The most complete pattern of the action of ionizing irradiation during the whole period of plant ontogeny can be obtained in field experiments, however, they do not allow the investigation of the effect of g-radiation and postradiation treatments at the initial, most important stages of plant development.
Postradiation treatment with stimulating factors is aimed at activating the repair system, which must increase the survival ability through somatic protection of initial cells maintaining the critical weight. In the course of postradiation regeneration, biologically active substances contribute to the recovery of tissues, including meristems that might serve as a test-system for estimating the degree of repair process activation. Compensation of phytohormonal insufficiency induced by radiation should also be defined in the action of exogenous biologically active substances.
Laboratory experiments were carried out to study the possibility of eliminating the damaging action of g-radiation during radiation mutagenesis in two types of maize: 1. a radiation sensitive line � 19-3-3 ( S. T. Chalyk's breeding ); 2. radiation resistant hybrid Moldavskiy 401 ( bred at the Research Institute "Porumbeni" ). Air dried seeds were exposed to a sublethal g-radiation dose followed by a treatment with biologically active substances "Crossing" and "Phytostim". Also, seeds were exposed to electrical field (EF) currents at industrial frequency and super-high frequency (SHF) for postradiation treatments.
The table summarizes the measurement data on the length of maize roots and coleoptiles at 8 days after the g-radiation and postradiation treatment. Inhibition and termination of root and coleoptile growth were observed to occur in the treatments under study, in the treated control to a larger extent, accompanied by practically complete absence of meristem cell division. The data prove that the joint action on seeds leads to a partial elimination of the damaging action of g-radiation. The difference is most reliable in the treatments with "Crossing" and SHF, even at the significance level of 1%. In these treatments, resumption of root growth due to regeneration of the initial meristem was observed at 7�8 days. Part of the plants survived due to regeneration of secondary meristem. Similar phenomena, but less contrasting, were observed in the experiment involving hybrid seeds where the degree of seed hybridity was effective. The postradiation treatment with the chemical stimulator "Crossing" appeared to be the most effective.
The process observed in the postradiation regeneration is supposed to be as follows: Regeneration from the secondary meristems resulting from the redifferentiation of particular cells, as well as the currently carried out cytological studies, indicated an increase in repopulation in the group of initial cells, i.e., the exogenous biologically active factors contributed to the regeneration of damaged cells, accounting for the reduction of their elimination.
Table 1. The influence of postradiation treatments on the growth of
maize seedlings.
| TREATMENT | ROOTLETS | COLEOPTILES | ||
| x±Sx, sm | % to the control | x±Sx, sm | % to the control | |
| Control | 8.1±0.53 | 100 | 2.9±0.27 | 100 |
| Radiated control | 4.5±0.66** | 55.6(100) | 1.8±0.19** | 62.1(100) |
| Radiation + Crossing | 7.0±0.61** | 86.4(+55.6) | 2.2±0.13 | 75.9(+22.2) |
| Radiation + Phytostim | 6.7±0.64* | 82.7(+48.8) | 1.9±0.15 | 65.5(+5.5) |
| Control | 7.4±1.36 | 100 | 3.7±0.51 | 100 |
| Radiated control | 3.9±0.51* | 52.7(100) | 3.8±0.57 | 102.7(100) |
| Radiation + EF | 6.0±0.73* | 81.1(+53.8) | 3.9±0.41 | 105.4(+2.6) |
| Radiation + SHF | 7.9±0.72** | 106.7(+102.5) | 4.8±0.54 | 129.7(+26.3) |
The difference with control is significant, * - p<0.05 , ** - p<0.01.
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