INFLUENCE OF GIBBERELLIN AND AUXIN 



337 



Influence of Auxin on Photoperiodic Induction in Long-Day Plants 



In long-day plants, the only information on auxin effects on photo- 

 induction that was available for quite some time was similar to the 

 original observation of Dostal and Hosek (1937), that is, auxin may 

 reduce the effectiveness of photoinduction. Von Denffer and Griindler 

 (1950) found that auxin treatment delayed flower formation in long- 

 day-grown Calendula officinalis; von Denffer and Schlitt (1951) 

 showed that UV irradiation, which, somewhat similar to ethylene 

 chlorohydrin treatment, seems to decrease the auxin content of plants, 

 promoted flower formation in long-day-grown flax and Statice bon- 

 duelli. In many other long-day species, neither of these treatments had 

 any specific effect on the flowering response, but this may well be due 

 to quantitative differences in the sensitivity of the plants to auxin. 



A few years ago, however, Liverman and Lang (1956) were able 

 to demonstrate a promotive effect of auxin on photoinduction in two 

 long-day plants, Hyoscyamus niger and Silene armeria. When these 

 plants were grown in short days (8 hr of light daily), but were given 

 subsaturating intensities of supplementary light during the night period, 

 simultaneous auxin treatment resulted in flower formation, or increased 



10 15 20 



CONCENTRATION OF lAA IN mg/l 



25 



Fig. 1. Promotion of photoinduction in annual Hyoscyamus niger by 

 simultaneous treatment with auxin. The plants received various levels of 

 supplementary light during the 16-hr night period and were sprayed with 

 auxin daily before the start of the dark period. 



