THE SYNTHESIS OF FLOWERING HORMONE 165 



flowering hormone synthesis. At 20°C flowering hormone synthesis 

 is essentially optimum, and there is virtually no destruction. At 25°C 

 destruction seems to equal synthesis during the B period at 30°C 

 destruction, although delayed for an hour or two during the A period, 

 becomes extremely important, and at 35°C destruction is so pre- 

 dominant (right from the beginning) that no net synthesis can occur. 

 The experiment was repeated in July, 1962, and the results are 

 shown in Fig. 9-7. The basic observations mentioned above are all 

 evident here, but there are some interesting diff'erences. Unfor- 

 tunately, effects upon timing are not clear cut. Flowering stage is 

 generally much higher, and the breaks between the A and the B parts 

 of the curves are not sharp. In general, the effects of temperature 

 are less striking, and the temperature range for a given response is 

 much broader. Most of these diff'erences can probably be attributed 

 to the high light intensities and better photosynthesis of the summer 

 months and to the high greenhouse temperatures. 



2. ^'Interruption'' of the Dark Period with High or Low Temperatures 

 Temperature has been used as a research tool in much the same 

 way as light to "interrupt" an inductive dark period. For example, 

 B. Schwemmle (1), then at Gottingen in Germany found that flowering 

 was inhibited most in long-day plants when the dark period was 

 "interrupted" near the middle with 3 hr of low temperature (5°C) 

 and least when the "interruption" occurred near the beginning or 

 the end of the dark period. Short-day plants reacted least when 

 "interrupted" near the middle and most when "interrupted" near 

 the ends. High temperatures (35°C) acted in an opposite way. 



We have taken this same approach, using our growth chambers, 

 floral stages, etc., to "interrupt" with 2-hr periods of low or high 

 temperature. The results are shown in Fig. 9-8. Our high tempera- 

 ture curve is similar to his, but low temperatures actually seemed to 

 promote flowering, regardless of when applied. It is interesting that 

 high temperatures had virtually no effect until after the critical dark 

 period, after which they inhibited almost as effectively as light. This 

 result has been obtained with a number of species (22). Apparently 

 high temperatures can destroy hormone only after it is made, and 

 timing is resistant to such temperatures. 



