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MISCELLANEOUS PUBLICATION 1271, U.S. DEPARTMENT OF AGRICULTURE 



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LOG OF EXPOSURE (log einsteins/cm 2 ) 



Figure 5. — Phototropic response of oat coleoptiles to vari- 

 ous quantities of blue light (435.8 nmi applied at 

 constant intensity (1.4 X 10- u einsteins em.- 2 see.- 1 ) 

 for various duration times (57). 



equal to short exposures at high intensities, pro- 

 viding the total quantity of light is the same in 

 both cases. Recipirocity does hold over an enor- 

 mous range of exposure times and intensities for 

 first positive curvature, but this is not true for 

 second positive curvature (51), where duration 

 of illumination appears to be more important 

 than total exposure. There arc other complica- 

 tions. Previous exposure to red light greatly re- 

 duces the sensitivity to blue light for the first 

 positive and negative, curvature — but sensitivity 

 of second positive curvature is increased. And 

 we still don't know how the auxin moves in re- 

 sponse to unilateral light. 



Flower Initiation 



Flowering in response to photoperiodism was 

 discovered by Garner and Allard in 1920 (23). 

 Responses to day length prove to be of wide- 

 spread occurrence in both plants and animals, and 

 there is typically a great variety of response, de- 

 pending upon species. We now know that there 

 are plants that flower not only in response to 

 short days and to long days, but also in response 

 to long days followed by short days or to short 

 days followed by long days. Some plants are in- 

 sensitive to day length, but flower in response to 

 prolonged low temperatures. Others require such 



a vernalization treatment followed by exposure to 

 long days (typically) or to short days (in a few 

 cases). Some plants are quantitative in their re- 

 sponse, flowering under virtually any conditions 

 but being promoted in their flowering by ex- 

 posure to short days, long days, low temperatures, 

 and so forth. Other species are absolute in their 

 requirements, remaining vegetative until suitable 

 conditions have been provided. There are numer- 

 ous interactions between day length, tempera- 

 tures, and other factors. Some species, for ex- 

 ample, flower in response to long days at one 

 temperature but are day-neutral at another tem- 

 perature. Age of the plant is often important. A 

 Chenopodium seedling can respond as a minute 

 organism on filter paper in a petri dish, btit many 

 other plants must have several mature leaves be- 

 fore they will respond to suitable environmental 

 conditions. 



Virtually all of these responses are known 

 among range plants. They are extremely im- 

 portant whenever a plant native to one region is 

 moved to another where daylengths may be dif- 

 ferent at given times of year (if latitudes are 

 different), or where different temperatures or 

 other factors might influence flower initiation, de- 

 velopment, or both. Olmsted (40) showed in pio- 

 neering studies with range grasses that varieties 

 within a species (especially Bouteloua curtipen- 

 dida [Michx.] Torr.) differ greatly in their re- 

 sponses to photoperiod, depending upon where 

 samples are collected. Southern varieties exhibited 

 a short-day response, while northern varieties 

 were long-day plants. Both flowering and vegeta- 

 tive growth of each variety seemed well adapted 

 to the region where the plant was native. These 

 studies were subsequently confirmed and greatly 

 extended by McMillan (34). who used 12 species 

 of grasses in his work. 



The modern analytical approach to a study 

 of photoperiodism was initiated in 1939 by Ham- 

 ner and Bonner (23). Among many other things, 

 they carefully investigated the relationship be- 

 tween critical day and critical night, using X a li- 

 thium strumarktm L. (cocklebur). This short-day 

 plant responds to a single inductive cycle. On a 

 24-hour cycle, flowering occurs if the day is less 

 than about 15 hours and 40 minutes (critical 

 day). Hamner and Bonner found that if the 

 night is longer than about 8 hours and 20 



