone of roots, is the basis of the explanation of the downward curvature. We assume that the internal auxin concentration is supra-optimal for growth (point A on the curve of^ Figure 1). An addidonal supply of auxin to the. Figure 1. Schematic representation of the relationship between growth rate and internal auxin concentration in roots. Intepnal concentration of auxin lower side of a main root placed in the horizontal position will thus retard the growth rate of that side, and consequently the root will curve down- ward. The theory seems to be well founded since both the higher auxin concentration in the lower half of the root and the retarding effect of added auxin on the rate of root elongation have been demonstrated experimentally. There are, however, various points in this theory which need clarification. 1. Firstly, is the internal auxin concentration really supra-optimal ? This problem has been approached by observation of the effect of decapitation on the rate of elongation of roots. In several cases, decapitation results in an increase of the rate of elongation and this has been interpreted as a con- sequence of reducing the supra-optimal, internal auxin concentration to values closer
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