Metabolism and mode of action 



and consistent lowering of free I AA levels, in most cases to vanishingly small 

 values. Concentrations were thus reduced from levels markedly inhibiting 

 root section growth to levels causing stimulation, i.e. a reduction to at least 

 a thousandth of the control value. This effect is highly significant. These 

 startling results offer an explanation of many of the observed physiological 

 actions of TIBA. Its stimulating action on flower production, now established 

 by many independent workers, coupled with this lowering of free auxin, 

 supports the current theories associating flowering with low auxin levels. 

 Its inhibition of extension growth in shoots could be due to the same lowering 

 of auxin levels. On the other hand, such an effect might be expected to cause 

 a stimulation of the growth of attached roots if endogenous auxins are at 

 supra-optimal levels for growth in such roots. Such stimulations, however, 

 have never been observed. It is, of course, possible that any such stimulation 

 might be offset by a direct inhibitory action of TIBA at those concentrations. 

 Its abolition of positive geotropic response is strong evidence that lAA plays 

 an important role in geotropic phenomena in roots. Synergisms of lAA 

 action by low concentrations of TIBA reported in coleoptiles and pea shoots 

 by Thimann and Bonner (1948) and in roots by Aberg (1953) are difficult to 

 reconcile with lowered auxin levels. The most rational explanation of these 

 conflicting responses is that TIBA has a number of different and discrete 

 actions in plant cells depending on the concentration employed. A slight 

 synergism of auxin action at very low concentrations might well give way to 

 this disturbance of free auxin levels as the concentration is raised, both effects 

 being modified and sometimes swamped by a direct, and perhaps unspecific 

 inhibiting action, particularly at the higher concentration levels. The next 

 steps towards solution of these problems are obviously investigations on the 

 relationship between the intensity of this auxin-lowering effect and the 

 applied TIBA concentrations. 



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