Photoperiodism and Light Quality 



33 



substantially alike; the most effective wavelengths are in the 

 orange-red range, 6000-6800, with a maximum at 6400-6600 and 

 a steep drop beyond 6800 A. Blue light is much less effective and 

 green is almost completely ineffective. Such results indicated that 



Fig. 3-2. Effects of various amounts of light given as dark-period interruptions 

 on inflorescence primordium development in the LDP barley (Hordeum vulgare 

 var. Wintex). Three-week-old plants were grown for 9 days with 12 H-hour 

 dark periods interrupted in the middle with various energies of light, then 

 allowed to grow for 19 days with uninterrupted dark periods. These dissections 

 show the apices greatly magnified ; that at the far right was about 3 mm high. 

 Relative energies used for the night interruptions ranged from none (extreme 

 left) through 25 (middle) to 400 (extreme right) foot-candle minutes of white 

 light. The study of similarly graded responses to various energies at various 

 wavelengths indicated the effectiveness of the wavelengths tested. (Photograph 

 from Borthwick, Hendricks, and Parker [1948], Bot. Gaz., 110: 103-1 18, courtesy 

 of Dr. H. A. Borthwick, U. S. Department of Agriculture.) 



light-breaks inhibiting the flowering of SDP were probably ab- 

 sorbed by the same pigment as those promoting flowering in LDP. 

 The nature of the pigment remained a subject of speculation since 

 no known pigment in higher plants had an absorption spectrum 

 with a peak only in the red region. Further information came from 

 outside photoperiodism proper, and it is therefore necessary to 

 digress. 



