GROWTH SUBSTANCES-RED LIGHT INTERACTIONS 189 



tion of a growth-active complex unless the ad hoc assumption is made 

 that this complex inhibits internode elongation. 



Klein et al. (1956) found that red irradiation decreased the sensi- 

 tivity of excised bean hypocotyl hooks to lAA; the reverse was also 

 true. Since red Hght and auxin cause opposite responses in the tissue, 

 no specific antagonism is necessarily implied by these results. 



Hilhiian and Galston (1957) reported that red and FR control the 

 apparent level of an in vitro lAA oxidase inhibitor found in the buds 

 of etiolated peas, but pointed out the difficulty of ascribing any direct 

 physiological significance to this control. While the changed auxin 

 sensitivity of internode sections from red-grown plants might be 

 ascribed to a change in lAA oxidase activity, this seems unlikely for 

 several reasons. No such chanse could be detected in internode tissue 

 in the work cited. In addition, Galston and Hand (1949), Kent and 

 Gortner (1951), and the resuhs reported here all indicate that the 

 effects of light on apparent auxin sensitivity are as marked when NAA 

 is used as the auxin as with lAA, although NAA is not attacked by 

 lAA oxidase, at least in vitro (Stutz, 1957; also unpublished experi- 

 ments in this laboratory) . 



A relation between gibberellin action and photomorphogenesis has 

 been suggested by several investigations. Lona (1956) and others have 

 observed that GA promotes dark germination of certain red-light- 

 requiring seeds. The specificity of this effect is doubtful, since kinetin 

 also acts in a similar fashion (Miller, 1956) as do other agents. 

 Lockhart (1956) found that while intact dark-grown pea seedlings are 

 only slightly affected by gibberellin, plants exposed to red light respond 

 to gibberellin with an increased internode elongation which amounts 

 to a complete reversal of the red light inhibition. However, gibberelHn 

 does not reverse the red light effects on leaf and node development. 

 Vlitos and Meudt (1957) have suggested that GA may act by pro- 

 tecting against the light inactivation of endogenous growth hormones, 

 although Biebel (1942) found that the hypocotyls of bean plants 

 de-etiolated with red light contained no less Avena-2iCi\vQ auxin than 

 dark controls. Scott and Liverman (1957) reported that GA pro- 

 motes the expansion of etiolated bean leaf discs, as does red light. The 

 promotion was at least partially additive to that of red light, and was 

 reversed by FR except at one concentration. 



