606 ]. Kato 



gans, reported no difference between the effect of GA and mixtures 

 of GA and TIBA in various concentrations, and concluded that 

 auxin was not definitely responsible for GA-induced cell elongation. 



As to the mode of action of gibberellin, Pilet (21), Pilet and 

 Wurgler (22), and Stutz and Watanabe (25) think that gibberellin 

 operates through its effect on the lAA oxidase, namely through rais- 

 ing the auxin level in plant tissues. Nitsch (18) reported that GA 

 treatment of some woody plants increased their auxin content. How- 

 ever, my experiments using combinations of gibberellin with auxins 

 and anti-auxins have suggested that the effect of gibberellin involves 

 a physiological sequence different from that of auxin (12, 13). 



Brian and Hemming (4) have shown that GA had no stimulatory 

 nor inhibitory effect on the lAA oxidase prepared from etiolated pea 

 seedlings. Kato and Katsumi (14) also tested the effects of GA and 

 GAi on the activity of lAA oxidase prepared from etiolated pea 

 shoots. As is shown in Figure 5, neither was stimulating nor inhibit- 

 ing. GA had no effect even at high concentrations and at various pH 

 values. 



According to my unpublished experiments, 0.1 to 1.0 mg/1 solu- 

 tions of crude gibberellin accelerated the multiplication of fronds of 

 Lemna paucicostatn, 0.5 mg/1 being the optimum. Fronds grown in 



100 



3 

 TIME IN HOURS 



Fig. 5. Lack of effect of gibberellic acid and gibberellin A on the activity of indole- 

 3-acetic acid oxidase (14). 



