34 • Photoperiodism: Attempts at Analysis 



THE RED, FAR-RED REVERSIBLE SYSTEM 



It had been known for a long time, in a general way, that the 

 germination of many seeds was affected by light. Flint and Mc- 

 Alister (1935, 1937) had found that the germination of lettuce, 

 Lactuca sativa, was promoted by red light. If seeds previously ex- 

 posed to enough red to cause subsequent germination were exposed 

 to either blue or near-infrared (7000-8000 A) light, germination was 

 inhibited. This work was taken up again by the Beltsville group 

 (Borthwick et al., 1952a, 1954). They determined an action spec- 

 trum for the promotion by red, which showed maximum activity 

 at about 6500 A and resembled the light-break action spectra, and 

 also an action spectrum for the infrared (now called far-red) inhibi- 

 tion, which showed a maximum around 7350 A. More important, 

 however, were observations leading them to postulate the existence 

 of what is now known as the red, far-red reversible pigment system. 



Some data taken from the 1954 paper illustrate what is meant 

 by red, far-red reversibility. Groups of lettuce seeds were allowed 

 to imbibe water in darkness at 20° C for three hours, subjected to 

 various brief light treatments, then kept in darkness at 20° C for 

 two days, after which the number germinating in each lot was 

 counted. The light treatments were either 1 minute of red (R) or 

 4 minutes of far-red (F) at previously established intensities, or 

 combinations of these in immediate succession. In typical results, 

 treatment R alone caused 70 percent germination, and the treat- 

 ment RF (red followed immediately by far-red) caused 7 percent, 

 almost the same as germination in darkness. Such alternations 

 could be carried much further: the treatment RF, RF, RF, R gave 

 81 percent, and the treatment RF, RF, RF, RF, 7 percent again. 

 The germination depended simply on whether R or F was given 

 last, as if a switch were thrown one way or the other by the 

 different radiations. Any red effect was reversed by far-red given 

 immediately after, and vice versa. Similar results could be obtained 

 even when the seeds were chilled to 6° C during the period of light 

 treatments. This temperature-independence and a number of other 

 observations led to the suggestion that the two opposed light 

 effects might be mediated by the same pigment. The basic assump- 

 tion is that the pigment can exist in two forms, a red-absorbing 



