PHOTOPERIODIC EFFECTS IN WOODY PLANTS 239 



Table I. Quantitative Changes in Amount of Endogenous lAA (?)" Caused by 

 Photoperiodic and Gibberellic Acid Treatments 



Endogenous lAA (?)" in /xg X 10~^ 



In tips (+ unfolded leaves) of sumac 



Controls 



Continuous long days 



After 1 week of short days 



After 2 weeks of short days 

 Treated with GA (10 /xg plant) 



Long days: 1 week after GA treatment 



A^fter 1 week of short days, 1 week after GA 

 treatment 



After 2 weeks of short days, 2 weeks after 

 GA treatment 



In tips (above the first two true leaves) of dwarf 

 red kidney bean seedlings: 



Controls 



Two days after GA treatment (5 /xg plant) 



" Auxin moving to the L\A position when chromatographed in isopropanol (80) + am- 

 monia (1) + water (19) (V/V). 



chromatographs at the lAA position had increased markedly (Fig. 

 10). A large increase in the concentration of this auxin was found 7 

 days after the application of gibberellic acid (GA), even though the 

 plants were growing under short days. Thus, a second result of this 

 investigation is that gibberellic acid, which increases stem elongation 

 and prevents, at least temporarily, short days from stopping growth, 

 also produces a clear-cut increase in the level of endogenous auxin. As 

 a matter of fact, we have found that only 2 days after the application 

 of gibberellic acid to dwarf kidney bean seedlings, the level of an auxin 

 that chromatographs at the position of indoleacetic acid increases ten 

 fold (Table I). The results may help, possibly, to understand the mode 

 of action of gibberellic acid. 



In short, the experiments summarized in Fig. 10 show that, when- 

 ever growth is kept active, either by means of long days or with gib- 

 berellic acid, a relatively high level of endogenous auxins can be 

 found. As far as the photoperiodic mechanism is concerned, however, 

 this is not the entire story. If we use a test which detects inhibitors, 



