TIMING AND THE FLOWERING PROCESS 



147 



way. The critical night was determined for plants receiving 2 hr of 

 low temperature (10°C) at the beginning of the dark period, or 

 between the fifth and seventh hours after the beginning of the dark 

 period, or not at all. The results in Fig. 8-6 show that timing was 



8-0 

 7-0 

 6-0 



S 5-0 + 



_i 4-0 

 < 



o 



^ 3-0 



2-0 + 

 1-0 

 



:•" 



3 Inductive 

 cycles 



/^8' 



Control (23°C)/® 



/ 



^ / 



'/ ICC During the / 



• 5'^' and 6'" hours / 



/ / 



/ 



/ 



/ 

 / 



/!0°C During the 

 y first two hours 



/ 





8 • 9 10 II 



LENGTH OF THE DARK PERIOD IN HOURS 



Figure 8-6 



Flowering response of cocklebur to various night lengths as influenced 

 by 10°C treatments applied during the first 2 hr of the dark period or 

 between the beginning of the fifth and the end of the sixth hours, as 

 compared to controls with no 10°C treatments. Plants were treated 

 with 3 dark periods, beginning July 18, 1962. Temperature during the 

 dark period other than treatment times was 23°C. Data previously 

 unpublished. 



indeed slowed by low temperature at the beginning of the dark 

 period, but not by low temperature given later on. It is also easy to 

 see why low temperature inhibition might be diflftcult to observe 

 with a 16-hr dark period: after 12 hr or so, flowering occurs at a 

 saturation level in spite of the low temperature treatment. These 

 results provide good support for the idea that the phytochrome 

 conversion aspect of timing is limited to a short interval of time just 

 after the beginning of the dark period. 



