INACTIVATION OF PLANT GROWTH SUBSTANCE BY 

 LIGHT 



By PAUL R. BURKHOLDER, 



Department of Botany, Connecticut College 



AND 



EARL S. JOHNSTON, 



Division of Radiation and Organisms, 

 Smithsonian Institution 



(With Two Plates) 



INTRODUCTION 



The influence of general illumination upon the size and form of 

 plants, as well as the induction of phototropic curvatures by unilateral 

 light, has been explained in recent years on the basis of special growth 

 substances. Since rate of growth is proportional to the concentration 

 of growth substance (auxin) within certain limits (Went, 1928; 

 Nielsen, 1930; Soding, 1936; cf. Boysen Jensen, 1936 a), differential 

 distribution of these growth-regulating substances should be expected 

 to result in unequal enlargement of the constituent parts of a growing 

 tissue or organ. The commonplace bending of plant shoots toward 

 light incident from one side has been ascribed to the unequal distri- 

 bution of growth-promoting substance on the illuminated and shaded 

 sides. More growth substance occurs on the shaded than on the 

 illuminated side, hence the rate of elongation in this region is relatively 

 greater, and the organ is forced to bend. 



The precise manner in which light influences the role of growth 

 substances in a growing plant is not easy to explain. Light may 

 modify the quantity of growth substance present by photo-formation 

 or -destruction. The extent to which translocation and effective 

 action of growth svtbstance are influenced also constitutes an im- 

 portant aspect of the problem. Various investigations up to the 

 present have emphasized the effects of light upon movement and 

 activity of growth regulators, especially in relation to phototropic 

 phenomena ; on the other hand, photo-synthetic and -destructive re- 

 actions have not received sufficient attention. For an extensive dis- 



Smithsonian Miscellaneous Collections, Vol. 95, No. 20 



