CALDECOTT AND NORTH: RADIO-SENSITIVITY OF SEEDS 381 



ship between seedling height and genetic injury held true when seed- 

 lings in populations of X-rayed seed, which had been subjected to 

 post-irradiation storage, were subdivided into different height classes 

 and analzed for frequencies of interchanges and mutations. Such a 

 study was conducted and the data presented show beyond any reason- 

 able doubt that there is a good correlation between seedling injury 

 and genetic injury (Figures 3 and 4, Table 1). Indeed, they warrant 

 the conclusion that the relationship is one of cause and effect. 



At the present time there is no obvious explanation as to why 

 seeds in a population undergo different degrees of storage injury 

 (Figure 2), although it seems likely that it is associated with either 

 an environmental variable that has not been elucidated, to perme- 

 ability of the cell, or to a different physical state of radiosensitive sites 

 within the nuclei of different seeds. These possibilities will be 

 explored later. 



Basing his arguments on the first root tip cell divisions in the 

 germinating seed, Wolff (29) came to the opposite conclusion to that 

 presented here, viz., that changes in seedling height during storage 

 were not correlated with radiation-induced chromosomal damage. He 

 suggested that, "the pattern of damage can be correlated to the 

 behavior under similar regimes of long-lived radiation-induced radi- 

 cals". Presumably the injury resulting from such radicals was con- 

 sidered to be nonoenetic in nature because it did not become mani- 

 fest in the form of chromosome breaks. It is unlikely that the differ- 

 ence between Wolff's results and those presented here is due to the 

 method of analyzing for genetic injury, because root-tip studies similar 

 to Wolff's that have been conducted in this laboratory confirm the 

 interchange and mutation data that are presented. There must be 

 some other explanation for the difference and this problem should 

 be re-examined. 



Early work demonstrated that the relation between interchange 

 frequency and dose was exponential when Tradescantia microspores 

 were subjected to X-irradiation (22) and linear when they were bom- 

 barded with neutrons (16). From these data it was concluded that, 

 when biological material is treated with neutrons, the dense clusters 

 of ionization that occur usually break two chromosomes simultane- 

 ously which then have a good probability of exchange; thus, the one 

 to one relation between interchange frequency and dose. In contrast 



