EXPANSION OF THE GENOTYPIC VARIABILITY IN WATERMELON BY PHYSICAL MUTAGENESIS

Studies have been conducted on 18 promising watermelon genotypes to expand the genotypic variability of watermelon by induced mutagenesis. Air-dried seeds were irradiated with a closed Co γ-source «Doslidnyk» (Department of Molecular and Medical Biophysics, Faculty of Radiophysics, Biomedical Electronics and Computer Systems, V.N. Karazin Kharkiv National University of MES of Ukraine). The patterns of influence of the irradiation doses on plant growth and development have been determined, both in individual genotypes and for the whole sample. Sources of economically valuable traits have been identified.

Results and discussion. The breeding accessions were evaluated for their morphological and biological characteristics. Careful screening of the breeding accessions for the morphological features of plants and fruits revealed morphobiological changes in plants and their organs. The results show that some plants were affected by the mutagen, and the exposed specimens had various morphological changes.
At the initial stage of morphogenesis -vining -plants with an increased number of nodes were seen. During the stem formation, the stem fasciation followed by stem bifurcation was observed, In the seed setting phase, plants with shortened internodes and paired arrangement of primordiums in alternate nodes, as well as 2 primordiums per node, were noted. A spontaneous mutation -fringed leaves in line 46 -was also noted. No severe morphoses in fruit color were observed during the genotype screening in the biological maturity phase.
The most informative economic useful features in breeding for increased levels of economically valuable traits are yield, marketability, average weight of marketable fruit, the growing period length, order of the 1 st female flower formation, the "seedlings -to female flowering" period, the "female flowering -maturation" period, and the stem length.
Each of these features was statistically analyzed for its expression patterns and levels in each of the 18 genotypes, depending on the variants of exposure to the mutagen (different doses ofirradiation -150 Gy, 200 Gy and 250 Gy) . The analysis revealed the different genotypes had different responses to the mutagenic factor. The patterns in influence of some irradiation doses on the plant growth and development have been established both for individual genotypes and for the entire sample.
The average number of clusters is the most informative for all the traits. We have determined that their most appropriate number was 5, as clear patterns were observed with 5 clusters.
Based on the results, the sample of the lines and variants was divided into separate groups (clusters) according to the expression levels of economically valuable features.
The figures illustrate the effect of gamma-irradiation on the expression of a particular feature with grouping (clusterization) of the accessions in comparison with the control. The clusterizattion of the breeding accessions by yield compared to the control is shown in Figure 1. The genotype clusterization by marketability is shown in Figure 2. Cluster 2 included the greatest number of genotypes (38.9%), which gave slightly increased yields after 150 Gy, slightly decreased yields after 200 Gy in comparison with the control, and showed a slight upward trend in the yield after 250

Fig. 2 Clusterization of the genotypes by marketability
The gradual increase in marketability after 150 Gy to 250 Gy in cluster 3 genotypes (K 107350/222 and K 107348/226 [11.1%]) compared to the control is useful for breeding practice.
The genotype clusterization by the average weight of marketable fruit is shown in Figure 3. The clusterization of the genotypes by the "seedlings -maturation" period length is shown in Figure 4.   Figure 6 shows the clusterization of the genotypes by the "seedlings -female flowering" period length. As to this feature, the genotype clusterization was as follows: clusters 2 (22.2%), 3 (22.2%), 5 (22.2%) and 4 (27.8%) contained the greatest numbers of genotypes; all of them had more days from mass germination to female flowering than in the control, which extended their growing periods. The highest level of this feature was observed in cluster 1 genotype (K SESHar/262) after 200 Gy, but the growing period in this genotype was the shortest. There was also a slight reduction in this period in cluster 4 genotypes (K 107348/226, K 107612/230, K 105468/234, K 105415/290, K 104930/294) after exposures 2 and 4.
As to the "female flowering-maturation" period, the genotype clusterization was as follows: The stem length was measured 2 twice during the growing period. The first measurement was made during the female flowering phase of the genotype population in order to determine the effect of irradiation during the initial grow on the plant growth and development.
The genotype clusterization by the stem length during the female flowering phase was assessed. Clusters 1 (27.8%), 3 (27.8%) and 4 (22.2%) included the greatest numbers of genotypes. All the genotypes of these groups had inhibited (to a various degree) growth and development during this phase. The growth in cluster 2 genotype K 107346/298 was drastically inhibited after 200 Gy, however, almost normal after 250 Gy related to the control. Therefore, 200 Gy irradiation of genotype K 107346/298 is effective in breeding for short stems. A significant elongation of the stem in cluster 5 genotypes irradiated at 200 Gy is interesting for breeding practice. These are genotypes K 107350/222, K 107348/226, and K 107612/230. At the same time, the clusterization of the genotypes by the stem length at the end of the growing period was as follows: clusters 1 (27.8%), 3 (27.8%) and 4 (22.2%) included the greatest numbers of genotypes. All the genotypes of these groups had inhibited (to a various degree) growth and development during this phase.
The possibility of cluster 1 genotype K 106806/246 -based breeding for short stems with gamma irradiation at 150, 200, and 250 Gy was evaluated for breeding practice. In breeding for long stems, it is advisable to use doses of 200 Gy and 250 Gy on cluster 3 genotypes K 107350/222, K 107352/270, and K 104930/294, as well as doses of 150, 200, and especially 250 Gy on cluster 5 genotypes K SEL2/238, K 104945/250, K SEMak/258, K 104937/274, K 107342/278, K 105206/286, and K 105415/290. Genotype K 107350/222 was proven to be a possible source of a long stem, since the espression of this feature was stable both in the female flowering phase and at the end of the growing period.

Conclusions.
The study results show that -irradiation had a depressing effect on the majority of genotypes (late maturation, long or short stems, altered order of the 1 st female flower formation, extended phases of the growing period), however, as we noted above, both individual genotypes and separate groups (clusters) of them, in which expression of traits is opposite (alternative), have been identified. Due to this, the peculiarities of the trait expression were revealed; the patterns of practical use of gamma irradiation in breeding programs were defined to expand the genotypic variability range; and M 1-3 accessions were selected for use in breeding to improve breeding features.
Basing on the experimental data, we recommend using the identified sources of economically valuable features in breeding with subsequent evaluating the trait expression stability in the following generations: -