الفهرس 
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Abstract
1-Rice (Oryza sativa L.) represents one of the most important food crops all over the world and it is the main source of food for about half of the population in the world (Rao et al., 2019). In Egypt, it represents the second important cereal crop after wheat with a cultivated area of 1.074 million fed. in 2021 season (Including water-saving rice strains such as dry rice and rice grown on relatively high salinity water) produced 7.2 million tons of grain yields/ fed (4.5 million tons of milled rice) (FAOSTAT, 2020).
The successful plant breeding programme depends mainly upon the choice of most appropriate parents for hybridization process. Combining ability is an effective tool for rice breeders since it provides some important genetical information about the studied traits. The line × tester analysis model is widely used to determine general (GCA) and specific combining ability (SCA) effects. General combining ability is a function of additive gene action, while specific combining ability is a function of non-additive genetic variance. Additive genetic variance was reported to be important in the inheritance of rice grain yield and other related traits (Dharwal et al., 2017 and Gowayed et al., 2020). On the other hand, non-additive was reported to be more important in governing earliness and grain yield plant-1 (Fahmi et al., (2018), Raja et al., (2018), Sudeepthi et al., (2018), Panchal et al., (2019), Abo-Yousef et al., (2020) and Hussein (2021). Meanwhile, both types of combining ability were influenced by planting dates for earliness, yield and most of its attributed traits (Okaz et al., 2016).
Determination of the level of genetic diversity in different field crops is of great importance for selecting the most suitable parents and maximum utilization of heterosis (Luo et al.