Dry seeds of Arabidopsis thaliana were exposed to accelerated4 He, <tex-math>${}^{7}{\rm Li}$</tex-math>,12 C,16 O, <tex-math>${}^{20}{\rm Ne}$</tex-math>, and <tex-math>${}^{40}{\rm Ar}$</tex-math> ions from the Berkeley heavy-ion linear accelerator (HILAC). The small size and simple geometry of the seed embryo make it possible to locate the Bragg peak for the ions within the stem meristem region. For reference, two low linear energy transfer (LET) radiations were used:4 He ions at full energy (LET = 18 KeV/μ) and 250 kVp x-rays (<tex-math>${\rm LET}_{{\rm avg}}=2\ {\rm KeV}/\mu$</tex-math>). The track average LET within the meristem for the ions ranged from 72 to 1890 KeV/μ (4 He to <tex-math>${}^{40}{\rm Ar}$</tex-math>). Biological end points scored were tumor induction, inhibition of plant dry weight increase, and somatic mutation induction. Cross-section analysis of the data indicates that sensitive sites for the three effects studied are multiple in number or complex in structure with effective thicknesses of a few Å to a few tens of Å. The relative biological effectiveness (RBE) compared with x-rays, for each criterion of response, showed the same pattern of variation throughout the range in LET, namely, a low RBE at 18 KeV/μ, a maximum at 72-174 KeV/μ, and a gradual decrease from 409 to 1890 KeV/μ. The highest RBE values were around 30. The results are consistent with the conclusion that the primary sites of damage for the different responses are the same-probably DNA.

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