Abstract: 【Objective】 The combining ability of parents and heterosis of agronomic and quality traits in two-line hybrid flax were analyzed to provide theoretical basis for a two-line flax crossbreeding program.【Method】 A incomplete diallel cross using eighteen flax genotypes including seven thermo-sensitive male sterile lines and eleven restorer lines with 77 (7×11) hybrid combinations was carried out to estimate effects of the general combining ability (GCA), special combining ability (SCA) and heterosis for fourteen main agronomic and quality traits in two-line hybrid flax. Data analysis was processed with DPS (Data Processing System). 【Result】Analysis of variances for the studied traits indicated highly significant differences among the genotypes. The general combining ability (GCA) effects was significant rather than specific combining ability (SCA) for most traits, including plant height, technical length, branches per plant, stems per plant, seeds per capsule, 1000-seed weight, oil content, oleic acid, linoleic acid, stearic acid and palmitic acid. The heredity of these traits was mainly controlled by additive genes. Both GCA and SCA effects were significant for some traits, including capsules per plant, yield per plant, and linolenic acid. The heredity of these three traits was controlled by additive and non-additive genes, of which the additive genes played a dominant role. The narrow-sense heritability of agronomic and quality traits was ranged as follows: technical length>1000-seed weight>plant height>seeds per capsule>branches per plant >yield per plant>stems per plant>capsules per plant, and oil content>linolenic acid>stearic acid>palmitic acid>linoleic acid> oleic acid. The general combining ability (GCA) and special combining ability (SCA) have greater differences among different traits with the same parent/cross or among different parents/crosses with the same trait. Meanwhile, there is no direct relationship between GCA and SCA. Among eighteen parents, the male sterile line 113S, 16-1-1-2, 24-1-1-1, T-11, 1S and restorer line Longya10, Tianye9, Lunxuan3, Macbeth, AC Watson were good parents with better GCA effects on some agronomic and quality traits. The heterosis over male parent and over control cultivar were widely observed in seventy-seven hybrid combinations. The correlation analysis revealed that heterosis of main agronomic and quality traits in two-line hybrid flax was significantly related to general combining ability (GCA) and specific combining ability (SCA). Both or one of parents have high GCA or have high SCA in strong heterosis combinations. Meanwhile, the parental GCA was significantly greater than SCA, and the GCA in restorer lines was significantly greater than in male sterile lines.【Conclusion】Heterosis in two-lines hybrid flax was closely related to the general combining ability (GCA) and special combining ability (SCA). Selection for restorer lines with higher general combining ability (GCA) was the key of parent selection in two-line flax cross breeding program.

Key words: flax, two-line hybrid, general combining ability, specific combining ability, heterosis