In ornamental plants, flower architecture is an important feature and double-flowered varieties are particularly highly valued by the horticultural industry (Scovel et al., 1998). Double-flowered varieties have been characterized anatomically and genetically in several plant species, including Chamelaucium ucinatum and Nicotiana alata (McComb et al., 1996; Zainol and Stimar, 2001; Zainol et al., 1998). Petunia is a profuse flowering annual with large, richly colored flowers and exists as a diverse array of varieties (Griesbach, 2007). As the most popular petunia varieties, double-flowered petunias had been well studied from an anatomical angle, but the genetic background of the double-flower form in petunia has not been investigated (Natarella and Sink, 1971; Sink, 1973; Vanderkrol and Chua,1993). Certainly, there have also been no studies in petunia regarding the relationship of ploidy with flower form (Meyerowitz et al., 1989). Currently, breeding for new varieties of double-flowered petunia is impeded by the fact that crosses between many single-flower and double-flower cultivars rarely produce double-flowered progeny. Furthermore, few lines reliably produce 100% double flowers and, in general, vegetative propagation is the most successful method of multiplying double-flowered plants of Petunia. Also, semidouble flowers appear to be a rarity within Petunia hybrida varieties. Hence, the genetic basis of the double-flower phenotype in petunia is poorly understood and this is a problem for breeding programs aimed at this important characteristic.
Polyploidy is a prominent and significant force in plant evolution (Adams and Wendel, 2005) and has played a major role in the evolution of flowering plants (Masterson, 1994; Otto and Whitton, 2000). The manipulation of ploidy can also be a valuable tool for plant breeding programs. Polyploids often generate variants that may contain useful characteristics and provide a wider germplasm base for breeding studies (Ramanna and Jacobsen, 2003; Thao et al., 2003). Since the discovery by Blakeslee and Avery (1937) of the effectiveness of colchicine for the achievement of chromosome doubling in plants, colchicine has been successfully used to obtain polyploid plants of many agricultural and ornamental crops, e.g., Phlox subulata L. (Zhang et al., 2008), Zizyphus jujuba Mill. ‘Zhanhua’ (Gu et al., 2005), hop (Roy et al., 2001), and bread wheat (Soriano et al., 2007). Similarly, Power and Sink (1978) produced triploid and tetraploid lines in petunia and documented the relationship of ploidy with various phenotypic characters, but because the study was conducted with single-flowered lines only, this did not provide information regarding the double-flower trait.
Kermani et al. (2003) conducted a study in Rosa, which indicated that chromosome doubling was positively reflected in the extent of flower complexity. However, there is otherwise little in the literature regarding the relationship between ploidy level and petaline type as relating to single- or double-flower architecture.
In this article, we describe the creation of a ploidy series based on double-flowered lines of Petunia hybrida. The various ploidy levels were generated through the colchicine-mediated induction of polyploid plants and cross-hybridization between double-flower tetraploid and single-flower diploid plants. We describe the major phenotypic characteristics among the various polyploid lines, including flower form. From this analysis, we note that an unusually high frequency of semidouble-flowered plants was generated from crosses involving a polyploid double-flower plant and a diploid single-flower Petunia cultivar and investigate the relationship between ploidy level and petaline type.
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