Combining ability and heterosis were estimated for ornamental and vegetative traits of four petunia (Petunia hybrida Hort.) inbred lines viz. L5 (P1), L8 (P2), L11 (P3) and L17 (P4) and their diallel hybrids in horticultural research farm of Ferdowsi University of Mashhad, Iran in 2011. The measured traits were plant height at first flower, number of leaf to first flower, flower diameter, flower tube length, internode length, stem diameter, plant height at flowering stage, plant spreading, number of branches per plant, leaf length and width. The results of combining ability for all studied traits revealed that both additive and non-additive gene effects contributed to the inheritance of the traits. Estimates of general combining ability effects showed that parent P3 was a good general combiner for most of the studied traits. For flower diameter, hybrid combination P1 × P2 had the highest significant positive specific combining ability effects. Reciprocal effects were significant for all traits and hybrid combination P1 × P2 had the highest significant positive reciprocal effects for flower tube length and plant height. Heterosis was found significant relative to both the mid parent and batter parent for all traits. For flower diameter, the highest positive values of heterotic effects were recorded in hybrid combination P2 × P3 both relative to the parental mean (37.3%) and relative to the better parent (33.9%). It is obvious that heterotic effects represent an important resource in hybrid breeding of petunia.

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