Five rice lines and three testers were crossed in line x tester mating design and the resultant fifteen hybrids along with their eight parents were evaluated under normal and drought conditions (irrigation every 12 days). This investigation was undertaken at the experimental farm of Rice Research and Training Center (RRTC), Sakha Agricultural Research Station, Kafr El-Sheikh, Egypt during 2019 and 2020 growing season. Genetic diversity, combining ability, heterosis, and antioxidant enzymes were studied. Four primers were used to study genetic diversity. The results revealed a total number of 7 alleles were ranged from one to three alleles per locus, the average of major allele frequency, gene diversity, and (PIC) were 0.607, 0.45, 0.46), respectively. The GD dendrogram revealed the close similarity among the genotypes; IET 1444, GZ1368 and G177 clustered together in the main cluster. On another hand, NP856-9 diverged in sub cluster and the other genotypes G178, G179, A22, and G182 were clustered together. The drought had an intensive inhibition on studied traits, plant height, chlorophyll content, grain yield plant-1 and 1000 grain weight. Otherwise, all genotypes were more earliness under drought than under normal. Highly significant differences were detected among genotypes and their partitions for all studied traits. Both additive and non-additive are important in the inheritance of studied traits. The parents GZ 1368 under normal irrigation and combined data and IET 1444 under stress condition seemed to be the best combiner for grain yield plant-1. The cross NP856-9 X GZ 182 revealed the highest significant and positive SCA effects for chlorophyll content and grain yield/plant. The most desirable midparent and better-parent heterosis for grain yield plant-1 were detected for the crosses IET1444 x G182 and NP856-9 x G182, respectively in the combined data. Results indicated that the activity of antioxidant enzymes: CAT, APX, SOD, and MDA enhanced under drought conditions. Similarly, proline accumulation increased due to water stress.
