Breeding rice for abiotic stress tolerance is very important objective for rice breeders. Significantly reduce in yield due to drought-stress in most world countries, especially suffering from water lace as well as Egypt. Evaluating combining ability effects would provide valuable information that can be used for developing new drought tolerant lines. This study was conducted at Sakha Agricultural Research Station during 2014 to 2016 rice growing seasons to determine combining ability and gene action for grain yield and other important traits to identify the superior varieties/parents and crosses under both drought and optimum conditions. Six F 1 hybrids were generated by crossing four rice genotypes in a half diallel mating scheme in addition to their six F 2 populations after self-pollination. The four populations (P 1 , P 2 , F 1 and F 2 ) for all the six crosses were evaluated in a replicated field trial under drought and normal conditions. Drought stress significantly decreased the mean values of all the studied traits for parents and their hybrids compared with the control condition. Significant differences were observed among parents and their hybrids for most traits under both research conditions. General combining ability (GCA) and specific combining ability (SCA) means squares were highly significant for grain yield and most traits under test environments. The non-additive gene action played an important role in the inheritance of most studied traits. The parental genotypes NERICA3 and NERICA4 appeared to be the best general combiners for earliness and grain yield. The cross combinations; Giza178×NERICA3; Giza178×NERICA4 and NERICA3×NERICA4 in both F 1 and their F 2 generations were distinguished as specific combiners for grain yield and other related traits under normal and drought conditions. These results emphasized by positive heterosis over high parents followed by high values of inbreeding depression. Hence, these hybrids would be valuable in rice breeding for improving grain yield under drought stress condition.

A Study on combining ability and heterosis were conducted on 15 F1 hybrids along with eight rice genotypes (five lines and three testers) to understand the pattern of inheritance of yield, its components and some root traits for selecting superior genotypes. The experiment was carried out using line x tester mating design, during 2010-2011 rice growing seasons at the experimental farm of Rice Research and Training Center, Sakha, Kafr El-Sheikh, Egypt. Analysis of variance revealed significant differences among genotypes, lines, testers and line x tester interactions for days to 50% heading, plant height, panicle length and weight, number of filled grains/panicle, number of panicles/plant, number of roots/plant, root length and volume and grain yield/plant traits under water stress conditions, indicated that the genotypes had wide genetic diversity for the studied traits. Significance of the means of sum squares due to lines and testers indicated a prevalence of additive variance. However, significant differences due to interactions of line x tester for the above mentioned traits, indicating the importance of both additive and non-additive variance in the inheritance of these traits. The mean squares due to GCA as well as SCA were significant for all studied traits except 100-grain weight and root/shoot ratio. Thus, the significance of GCA (variances due to lines and testers) and SCA (variances due to lines x testers) implied that both additive and non-additive types of variation was available for all the traits, yet additive genes were more important than the dominant genes because variance due to GCA was higher than that of SCA for all mentioned traits. Moreover, the ratio of GCA and SCA variances was greater than unity for all the traits studied that revealed the preponderance of additive gene action over the nonadditive gene action. IET 1444, Morobrekan and Giza 178 rice genotypes were the best general combiners due to highly significant GCA effects for number of roots/plant, root volume and root length traits. Moreover, Wab 450-JR-4-1-1 as tolerant to water stress conditions was the best general combiner for panicle weight, number of filled grains/panicle, otherwise, Sakha 102, rice genotype was the poorest combiner comparing with other rice genotypes for all the studied traits, except days to 50% heading. The crosses, Sakha 105 x IET 1444, Sakha 103 x Sakha 102 and Morobrekan x Wab 450-JR-4-1-1 were found to be the best cross combinations for most of the studied traits; these crosses had high and significant SCA in desirable direction under water stress conditions. Highly significant and positive estimates of heterosis and heterobeltiosis were recorded in Morobrekan x IET 1444, Sakha 105 x IET 1444 and Sakha 104 x Sakha 102 for number of panicles/plant, panicle length and number of roots and their volume/plant, respectively. The highest estimated values of phenotypic (Vph) and genotypic (Vg) variances were observed for number of filled grains/panicle, root number/plant and root volume indicating better scope...

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