Studies on plant genetic variability induced by combining low and high doses of ionizing radiations
--Romanova, IM

Pollination with pollen exposed to low and high doses of irradiation, the high doses being lethal, is a new technique to develop maize mutants. The present research was done with the aim of studying variability of maize qualitative and quantitative traits in M1-M4. Regularities of quantitative trait variability were studied involving statistically processed measurement data for 16 traits in 100 randomly drawn plants.

The inhibitory effect caused by the mutagenic action of gamma-irradiation of pollen, on quantitative traits, was expressed most in M2. When the pollen of K167 bm2 irradiated with low (5 Gr) and high (500, 800, 1000 Gr) doses was used as donor, followed by additional selfing, statistically significant decreases in all the plant traits studied were observed, except for leaf width and ear diameter. Changes in the traits plant height, leaf number, leaf length, tassel length, kernel number, ear length, kernel weight, and ear weight occurred with all the doses of gamma-irradiation. At the same time, the doses of 5+1000 Gr led to termination of changes in the traits node height of the first ear attachment, developed ears, number of branchlets on the tassel, and kernel height and diameter. The data obtained showed effects on 14 of the 16 quantitative traits studied. The traits of reproductive structures, i.e. the number of kernels per ear, kernel and ear weight, and the length of branchlets in the tassel, showed the greatest modification.

The DNA fragments of the pollen exposed to high doses of gamma-irradiation became mutagens and caused a high rate of mutations. Chlorophyll mutations were produced at doses of 5+500 Gr, 5+800 Gr, and 5+1000 Gr. Higher doses of gamma-irradiation resulted in a marked increase in the frequency of chlorophyll mutations, as well as in subsequent generations with a maximum in M3 and mutation decrease in M4. This was especially characteristic of the percentage of modified plants - the frequency of chlorophyll mutations increased three-fold for the dose of 5+500 Gr, and four-fold for 5+500 Gr, 5+1000 Gr in M3. Chlorophyll mutations are a signal to detect mutations for other genes.

The highest increase in the total frequency of mutations was observed in cases of direct three-fold pollination of the recipient P346 with the donor pollen K167 bm2, irradiated with the dose of 5 Gr, followed by high doses and additional pollination with non-irradiated pollen as opposed to reverse irradiation with high doses, followed by a low one, and additional pollination with the recipient's own pollen.

Direct irradiation with the doses of 5+1000 Gr induced the highest genetic variability, i.e. for four groups of mutations, and reached 66.4% as well as the widest spectrum of variability, i.e. 26 traits, as opposed to reverse irradiation (1000+5 Gr), when the spectrum of variability included 16 traits, and variability reached 17.6 %. Forms with increased number of ears, or huge ears, present a significant interest among the mutations produced. The lemon-yellow line P346 yielded forms with dark yellow, red, light brown, and dark brown kernels.

Pollen exposed to combined low (5 Gr) and high (100-1000 Gr) doses of gamma-irradiation induces peculiar mutagenic activity with various frequencies and with different doses of irradiation. The doses 5+200 Gr, 5+500 Gr produced red kernel mutations changing the kernel coloration due to the pericarp pigmentation.

Hence, combined irradiation of maize generative structures with low and high doses produces specific mutation types characteristic of chemical mutagenesis.


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