sparrow: cytogenetic effects of ionizing radiations 95 



of the radiation response. Dose rate, dose fractionation (see Section 

 VI, 4), linear ion density (see below), and temperature are probably 

 the most important of these variables, but several others also can 

 cause a significant modification under appropriate circumstances 

 (Table 6). For instance, small doses of ultraviolet in combination 

 with 1 rad of X-rays can give a yield of aberrations roughly equal to 

 that normally obtained from 100 rads of X-rays alone (85). Similar 

 but much less dramatic synergistic effects have been known for some 

 time (see references in Table 6). It is difficult to make generalized 



Table 6. — Physical Factors Affecting the Radiosensitivity of Plants.* 

 Factor References 



A. Dose fractionation 21,39,49,93,125,137,199,202 



B. Dose rate 21,39,86,93,125,137,183,199 



C. Linear ion density 21,26,57,88,93,125,202 



D. Previous exposure to ionizing radiation 202 



E. Exposure to other radiations 



1. Ultraviolet 183(page 382) 



2. Infrared 49(page 739),202(page 374) 



3. Visible light 160,202(page 358) 



F. Exposure to ultrasonic energy 125, 137 (page 19) 



G. Bioelectrical potential 160 



H. Temperature 49(page 741),202(page 360) 



1. Centrifugation 137 (page 19) 



J. Pressure, hydrostatic 160 



K. Phase state 202(page 353) 



*After Gunckel and Sparrow (62). The references in this table are held to a minimum and are in 

 many cases to review papers rather than to the original research. References to many original papers 

 can be found in the reviews cited or in the bibliography by Sparrow, Binnington, and Pond (160). 



statements concerning these factors. Other conditions of the experi- 

 ment and the effect studied will determine, in part, the degree and 

 direction of modification. The articles referred to in Table 6 contain 

 extensive discussions of the various physical factors. 



2. Ionization density, LET and RBE 



It is well known that different radiations or the same radiation 

 at different energies behave differently with respect to the average 

 number of ionizations produced per micron of tissue traversed. (See 

 Figure 14 and Table 2.) Since each ionization is estimated to require 

 32.5 ev, it is apparent that the denser the ionizations along the track, 

 the higher the rate of energy transfer from the ionizing particles to 



