Twenty-one hybrids developed from crossing seven lines and three testers were assessed for the extent of combining ability effects, heterosis and genetic parameters for yield and related contributing traits. The analysis of variance uncovered that there were highly significant differences for all the studied traits except panicle weight among parents and parents vs. crosses. The two promising lines GZ 9461-4-2-3-1 and GZ 1368-S-5-4 were seen as a good general combiner among lines for most of the traits, while among the testers, the parental variety Sakha S 300 was superior. Two crosses out of hybrids, GZ 9461-4-2-3-1 / Milyang 349 and GZ 1368-S-5-4 / IR14K642 exhibited positive and highly significant SCA effects for grain yield plant -1 , respectively. The maximum positive and highly significant mid-parent heterosis and heterobeltiosis for grain yield plant -1 were observed. The five crosses GZ 1368-S-5-4 / Sakha S 300, GZ 1368-S-5-4 / IR14K642, GZ 9461-4-2-3-1 / Milyang 349, GZ 9461-4-2-3-1 / Sakha S 300 and GZ 9461-4-2-3-1 / IR14K642 recorded the highest values and maybe extensively used in rice programs by adopting a heterosis breeding strategy. The additive gene action was noticed for days to 50% flowering, plant height, flag leaf area, number of panicles plant -1 , panicle length, panicle weight, panicle density index, spikelets fertility, 1000-grain weight, and grain yield plant -1 . Grain yield plant -1 was highly significantly and positively correlated with plant height, flag leaf area, number of panicles plant -1 , panicle weight and panicle density index indicating the significance of these traits as determination criteria in yield enhancement.

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.

An understanding the nature and relative extent of gene actions and combining ability is useful for a breeder to develop superior rice hybrids. In this association, an attempt was made to identify good general and specific combiners for selecting better parents and better cross combinations for developing high yielding and short duration rice hybrids. Four cytoplasmic male sterile lines viz., IR69625A, IR70368A, IR58025A and IR79575A were crossed with ten elite testers, 40 hybrids along with their fourteen parents were evaluated for grain yield and its related traits at the experimental farm of Rice Research and Training Center, Sakha, Kafr El-Sheikh, Egypt, during the rice-growing seasons of 2018 and 2019. The differences among genotypes, parents and crosses were highly significant for all studied traits. The specific combining ability (SCA) played the major role in determining inheritance traits, revealing that the largest part of the total genetic variability associated with most traits was a result of non-additive gene action. The parental line, IR58025A was demonstrated as a great general combiner for grain yield per plant and most of yield components and therefore could be utilized in future breeding programs to improve grain yield. HHZ12-SAL8-Y1-SAL1, HHZ5-Y7-Y2-SUB1 and HUA564 were seen as a good general combiner among testers for grain yield per plant and some studied traits. Four crosses, namely IR58025A / HHZ8-SAL9-DT2-Y2, IR70368A / WEED TOLERANT RICE1, IR70368A / HHZ12-SAL8-Y1-SAL1 and IR70368A / HHZ5-Y7-Y2-SUB1 exhibited significantly good specific combining ability effects for grain yield per plant. Hence, it can be seen that non-additive genetic effects are controlling the inheritance of the studied traits and indicated that selection for the traits inherited with this manner should be performed in the further generations.

Salinity is a major abiotic constraint faced by farmers in many rice-growing regions of the world and improving grain yield in rice is the most important breeding objective. Thirty-five crosses were generated in a line x tester mating design by crossing seven lines with five testers and were evaluated in a randomized complete block design at the greenhouse lysimeter of Rice Research Department, Sakha, Kafr El-Sheikh, Egypt, during the two rice-growing seasons of May, 2021 and 2022. Analysis of variance revealed a highly significant difference among testers and lines for all the studied traits. Variances of specific combining ability were higher in magnitude than the corresponding general combining ability. The lines Sakha 107, Sakha 104, and Giza 182 including testers SAL 010, and CSR 28 were the best general combiners for grain yield plant -1 under saline conditions. The variety Giza 179 followed by Sakha 107 was the best general combiner for 100-grain weight under both normal and saline conditions. Two crosses Giza 179/MTU 1010 and Sakha 107/SAL 010 exhibited highly significant positive estimates of specific combining effects for grain yield plant -1 , respectively. Estimates of narrow sense heritability for all the traits under study were low which indicated a preponderance of non-additive gene action governing these traits. By selecting the appropriate genotypes and phenotypes, the outcomes will be beneficial in breeding salt-tolerant cultivars at both the seedling and reproductive stages.

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