Occurrence and Role of Growth Hormones 



85 



quence auxin flowing Iroin the apex would be shunted to the shaded 

 side, and unilateral growth would ensue. The investigators point out 

 that the tropic stimuli may exercise their effects by altering the bio- 

 electric potential of the plant part, which in turn may alter proto- 

 plasmic streaming and hence auxin transport. 



Quite a different type of auxin transport which is non-polar is 

 known to exist, and this is discussed in chapter VI. 



AUXIN INACTIVATION 



It would be logical to expect that there must be some mechanism 

 in the plant by which auxin may be disposed of, inactivated, or de- 

 stroyed. Early workers in the field of phototropism demonstrated that 



400 420 440 



WAVELENGTH 



460 



480 



500 



520 m^j 



Fig. 41. Action spectra for photo-inactivation of indoleacetic acid by pure riboflavin 

 and by breis of pea plants (Galston and Baker, 1949). 



the exposure of the Avena coleoptile to light resulted in a reduction 

 in the total amount of auxin present (e.g. Went, 1928). More recent 

 observations have led to the conclusion that light-activated processes — 

 both enzymatic and non-enzymatic — can inactivate auxin in plants 

 (Tang and Bonner, 1947; Galston, 1949). 



Photoinactivation of auxin has been studied both in vitro and 

 in vivo. The inclusion of either pea brei or riboflavin with indoleacetic 

 acid results in a disappearance of auxin when exposed to light. Studies 



