PHOTOPERIODIC CONTROL OF FLOWERING 



H. A. BORTHWICK 

 Agricultural Research Service, United States Department of Agriculture, 



Beltsville, Maryland 



Plants are responsive to very small differences in photoperiod such as 

 occur in nature from day to day or from one latitude to another. 

 This responsiveness reveals one important feature of the photoperiodic 

 mechanism that regulates flowering: it operates as a graduated control. 

 The extent to which plants are able to detect and respond to small 

 photoperiodic differences is illustrated by a recent experiment with 

 several varieties of soybean (Glycine max (L.) Merr.). 



In this experiment, performed at Beltsville, Maryland, we attempted 

 to simulate the natural photoperiods of three different localities in 

 Maryland, Virginia, and North Carolina, in which field plantings of soy- 

 bean were grown. The latitudes of these places are 39°N, 36°40'N, 

 and 34°20'N, respectively. The daily photoperiod, the period from 

 sunrise to sunset plus the parts of the morning and evening twilight 

 during which the light intensity was 2 ft-c or greater, was calculated 

 for each latitude and date. The amounts of twilight included in the 

 photoperiods proved to be 20.9% of the twilight periods listed for the 

 appropriate latitudes and dates in the American Nautical Almanac. 



The soybeans were planted in soil boxes on trucks that could be 

 kept outside in the daytime and in dark rooms at night. Incandescent 

 lamps on time switches were used to complete the photoperiods after 

 the trucks were moved inside. The switches were reset at 2-day inter- 

 vals to maintain the calculated natural changes in day length of the 

 three latitudes. Four plantings of six varieties were made at 2-week 

 intervals beginning May 23. 



The mean times of flowering and maturity for all varieties became 

 progressively earlier with decreasing day length (Table I) even though 

 the difference between photoperiods 2 and 1 or 3 for any date was 

 about 17 min near the end of June, and became less as the season 



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