Vernalization and Photoperiodism — 72 — A Symposium 



Photoperiodic schedules have been found for barley, variety V/intex C. I. 

 no. 6127, in which a brief period of irradiation applied near the middle 

 of each dark period induced spike formation and stem growth. In the 

 absence of such dark period interruption the plants failed to form spikes 

 and remained in the rosette condition. The photoperiodic treatment used 

 in these experiments consisted of llj^-hour photoperiods and 12j/2-hour 

 dark periods applied in growth chambers (16). 



Both the formation and development of spikes and the elongation of 

 stems were induced by interrupting the dark periods with incandescent 

 filament radiation applied for a period as brief as 30 seconds in the middle 

 of each dark period. The energies required to induce spike formation and 

 stem elongation in barley by such treatments were of the same magnitude 

 as those required to prevent floral initiation in Biloxi soybean and cockle- 

 bur under closely similar experimental conditions. 



Experiments with the spectrograph indicated a spectral region of a high 

 degree of effectiveness in promoting flowering of barley between about 

 5400A and 7200A. The blue end of the spectrum is markedly less effec- 

 tive. Approximately equal flowering response required about 250 fold 

 more energy in the region of 4800A than in the region of 64O0A. Barley 

 is relatively less responsive than soybean and cocklebur to radiation in the 

 blue violet. 



The effects of filtered radiation from various parts of the spectrum 

 on flowering of both long-day and short-day plants have been investigated 

 by a number of workers. Rasumov (19) and Withrow and his co- 

 workers (23, 24, 25) found that red radiation very effectively inhibited the 

 flowering of short-day plants and promoted the flowering of long-day 

 ones when used as a supplement to short photoperiods of natural light. 

 They found blue radiation to be ineffective except in the case of china aster, 

 Callistephus chinensis var. Heart of France, which Withrow and Bene- 

 dict (23) found to respond to all wave lengths. Katunskij (7) and 

 Kleshnin (8) on the other hand found that all parts of the visible spectrum 

 were effective provided sufficient energy was applied. 



FuNKE* (6), working with a large number of plants, found four differ- 

 ent types of response to filtered radiation. In one type, red and white 

 radiation were effective but blue was not, a result similar to the findings 

 of Rasumov and Withrow. In a second type, white, red, and blue were 

 effective, as was found by Kleshnin and Katunskij. In the third type, 

 white was effective but red and blue were not ; and in the fourth, blue and 

 white were effective but red was not. Funke did not suggest any ex- 

 planation for the last two types nor can we, in the absence of more complete 

 information as to energies applied, characteristics of filters used, and more 

 detailed knowledge of the behavior of the plants. 



Lack of agreement of results of these various workers can be attributed 

 to several causes. Wide differences in energies must be employed to dif- 

 ferentiate between effects in different parts of the spectrum. In most 

 instances the failure of a response in the blue part of the spectrum seems 

 to have been the result of inadequate energy. Another reason for lack of 

 agreement seems to have been the fact that the filters used by various 

 workers differed in their light-transmitting characteristics. In general it 



* See also his contribution on page 79 of this volume. 



