<br/>The parameters used were survival of irradiated leaves of <em>S. ionantha</em> , production of adventitious plantlets at the base of the petiole and mutation frequency.<p/>The differences between unfiltered X-rays from a 250/25 deep therapy apparatus and fast neutrons from a degraded fissions spectrum was slight and sometimes negligible. A striking dose rate effect was observed, especially with X-rays. In fractionation experiments a protective mechanism was induced in the leaves by pre-irradiation with either type of radiation. This mechanism reached a maximum after 8-12 h. Then the extent of the protection was equivalent to 3 krad X-rays or 1.5 krad fast neutrons after a single pre-treatment with the optimum initial dose of 500 rad, X-rays or fast neutrons. A 10-15 fold irradiation with this dose, separated by 8-h intervals, induced a protection of 6-7 krad X-rays or 3-4 krad fast neutrons.<p/>Several of the treatments result in a higher number of mutants per 100 irradiated leaves compared with the optimum acute dose of 3 krad X-rays or 1.5 krad fast neutrons.
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