THE LIFIvSPAN OF MUTAGENS PRODL'CEl) IN CIJ.LS BY IRRADIATION 



allow the ujas tensions in liic embryos Icj reach an ecjuilibrium with the 

 surroinidins^ niixtiu'c of gases. Post-storage was tested by comparing seed 

 germinated iuunediately after irradiation with seed held in the original 

 phials for a further two weeks before germination. In one set of experiments 

 post-storage was extended to one month after irradiation. As controls stock 

 seed was compared with seed stored for two weeks and four weeks in 0, 20, 

 and 100 per cent mixtures of o.xygen in nitrogen. 



All material was examined by germinating in petri dishes at 23°C and 

 fixing at the peak of the first mitotic divisions in the radicle. Squash prepar- 

 ations were made from seedlings selected at random and scored for number 

 of acentric chromosome fragments per cell. The remaining seed was left 

 in the petri dishes for comparisons of rates of growth. 



In a further preliminary experiment fresh seed and two-year-old seed was 

 irradiated with a dose of 4-8 kr in air and in nitrogen after pre-storage for 

 two weeks. Following irradiation the seed was leached using two cc of 

 water in small closed-system cyclic extractors with an atmosphere of air. 

 Fifty fresh seeds previously irradiated in air with 0-8 kr of X irradiation 

 were placed in the base of each extractor where they inbibed the Icachate. 

 The extractions were carried out for seven days at 3 to 5°C, after which the 

 seeds were germinated at 23°C in the leachate and examined as above. 

 Controls were run in which unirradiated fresh and aged seed was substituted 

 for the corresponding irradiated seed in the leaching column. 



RESULTS 



Ageing of the seed causes an approximately exponential increase in spon- 

 taneous breakage. The percentage of aberrant cells in seed stored in air at 



Equivalent 

 dose of 

 X-rays 

 in air 2 5 kR 



1 



2 3 



Years of storage 



air,20°C.A0Vorh 



Figure 1. Frequency of cells with chromosome aberrations in seed 

 stored in air for various periods, showing the exponential increase in 



damage with age 



room temperature for periods of up to four years is shown in Figure 1. The 

 approximate ecjuivalcnt dose of X irradiation, which must be given to fresh 

 seed to produce the same level of damage as that found in four-year-old seed, 



194 



