Abstract: 【Objective】Tagetes patula belongs to Tagetes genus of the Asteraceae family and it is an extremely popular annual plant for its great commercial and ornamental value. Heterosis breeding is an important approach to breed new cultivars. The heterosis and the genetic effects of horticultural traits of F1 hybrids of tetraploid T. patula were analyzed to establish a heterosis breeding system. 【Method】 Eighteen cross combinations of T. patula were made using six inbred lines K2, K3, K4, K5, K15, and K17 as female parents crossed with three male parents K6, K8, and K13 by following the North Carolina Design II incomplete diallel cross design method, then eleven horticultural traits of first flowering date, full-bloom stage, plant height, crown size, branch number, inflorescence number, pedicel length, diameter of inflorescence, diameter of disk florets, and number of ray florets, number of disk florets were investigated. Heterosis of horticultural traits was calculated using Excel software and combining abilities and heritabilities were analyzed using DPS software to chose primary parents and excellent combinations. 【Result】 The results of heterosis analysis showed that the crown size, number and diameter of inflorescence were improved over best parent and the full-bloom stage, plant height and ray floret number were improved over mid-parent, but the traits of first flowering date, branch number, pedicel length, diameter of disk florets and number of disk florets conferred negative best parent and mid-parent values. Analysis of general combination ability (GCA) showed that the line K6 was chosen as the preferred male parent due to the dwarf, compact and multiple flower traits conferred to the hybrid progeny; the line K13 was judged to be a good male parent for the production of hybrids with early flowering, large and double flower and long ornamental period; the line K3 was the preferred female parent for the production of hybrids of compact plant, early flowering, double flower and long ornamental period; the line K5 was the good female parent for the production of dwarf, compact, early flowering, multiple flower and long ornamental period hybrids; the line K17 was judged to be a good female parent for breeding dwarf, large and double flower hybrids. Analysis of special combination ability (SCA) suggested that SCA effects exhibited significant differences both in the combinations of the same parent and in the traits of the same combination. The crosses K2×K6 and K3×K8 were determined as the excellent combinations for the production of dwarf, multiple and double flower cultivars, exhibiting negative SCA effects in plant height and diameter of disk florets and large positive SCA effects in inflorescence number and number of ray florets. The cross K4×K13 was determined as the excellent combination for the production of compact, early flowering, large and double flower and long ornamental period cultivars, exhibiting negative SCA effects in first flowering date, plant height, crown size and branch number and positive SCA effects in full-bloom stage, diameter of inflorescence, number of ray florets. Analysis of heritability suggested that the traits in first flowering date, full-bloom stage, branch number, diameter of inflorescence, diameter and number of disk florets were primarily controlled by GCA variance and their broad sense heritability and narrow-sense heritability are above the average, whereas the GCA and SCA exhibited insignificant difference in the traits of plant height, inflorescence and ray floret number and the broad sense heritability performed higher, respectively. 【Conclusion】 The F1 hybrids of T. patula have quite distinct heterosis and it was of great value to breed early flowering, large, double and multiple flower, and long ornamental period cultivars. Among the parental lines, K6, K13, K3, K5, and K17 were chosen as primary parents. The crosses K2×K6, K3×K8 and K4×K13 were determined as the excellent combinations. The traits of first flowering date, full-bloom stage, branch number, diameter of inflorescence, diameter and number of disk florets were primarily controlled by additive effects, whereas plant height, inflorescence and ray floret number were mainly controlled by non-additive effects. In addition to first flowering date, horticultural traits were vulnerable to the environment.

Key words: Tagetes patula , incomplete diallel cross , heterosis , combining ability , heritability