100 Fundamentals of Auxin Action 



level in most roots is at the optimum or above it. Consequently tropic 

 stimuli which result in lateral movement of auxin and an attendant 

 increase of auxin content on one side inhibit growth on that side. In 

 the case of shoots, on the other hand, growth is stimulated instead. 

 Therefore, while stems turn toward light, roots turn away; and while 

 stems turn away from the force of gravity, roots turn toward it. A 

 clever demonstration of the difference in sensitivity between roots and 

 stems which produces this oppositeness of tropic response has been 

 made by Geiger-Hiiber and Hiiber (1945). They found that repeated 

 decapitation of roots, which drastically lowers the auxin content, 

 causes these structures to respond as stems, i.e. away from gravitational 

 pull. Likewise the addition of rather large quantities of auxins to 

 stems resulted in root-like responses to gravity, presumably because 

 the lateral redistribution of auxin furnished inhibitory levels of auxin 

 to the side toward the gravitational pull. 



Some provocative studies of root tropisms have been made by 

 Pilet (1951), who found that very young roots of lentil bean have a 

 suboptimal auxin concentration, and hence respond tropically in the 

 manner of stems. Roots less than 20 mm. long turn toward light, and 

 as they grow older their auxin content rises and a reversal of the 

 tropic behavior results. This situation is probably not common to 

 plants in general. Moewus and Moewus (1952) found that cress roots 

 contained suboptimal auxin concentrations either when very young 

 or when they had aged beyond the stage of optimal growth. It may be 

 that the lateral spreading of root systems may be due in part to weak- 

 ened geotropic sensitivity as the roots grow older and their auxin con- 

 centration is reduced. 



The action of auxins in other types of tropic responses such as 

 thigmotropism and traumatropism is discussed by Went and Thimann 

 (1937, p. 178) and Schrank (1951). 



INHIBITION EFFECTS 



With the first discovery of auxins, it was found that they could 

 inhibit the growth of coleoptiles, stems, and of course roots. The first 

 inhibition effect of auxin to be described as a positive force in plant 

 development was that of auxins upon lateral bud growth and the 

 resulting phenomenon of apical dominance. Thimann and Skoog 

 (1933) clearly demonstrated this function of auxin by establishing, 

 first, that the apex is the largest source of auxin for the plant; second, 

 that removal of the apex results in the loss of apical dominance and 

 stimulates the development of lateral buds; and finally that the artifi- 

 cial supplying of auxin after decapitation restores apical dominance. 



