The goal of this work was to find out the influence of stimulating doses on the radiation reaction of maize seeds at later ontogenetic stages, using combined irradiation of maize dormant seeds with stimulating doses.

Significant influence of pre-irradiation of dormant seeds is observed at the earliest stage of plant development, following the initial cell division, expressed in a nearly double decrease in the number of damaged cells. The number of damaged anaphases in the range of 100-300 Gr is reduced if the seeds have been pre-irradiated with a dose of 5 Gr at a 5 min or 24-hour interval between irradiations.

Observations over the growth and development of plants in a greenhouse experiment showed that air-dry kernels irradiated with a dose of 5 Gr initiated their germination two days earlier than the control. The seeds germinated at lower rates, and their emergence and plant height were nearly twice as low by the end of the vegetation period with irradiation at doses of 100 Gr, 200 Gr, 300 Gr in the treatments both with and without preliminary irradiation. This index is characterised by high variability of plants. In the treatments involving pre-irradiation with a 24-hour interval between irradiations (5+100 Gr, 5+200 Gr) the plants were on average 25% higher than in the controls (0+100 Gr, 0+200 Gr), as well as in the 5-min pre-treatment (5+100 Gr, 5+200 Gr), but the difference was not statistically significant.

The dose of 300 Gr, 5+300 Gr was lethal for the intervals of 5 min and 24 hr. The time interval between irradiations with stimulating and mutating doses was established to have significant importance. Thus, it was shown that the pre-irradiation of seeds with the stimulating dose of 5 Gr followed by the mutating dose after 5 min caused a sensibilizing effect, and in the case of additional irradiation after 24 hr with the same dose a distinct radioprotecting effect was induced. The combined irradiation of seeds with 5+100 Gr and 5+200 Gr gave rise to mutation forms with a changed growth pattern and even dwarf mutations. 

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