LIGHT AND THE PIGMENT 



119 



5 10 



MICR0WATTS/cm2 



5 20 25 



INCANDESCENT ENERGY 



0-1 0-2 0-3 0-4 



MICROWATT S/cm2 "req" LIGHT 



Figure 7-8 



The effects upon subsequent flowering of low intensity light applied 

 continuously during a 16-hr "dark" period. Experiment initiated on 

 April 26, 1962, by James Whitmore; light intensities measured by 

 Charles Curtis with an Eppley circular 8 junction bismuth-silver thermo- 

 pile (with and without a Schott interference filter transmitting only in 

 the red region between 641 and 669 millimicrons with a maximum at 

 655) — both of the Advanced Plant Physiology Class. Data previously 

 unpublished. 



reversal. The results with cocklebur are shown in Fig. 7-9. Complete 

 reversal of red induced lettuce seed germination is relatively easy to 

 demonstrate for a number of hours, but in cocklebur and soybean 

 the ability to reverse a red effect with far-red is rapidly lost, so that by 

 the end of 30 min no reversal with far-red can be observed. 



More recently, H. A. Borthwick and H. M. Cathey (19), also at 

 Beltsville, have performed related experiments with chrysanthemum, 

 a short-day plant. They found that in order for a continuous light 

 interruption to be effective in the middle of an inductive dark period, 

 it had to be about 4 hr long. If a number of flashes were given, 

 however, the total amount of light required for inhibition proved to 

 be much less. Light flashes given every few minutes for 4 hr were as 

 effective as continuous light given for 4 hr. 



