Gamma-ray-induced ESR signals in lettuce: Evidence for seed-hydration-resistant and -sensitive free radicals

Abstract

The effects of wetting and redrying lettuce seeds on γ-radiation-induced electron-spin-resonance (ESR) signals have been studied. Seeds were wetted and then dried at room temperature over a desiccant, followed by equilibration in an atmosphere until the water content for both the wetted-redried seeds and their unwetted controls was approximately 6-7 per cent. When γ-irradiated seeds were completely wetted and redried, most, but not all, of the radiation-induced signal was eliminated. The radiation-induced signal that persisted after wetting and redrying was approximately 8 per cent of the total radiation-induced signal in unwetted seeds. The derivative curve of the signal from un-irradiated seeds, both before and after wetting and redrying, has a peak-to-peak line width of 6·5 G. The corresponding line width of the radiation-induced signal for doses less than 1 Mrad was not detectably different from the endogenous signal width. At very high doses (5-20 Mrad), the radiation-induced, water-resistant signal was from 4 to 12 per cent wider and the radiation-induced signal in unwetted seeds was from 9 to 20 per cent wider. No difference in g values (2·006-2·007) were detected resulting either from irradiation or from wetting and redrying. Studies of power saturation, however, did indicate a difference between the radiation-induced resonances that persisted after wetting and redrying vs. the radiation-induced resonances that were present in unwetted controls. These results indicate a difference in the molecular environment of such radiation-induced unpaired electrons that persist after hydration vs. the molecular environment of radiation-induced unpaired electrons in unwetted seeds. Reirradiation of seeds that had been wetted and redried after receiving 8 Mrad was as effective in inducing resonances as was irradiation of control seeds that had not been irradiated before having been wetted and redried. This indicates that the radiation-induced resonances that persist after hydration and redrying do not influence the subsequent induction of resonances by radiation. This last finding also suggests that the radiation-induced signal that disappears on seed hydration can be reversibly eliminated by hydration and reinduced by reirradiation. © 1967.