80 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



Modification of X-ray damage by visible radiant energy. — The 

 damaging effects of X-rays and other forms of ionizing radiation to 

 living cells are due chiefly to the breaking of the chromosomes and 

 interference with normal cell division. Young, rapidly dividing cells 

 are most susceptible to X-ray damage and evidence three types of 

 aberrations — chromatid break, isochromatid break, and chromatid 

 exchange. 



It has previously been reported that far-red radiant energy, when 

 used as a treatment supplementary to X-rays, increases the frequency 

 of chromatid aberrations. This year, Dr. C. C. Moh and Dr. R. B. 

 Withrow have extended the study on the interaction between red and 

 far-red radiant energy at the level of the cell nucleus. 



Root tips of broad bean (Vicia faba) were pretreated with a 3-hour 

 exposure of red (620-680 m/x) and/or far-red and near infrared (710- 

 960 m/t) radiant energy, and then irradiated with 120 roentgens of 

 X-rays. As compared with the control (X-rays only), root tips 

 receiving far-red treatment yielded 30 to 40 percent more chromatid 

 breaks and chromatid exchanges. Those receiving far-red followed 

 by red energy showed no appreciable increase in aberrations. These 

 results indicate that red radiant energy inactivates the action of the 

 far-red exposure. In a second series of experiments, root tips were 

 irradiated with red energy for 3 hours, followed by a 3-hour exposure 

 to far-red energy. As compared to the control, the red plus far-red 

 treatment resulted in an increase of chromatid breaks and chromatid 

 exchanges amounting to about 20 percent. It would appear that 

 application of far-red energy after the red treatment could not com- 

 pletely overcome the inactivating effect of the red. In a third series, 

 root tips were exposed for 3 hours to red and far-red energy simul- 

 taneously by using a broad waveband from 620 to 960 nut. No 

 increase in any type of chromatid aberration was found. 



PUBLICATIONS 



Klein, W. H., Withkow, R. B., and Elstad, V. The response of the hypocotyl 



hook of bean seedlings to radiant energy and other factors. Plant Physiol., 



vol. 31, pp. 289-294, 1956. 

 Klein, W. EL, Withrow, R. B., Elstad, V., and Peice, L. Photocontrol of growth 



and pigment synthesis in the bean seedling as related to irradiance and 



wavelength. Amer. Journ. Bot., vol. 44, pp. 15-19, 1957. 

 Klein, W. H., Withrow, R. B., Withrow, Alice P., and Elstad, V. Time course 



of far-red inactivation of photomorphogenesis. Science, vol. 125, pp. 1146- 



1147, 1957. 

 Withrow, Alice P., and Wolff, J. B. Succinate oxidation by mitochondrial 



preparations from bean seedlings. Physiologia Plantarum, vol. 9, pp. 339-343, 



1956. 

 Withrow, R. B. An interference-filter monochromator system for the irradiation 



of biological material. Plant Physiol., vol. 32, pp. 355-360, 1957. 



