INTERACTION OF GROWTH SUBSTANCES AND 

 PHOTOPERIODICALLY ACTIVE RADIATIONS ON 

 THE GROWTH OF PEA INTERNODE SECTIONS^ 



WILLIAM S. HILLMAN 



Department of Botany, Josiah Willard Gibbs Research Laboratory, 



Yale University, New Haven, Connecticut 



A prominent role of the red-far red photomorphogenic system in plants 

 is the control of stem elongation, a process which has long been 

 investigated with particular emphasis on the action of growth sub- 

 stances. The use of excised tissues provides many advantages in study- 

 ing possible radiation-growth substance interaction, but, unfortunately, 

 most recent work on this question (see discussion) has employed mate- 

 rials (leaf discs, hypocotyl hooks, coleoptile sections) in which the 

 response measured is promoted by red light. The results are thus not 

 directly relevant to stem elongation, which is inhibited by red Hght. 



The work of Schneider (1941) suggested that there is no direct 

 interaction between auxin and red light action. Using Avena seedlings 

 grown either in total darkness or exposed to dim red light, Schneider 

 found that the growth of excised mesocotyl (first internode) sections 

 was greater in sections from dark-grown plants than in those from 

 plants previously exposed to red light. The absolute amount of the 

 inhibition was not affected by auxin except at supraoptimal levels, 

 which ehminated it entirely. For some reason, these results have not 

 been considered by more recent investigators of red light-auxin inter- 

 actions (e.g., Liverman and Bonner, 1953; Galston and Baker, 1953). 

 The results to be presented below confirm and extend them, and lead to 

 the conclusion that the effects of red and far-red radiation on pea 

 internode sections, at least, are at most very indirectly mediated by 

 growth substances such as indoleacetic acid or gibberellic acid. 



1 Research supported by the National Science Foundation under grant NSF 

 G-2009 to A. W. Galston. The writer is greatly indebted to Professor Galston 

 for his advice, particularly in the preparation of this paper. 



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