Ornamental sunflowers have become increasingly popular as cut flowers the world over (Blacquière et al., 2002; Devecchi, 2005). Despite its popularity as a cut flower, growers of cut sunflowers have long complained about the susceptibility of some cultivars to loss of petals soon after the flowers open. This ruins the appearance of the flowers and destroys their market value. This can happen within a day of the flowers’ opening and the petals flattening out. There has been no systematic study of this problem in literature, although sunflower growers have been actively selecting for cultivars that are less prone to this condition. Many researchers (Evensen et al., 1993a; Fernandez et al., 2000; Lease et al., 2006; McKenzie and Lovell, 1992; Patterson and Bleecker, 2004) have used physical methods such as measurement of break strength to study petal abscission. Break strength is a quantitative measure of the force required to detach an organ from the body of a plant. Past studies on the anatomy of petal drop in other species reveal that the process involves the separation of four to five rows of smaller transversely oriented cells that laid horizontally across the diameter at the juncture between the petal and the achene (Sexton, 1994; Sexton et al., 2000; Sexton and Roberts, 1982). Separation in tulip (Tulipa hybrid cv. Golden Apeldoorn and Tulipa kaufmanniana cv. Shakespeare) tepal abscission happens along a fractured line (Sexton et al., 2000).
The objectives of this study were to define and characterize an abscission zone, if present, at the base of petals of sunflower florets in cultivars that differ with respect to petal drop and to determine if differences in the nature and/or development of the abscission zone among cultivars were correlated with differences in timing with respect to petal drop. The hypotheses were that 1) a specific set of anatomical and microchemical details that might enhance the tendency of sunflowers to drop their petals can be used to characterize an abscission zone; and 2) the anatomical details that characterize the abscission zone will differ among cultivars in a way that correlates with differences in tendency to drop petals.
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