Combining ability for grain yield and brown spot disease resistance traits in rice, Oryza sativa L. under saline soil conditions

Combining ability analysis was carried out in rice through A 6 x 6 diallel set analysis excluding reciprocals involving 6 diverse parents for grain yield and brown spot disease infection traits, at Rice Breeding Salinity Program, Sirw Agricultural Research Station, during 2007 and 2008 summer seasons. The ratio of variances due to general combining ability and specific combining ability (ό2 GCA / ό2 SCA) was more than unity, indicating the preponderance of additive genes in controlling, chlorophyll content, plant height, number of days to heading, number of panicles/plant, both percentage and severity of brown spot infection, and grain yield/plant. Moreover, additive x additive type of gene interaction was played an important role in the inheritance of proline content and osmotic pressure. On the contrary, predominance of non-additive gene action chiefly controlled the expression of panicle length; number of filled grains/panicle and 1000-grain weight. Among the parents, Sakha 101 was found to be significantly superior general combiner for all the studied traits except, earliness, chlorophyll content and both percentage and severity of brown spot infection. The genotypes Sakha 102 and Sakha-103 were good general combiners for earliness, chlorophyll content, and both percentage and severity of brown spot infection, and Sakha 104 for panicle length, 1000-grain weight, grain yield, proline content, osmotic pressure and both percentage and severity of brown spot infection, Giza 177 and Giza 178 for earliness. Cross combinations; Giza 177 x Sakha 103, Giza 178 x Sakha 102 and Sakha 103 x Sakha 104 gave high SCAs effects for number of filled grains/panicle and grain yield/plant. Seven hybrids for both percentage and severity of brown spot infection and 3 hybrids for plant height and number of days to heading expressed desired significant negative SCA effects. In addition, Giza 177 x Sakha101 and Giza 178 x Sakha101 were found to be the superior cross combinations for panicle length, 1000-grain weight and chlorophyll content; and Sakha 101 X Sakha 104 for proline content and osmotic pressure. The values of mean degree of dominance (H1/D) . .5 exhibited over dominance with a value above the unity for all, yield and its component traits; and chlorophyll content, while, the other remaining traits were controlled by partial dominance. The ratio of dominant and recessive genes (KD/KR) in the parents was high than unity for all the studied traits except plant height, number of days to 50% heading, panicle length and proline content indicating that dominant genes were more than recessive genes in the parental lines. High estimates of broad sense heritability was recorded for all the traits, it was ranged between 94.4% and 98.7% for osmotic pressure and percentage of brown spot disease infection. In addition, high estimates of heritability in narrow sense were recorded for proline content, osmotic pressure and both percentage and severity brown spot disease infection.