The advantage of using the tobacco (Nicotiana tabacum var. xanthi) mutagenicity 
assay is the ability to analyze and compare on the same plants under identical 
treatment conditions both the induced acute DNA damage in somatic cells as 
measured by the Comet assay and the yield of induced leaf somatic mutations. 
Gamma-irradiation of tobacco seedlings induced a dose-dependent increase in 
somatic mutations from 0.5 (control) to 240 per leaf (10Gy). The increased yield 
of somatic mutations was highly correlated (r = 0.996) with the increased DNA 
damage measured by the Comet assay immediately after irradiation. With increased 
dose of gamma-irradiation, the averaged median tail moment values ( +/- S.E.) 
significantly increased from 1.08 +/- 0.10 (control) to 20.26 +/- 1.61 microm 
(10Gy). Nuclei isolated from leaves 24h after irradiation expressed tail moment 
values that were not significantly different from the control (2.08 +/- 0.11). 
Thus a complete repair of DNA damage induced by gamma-irradiation and measurable 
by the Comet assay was observed, whereas the yield of somatic mutations 
increased in relation to the radiation dose. Data on the kinetics of DNA repair 
and of DNA damage induced by gamma-radiation on isolated tobacco nuclei, and on 
nuclei isolated from irradiated leaves and roots are presented.