PHOTOREACTION AND ASSOCIATED CHANGES 433 



indicated by the low value of €r<i>X for the red-absorbing system 

 (Table II). Moreover, many lots of lettuce seed that will germinate in 

 darkness are readily induced to have a light requirement by holding 

 for the order of 24 hr at 35 °C. Despite this responsiveness of the 

 physiological action to temperature changes, the photoreaction can be 

 shown, as for lettuce seed, to have a constant rate independent of 

 temperature. 



RESPONSE TO HIGH RADIANT FLUX 



Experiments on the photoreversible system in which the object is 

 exposed to high radiant flux, such as sunlight, show that the pigment 

 system is not destroyed by a process such as a photooxidation. In the 

 case of internode elongation of the pinto bean, as discussed by Dr. 

 Downs, and of photoperiodic control of flowering of millet, as indi- 

 cated by Dr. Borthwick, the reversible photoresponsive system is pres- 

 ent and operative immediately after the termination of a photoperiod 

 of many hours with high light intensity. Growth features depend upon 

 the relative amounts of red and far-red radiation during, and particu- 

 larly at the close of, the photoperiod. Thus are to be explained the 

 marked relative effects of incandescent and fluorescent sources on 

 plant growth and maturation. 



A newly recognized effect of radiation at high flux density that 

 probably is important to growth of plants in sunlight, as well as to 

 several other photobiological responses, is illustrated by results in Table 

 III. The experiment was designed to test why Lepidium virginicum 

 seeds germinate on exposure to direct or diffuse sunlight when the 

 relatively greater sensitivity to far-red (inhibitory) than to red (promo- 

 tive) radiation indicates that they should be inhibited. The answer was 

 that during 1 sec in full sunlight the sensitivities were changed. By use 

 of a water filter to remove radiation in the general region of wave- 

 length >8000 A, it could be seen that the change was due to such 

 radiation, i.e., heat, even though the rise in temperature was not more 

 than the order of one degree after 8 sec exposure. 



An explanation is that the high radiant flux is supplying activation 

 energy that in the molecular surroundings of the point of absorption is 

 not in equilibrium with the black-body radiation. This effects, to a 



