RADIATION DAMAGE TO CHROMOSOMES 203 



Table 1. Typical results showiyuj the sjpiergistic action of ultra-violet 



and X-radiation 



Dose Cells scored Isoeliromatic 



(7o) 



Expt. 1-9 E 

 X-ray 

 u.v. 

 X-ray-u.v. 



Expt. 1-7 E 

 X-ray 

 u.v. 

 X-ray-u.v. 



(1960). These investigators have observed that it is possible to modify 

 X-ray induced chromosomal aberrations by pre- and post-treatment 

 with monochromatic u.v. radiation. The most extensive observations 

 in this field have been made with X-rays in the dose range 50 to 150 r 

 and ultraviolet exposure between 50,000 and 200,000 ergs/cm2. The 

 technique is to irradiate pollen under carefully controlled conditions 

 of humidity and examine chromosomal aberrations at metaphase in the 

 post-meiotic mitosis in pollen tube cultures. The magnitude of the 

 effect depends in a complex manner on the time intervals between 

 exposure to the different radiations as well as on the time between the 

 end of the irradiation treatment and sowing of the pollen. The time 

 interval between irradiation by X-rays and u.v. radiation can be as 

 long as two or more hours with no very extensive decrease of the effect. 

 For pre-treatment with u.v. radiation, the magnitude of the synergism 

 increases as the interval between radiation treatments is extended to 

 60 min and then decreases to its initial value as the interval is further 

 increased to 120 min. In the latest work of this group it was observed 

 that irradiation of the pollen with as little as 0-5 r of X-rays followed 

 by approximately 10,000-20,000 ergs/cm^ of ultraviolet results in a 

 definite synergistic effect. In this work, which is just being developed, 

 the wavelength dependence of u.v. necessary to produce the effect is 

 being determined as well as the dependence of the synergistic effect on 

 the linear energy transfer (LET) of the ionizing radiation used. More 

 recent studies also show that the magnitude of the effect is also de- 

 pendent on the amount of solar radiation the pollen receives prior to 

 the u.v. and X-ray treatments given in the laboratory. The potentiation 

 of the synergism thus appears to be a photobiological phenomenon. 

 This investigation reminds me very much of earlier experiments com- 

 bining infra-red with X-rays. Initiated some years ago in co-operation 



