الفهرس 
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Abstract
Summer squash (Cucurbita pepo L.) is one of the important cucurbitaceous vegetable crops. It is a monoecious and cross pollinated crop provides an ample scope for exploitation of hybrid vigor. Summer squash are the edible young fruit (several days past anthesis). The United States and Mexico are two countries of which C. pepo L. is native which are two of the largest producers of summer squash. Egypt is one of the countries in the Mediterranean and Middle East responsible for one third of the world’s production. Summer squash ranks high in economic importance among vegetable crops. The immature fruits used for culinary purposes in different parts of the world. The term pumpkin was applied to edible Cucurbita fruit that are round or nearly round, whereas zucchini applied to cylindrically shaped fruit. Breeding summer squash was conducted in increasing number of countries among them United States, Mexico, France, Italy, Spain and Egypt. The universal goals of crop breeding, i.e. increased productivity, earliness and disease resistance. Breeding summer squash appeared to be more intensive now than ever before. Large plants typical zucchini hybrid tend to have excessively fast fruit growth which resulted in oversizing of the fruit very quickly. UV irradiation is a method that given variations in plant materials. Irradiation caused ionizing or electric or magnetic disturbances that affect the internal structure of the cells leading to exhibit various physiological and morphological changes used for select the better genotypes to be used in plant breeding. The objective of this study was to induce genetic variations in both varieties of summer squash named Alexandarni and Dyana using ultraviolet irradiation to increase the genetic variations between both varieties as a tool for heterosis. Soaked seeds of both varieties were UV irradiated and then cultivated immediately in the field. Six crosses were done between M₁ plants in the summer season of 2019 to obtain M₁ seeds which planted in the summer season of 2021 to show the hybrid performance in M₂ plants. The results obtained in this study are summarized as follows : Insignificant differences were obtained between different genotypes for all vegetative traits including number of leaves per plant, leaf area, plant height, number of days to first female and first male flower appeared. The genotype extracted from hybridization between P2× M1” ” ” " ~ " ” ” ”P2 promoted the number of leaves per plant if compared with unirradiated parent. The genotypes derived from hybridization between P1× M1” ” ” " ~ " ” ” ”P1 , P1× M1” ” ” " ~ " ” ” ”P2 and P2× M1” ” ” " ~ " ” ” ”P1 promoted the leaf area /plant. The genotype obtained from hybridization between P2× M1” ” ” " ~ " ” ” ”P2 promoted plant height. The reduction shown in some vegetative traits of the plants affected by UV irradiation may resulted from the reduction in the amount of internal growth regulators affected by mutagenic effect by UV radiation. Significant differences were obtained between different genotypes in total chlorophyll pigments formed in leaves and fruits. UV irradiation induced negative effects in chlorophyll formation due to mutagenic effects in the genes related to chlorophyll formation. Total chlorophyll pigments formed in fruits was decreased in the genotypes affected by UV irradiation than the parental wild types. Some hybrids showed intermediate values of total chlorophyll formation in leaves as P1× P2 and P1× M1” ” ” " ~ " ” ” ”P2. In addition, the same trend was also obtained in fruits produced by hybrids P1× P2 and P1× M1” ” ” " ~ " ” ” ”P2 due to negative influences of irradiation on plant health and vitality.