Photoperiodism and Light Quality • 35 



form (or form with greater red than far-red absorption) and a 

 far-red-absorbing form. These two forms, call them P R and P F , 

 would be photochemically interconvertible, thus: 



red light 



Pr - v — I p» 



far-red light 



and the final physiological result would then depend on whatever 

 form remained after the last illumination, or on the ratio of the 

 two. 



THE RED, FAR-RED SYSTEM IN PHOTOPERIODISM 



Evidence for the red, far-red reversibility of photoperiodic 

 light-breaks was presented first by Borthwick et al. (1952b), using 

 Xanthium. Following this, Downs (1956) showed that the effects of 

 light-breaks were also far-red reversible in the LDP Hyoscyamus 

 niger and Wintex barley and the SDP Amarantlnis caudatus and 

 Biloxi soybean, and was able to demonstrate repeated reversibility, 

 like that in lettuce seeds, in both Xanthium and soybeans. A more 

 concrete account of some of these results may be illustrative at this 

 point. 



By this time, simpler light sources than the spectrograph had 

 been developed. The red source was simply white fluorescent light 

 (about 1000 foot candles at plant level) with an interposed filter of 

 two sheets of red cellophane. Far-red was obtained by filtering 

 either sunlight (8000 foot candles) or incandescent light (800 foot 

 candles)— both rich in far-red compared to fluorescent light- 

 through two layers each of red and blue cellophane. These cut out 

 almost all radiation of wavelengths shorter than 7000 A but allow 

 far-red to pass. Using these sources, Downs then conducted a more 

 detailed investigation of the time and energy relations of these 

 effects on Xanthium. Groups of plants were given various experi- 

 mental treatments for three 24-hour cycles. They were all then 

 placed under noninductive long-day conditions and allowed to 

 develop for some days, after which the flowering response was 

 scored as an inflorescence-stage index from (vegetative) to 7 

 (maximum response). 



The effect of red light in the middle of each dark period of 

 three successive 12 hours light-] 2 hours dark cycles was propor- 

 tional to the duration of exposure, that is, to total energy given. 



