CONTROL BY LIGHT 215 



breaks, as these brief exposures lia\ e been ealled. Since only short 

 exposures and relatively low energies are required ior effective liglit- 

 breaks, it became possible to obtain reasonal)ly accurate action 

 spectra for at least one of the light reactions in photoperiodisni. 



This work, done niainh' at the U.S. Department of Agriculture at 

 Belts ville bv a group including Borthwick, Hendricks, and Parker as 

 well as others, showed that red light in the region of 6400-6600 A 

 was the most effective for light-breaks, with no evidence of any other 

 important peaks in the action spectrum. 



A major advance took place in 1952, when the same group con- 

 firmed and extended observations made over 15 years earlier l)y 

 Flint and McAlister on the germination of lettuce seeds. Under 

 certain conditions, these require a brief light treatment in order to 

 germinate after being returned to darkness. Red light in the photo- 

 periodically effective region is also most active here. However, if 

 a brief treatment with the region around 7300 A— now generally 

 referred to as far-red— is given right after the red, germination is 

 prevented; if red light is given again, following the far-red, germi- 

 nation will take place. In short, if the seeds are alternately exposed 

 to red and far-red light, they will germinate or not, depending on 

 whether red or far-red came last. 



Red, far-red reversibility could be demonstrated equally well 

 even when the seeds were chilled to near 0° C during the irradi- 

 ation period, although, of course, thev had to be brought back to a 

 favorable temperature for observation of the germination response. 

 These data suggested that reversibility was a purely photochemical 

 matter; one could thus infer the existence of a pigment that re- 

 versibly changed its absorption spectrum, depending on the wave- 

 length absorbed last, and of which the two forms had different 

 physiological activities. Within the next few years the Beltsville 

 group, as well as others, was able to show that this red, far-red re- 

 versible photoreaction could control not only seed germination and 

 the light-break effect in photoperiodisni but also many other plant 

 processes too numerous to discuss here. This pigment system is 

 probably ubiquitous among higher plants at least, and these days 

 the plant physiologist suspects its presence wherever low energies 

 of red light appear to have an effect (for all the preceding, see, e.g., 

 Borthwick et al, 1956; Withrow, 1959b). 



The postulated reversibility of the pigment's absorption charac- 

 teristics seemed to provide a useful handle for its isolation and iden- 



