Light Periods Not Clocked 



To learn whether plants also could clock their light periods, the 

 scientists reversed the midnight treatment. But no plant altered its 

 flowering time when a day length was interrupted with brief darkness, 

 even midway at noon. 



A Boon to Growers 



Before breaks in dark periods were tested, plant growers had taken 

 care to extend day length by using electric current for some hours either 

 before sunrise or after sunset, to hold back flowering in short -day plants, 

 sonnetimes continuing this delaying treatment for many weeks. Chrysan- 

 themums were almost the only comnnercial crop that justified the extra 

 expense in light bills. Growers of chrysanthennums and some other 

 greenhouse crops welcomed the evidence that midnight light would give 

 more dependable results and at a bargain rate of an hour or less of current 

 per night. 



A Note on Terminology 



Since plants clock darkness, not light, it might seem realistic at this 

 point to adopt the adjectives "short night" and "long night" to describe 

 plants from the angle of their photoperiod timing. 



However, the Garner and Allard discoveries are so nauch a part of 

 all follow-up findings, and so often referred to, that basic terms they 

 introduced continue to be used for consistency. Plants, therefore, are 

 still commonly called long -day and short -day types. 



More Importance for Light 



Giving darkness its due has not relegated light to any minor role in 

 growth changes. During light periods, both the intensity and the color 

 of light are important to photoperiodic responses. 



High intensity of light during all or part of the light periods promotes 

 starch formation. Experiments have shown that during darkness a plant's 

 photoperiodic nnechanism draws on the carbohydrate reserves, and in 

 fact, the mechanism cannot operate if these reserves fall too low. Further- 

 more, in most plants, bright light speeds the initiation of a growth change. 

 Soybean plants have started to flower after only three or four cycles in 

 which 14-hour nights provided adequate darkness and each 10-hour day 

 provided bright light. 



Plants differ in their favorable responses to light intensities, as to 

 other factors that influence their photoperiodic reactions. Knowledge of 

 these light intensity effects is valuable for predicting how well a variety 

 may be expected to yield in a given latitude and climate. Adequate intensity 

 of light is rarely lacking for plants that flower and fruit in days of long, 

 bright light. Daylight intensity is sometimes less dependable for plants 

 that mature in short days. Indoors, light intensity can be naanipulated for 

 many significant effects. 



