Physiological Action of Gibberellin 607 



the presence of gibberellin were smaller and lighter green in color 

 than normal ones. With lAA, however, multiplication of the frond 

 was inhibited by I mg/1 and stimulated by 0.5 mg/1; fronds grown 

 in the latter concentration were larger in area and somewhat deeper 

 green in color than the controls. Therefore, the hypothesis that the 

 gibberellin action is due to a change in the auxin content in plant 

 tissues should be revised. There are many reports (1, 2, 3, 6, 9, 17, 20, 

 24,26) which do not conform with the hypothesis of Pilet (21), Pilet 

 and Wurgler (22), and Stutz and Watanabe (25). 



There remains, then, the problem of the mechanism of action of 

 gibberellin. Without entering into this difficult problem, I wish to 

 mention that some hormone-like factors are needed for the growth 

 effect of gibberellin. The necessity of auxin is already discussed. 

 Vlitos and Meudt (27) demonstrated, by using etiolated pea cuttings, 

 that some factor(s), regarded as existing in the shoot apex, is involved 

 in GA action. Brian and Hemming (4) and Galston and Warburg 

 (8) postulated that a third factor is required for gibberellin to be 

 effective. Since Wittwer and Bukovac (29) and Wittwer et al. (30) 

 showed that the photoperiod was an important factor controlling 

 the plant response to gibberellin, it is presumed that some factor(s) 

 produced under proper photoperiods is necessary for the gibberellin 

 action. 



The necessity of these factors should be kept in mind when the 

 action mechanism of gibberellin is considered and also when the 

 bioassay of gibberellin is attempted. It should also be noted, on the 

 other hand, that such factor(s) may be contained naturally in certain 

 kinds of strains of plants and not in others (19, 28). 



SUMMARY 



Gibberellin is a growth-promoting substance quite different 

 in nature from auxin. Its growth effect is not due to a change in 

 auxin level of the affected tissue. The reaction site of gibberellin in 

 plant tissues is different from that of auxin. 



ACKNOWLEDGMENTS 



The author wishes to thank Professor Joji Ashida for his cordial 

 guidance through the preparation of this paper. The crude gibber- 

 ellin and gibberellin A were provided through the courtesy of Pro- 

 fessor Y. Sumiki, Tokyo University, and gibberellic acid through the 

 courtesy of Dr. C. Leben, Eli Lilly Co. The author is greatly in- 

 debted to Professor R. L. Wain, University of London, and Professor 

 H. Burstrom, University of Lund, for kindly providing the author 

 with anti-auxins. 



