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 to the optimum. The operation itself may have an influence on 

 the growth rate, but one would expect that the effect of wounding would 

 most likely be a retardation of growth. In various cases, decapitation actually 

 decreases the growth rate; in those instances, however, in which a growth 

 acceleration was noted, the result favours the concept that the normal, 

 internal auxin concentration is supra-optimal. 



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