Petal Drop In Sunflowers: Varietal Differences And Possible Remedies
A complaint among sunflower growers is that the petals are easily knocked off the flower, which ruins its appearance and destroys its market value. Varietal susceptibility to petal drop among different sunflower genotypes, grouped into the red, bicolor, orange and yellow colored varieties, was measured using a novel technique. Mean break strength of varieties in the yellow group were higher and significantly different (p<0.05) from the orange and bicolor groups, which were in turn higher and significantly different from varieties in the red group. Mean vase life (12 days) of sunflower varieties in the yellow group was longer than the orange group (10 days), which in turn were longer and significantly different (p<0.05) from the bicolor group (9 days), and significantly different from the red group (8 days). There was no relationship between head diameter, petal length and abscission tendency in sunflowers. It was also observed that petal drop in all the sunflower varieties was ethylene insensitive. Three types of phytohormones (cytokinins, ABA and auxins) were present in abscission zone tissues of 5 sunflower genotypes, Procut Yellow Lite (PYL: yellow group), Procut Lemon (PL: yellow group), Strawberry Blonde (SB: red group). Procut Bicolor (PBC: bicolor group), and Moulin Rouge (MR: red group). Two groups of cytokinins were detected: the zeatin + zeatin riboside (Z + ZR) group and the iso- pentenyl adenosine (iPA) group. The levels of 'Z+ZR' in PYL and PBC were significantly higher (p<0.05) than in PL, SB, and MR, while levels of 'iPA' in PYL, PL and PBC were significantly higher than in SB and MR. It was also observed that PYL and PBC were significantly high in cytokinins but low in ABA. There were significantly low (p<0.05) levels of auxin in sunflower abscission zone tissues within 1 hour of flower opening in all genotypes. The genotypes with the shorter vase life (PBC, SB and MR) had the highest levels of ABA in their petals while those with the longer vase life (PYL and PL) had the smallest amounts of ABA in their petals. Petal detachment forces were significantly higher (p<0.05) when dipped in cytokinins (BAP-10 and Fascination- BA+GA4+7) than in the controls. At higher concentrations of up to 300 ppm, detachment forces decreased in Fascination treated plants more than in BAP-10 treated plants. When sunflower heads were dipped for 10 minutes in different concentrations of BAP-10 and BA+GA4+7 and kept for 14 days, abscission was delayed and vase life was extended by 4 and 6 days, respectively. Anatomical analysis revealed a differentiated region (the abscission zone) at the junction of the petal and achene consisting of cells with a different morphology from those above and below it. Cell division at the abscission zone of the short- lived variety occurred earlier a n d fa s t e r than that in the long-lived variety. These differences indicate that whereas the anatomical and cellular nature of the abscission zone is similar in the two lines (PYL and PBC), the tempo of development differs. Specifically, the abscission layer reaches full differentiation, or maturity, sooner in line PBC, hence its earlier petal drop, than in line PYL.
cut sunflowers; Petal drop; Varietal susceptibility
Wien, Hans Christian
Zitter, Thomas A; Pell, Alice N
Ph. D., Horticultural Biology
Doctor of Philosophy
dissertation or thesis