REVERSIBILITY OF THE MECHANISM SHOWN BY 

 RED AND FAR RED 



Concentrating on the red in sunlight, photoperiodic pioneers were led 

 to their next major milestone --finding that the unknown chemical works 

 a reversible shift. 



Seeds were the part of the plant to disclose how the hidden chemical 

 develops signals. Seed research provided photoperiodic specialists with 

 the leading clue of an experiment in which seed refused to germinate under 

 light rays of 7000 to 8000 A. This suggested that light in the far red part 

 of the red band gave '\vait" signals resembling darkness, worth special 

 investigation. In the early 1950's, the photoperiodic research team was 

 joined by Dr. Eben H. Toole, seed specialist, to begin systematic studies 

 of red, far red, and darkness. 



The reversible action of the plant's growth-triggering chemical was 

 demonstrated strikingly by responses such as those of small lots of lettuce 

 seed. First, 30 seconds of red light and then darkness caused a few of 

 the lettuce seed spread on a wet blotter to sprout in Z days. Prolonging 

 the red light exposure up to 16 minutes induced general sprouting in a 

 lettuce seed lot. Next similar lettuce seeds that had been given enough 

 red to assure readiness to sprout were given brief far red for intervals 

 up to 16 minutes --and this brief far red halted sprouting up to almost 

 100 percent. Then, even naore striking evidence: The seed response 

 was pushed back and forth by alternating red and far- red light, and the 

 last light always ruled, as shown in figure 2. 



Such rapid sequences of light are highly artificial, not duplicated in 

 nature. The importance lay in demonstrating the existence of a reversible 

 mechanism hidden in a plant. Putting it through paces showed what it 

 could do. The paces showed that far- red can do what darkness does, and 

 sometimes nnore. Far red gave signals so potent that they inhibited 



Lot 1 Lot 2 . Lot 3 Lot 4 



Figure 2. --These lettuce seed provided the dramatic and conclusive evidence that the growth-triggering 

 chemical in plants reacts reversibly to red and far-red light. In preceding tests, the scientists in- 

 duced small lots of lettuce seed to sprout by applying red light briefly, and held back sprouting in 

 similar lots by applying far red. The four lots pictured show the striking final test results of apply- 

 ing red and far red alternately: In lot 1, red induced sprouting. In lot 2, red followed by far red pre- 

 vented sprouting. In lot 3, a red, far red, red treatment induced sprouting, in lot 4, a red, far red, 

 red, far red sequence prevented sprouting. The light-sensitive chemical reacted rapidly to each form 

 of light--and the last light always ruled. In these laboratory tests, ail seed lots spread on wet 

 blotters had the same preparatory management of dark storage at 20 C. (68 F.). 



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