Estimates of Combining Ability for some Quantitative Traits During Different Sowing Dates in Rice

Half diallel crosses among six genotypes of rice (Oryza sativa L.) as well  as  their  parents were  cultivated  in  a  randomized  complete  block  design  to  investigate  general  combining  ability  (GCA)  effects  of  parents and  specific  combining ability  (SCA)  effects  of  crosses during different sowing dates. Analysis of variance in diallel analysis exhibited highly significant (P < 0.01) for most studied traits at the four sowing dates. The mean squares due to GCA and SCA were also highly significant for all studied traits during the four sowing dates, indicated that the importance  of  the both  additive  and  dominance  gene  actions  for  these  traits. Variances due to  GCA  were  greater  than  these  due  to  SCA  for  days to heading, panicle length and panicle weight traits during the second, third and fourth sowing dates and for grain yield/plant during the first and second sowing dates. However, the values of SCA variances were higher than the values of GCA variances for the other studied traits through the other studied sowing dates. Results showed significant differences among all the crosses means and their respective parental values for all examined traits at the four sowing dates. The most of parents and their F₁ crosses were displayed significant or highly significant GCA and SCA effects either negative for days to heading or positive for the other studied traits at the four sowing dates. Concerning  the four sowing dates, the variety Giza 179 was the best general combiner for days to heading, grain yield/plant, number of panicles/plant and 1000-grain weight traits while, the genotype CT 9882 was the best general combiner for panicle length and panicle weight traits. The best specific combinations during the four sowing dates were the cross Giza 178  x CT 9882 for days to heading and number of panicles/plant traits, the crosses Giza 178 x Sakha 105, Giza 178  x Sakha 106 and Giza 178  x Giza 179 for grain yield/plant and 1000-grain weight traits and the cross Giza 179 x CT 9882 for panicle weight and panicle length traits. Cluster analysis based on all studied traits resulted into two clusters from genotypes through all sowing dates in rice. The first cluster included the two exotic genotypes (CT9882 and CT 9506). The second cluster containing four genotypes, further divided into two sub clusters. Sub cluster-I containing the most earlier rice genotypes (Sakha105 and Sakha106). The Egyptian indica rice varieties (Giza178 and Giza179) were grouped together. The results of cluster analysis suggested that there is genetic diversity among the six genotypes for all studied traits. Hybridization among these genotypes in the two groups provided more possibility to having more genetic diversity and could be used in breeding programs to achieve maximum heterosis as well as earliness and yield improvement in rice. Generally, the parents involved in the previous combinations and the best crosses could be use in initiated the breeding program for growing at early sowing dates (first and second sowing dates) of rice crop in the north of Egypt.