Petal Drop in Sunflowers: Varietal Differences
The sunflower is an attractive ornamental plant that continues to gain popularity in the cut flower industry. Despite its popularity as a cut flower, a growing complaint among sunflower growers is that some of the varieties used as cut flowers easily lose their petals, which ruins their appearance and destroys their market value. Most of the dark-colored varieties like ‘Procut Bicolor’, ‘Chianti’ and ‘Moulin Rouge’ look attractive in bouquets, but they are hardly grown because of the petal abscission problem. They are delicate and hard to handle, and even when properly handled, may still lose their petals within one day of the flower opening and the petals flattening out. There has been no systematic study of this problem although sunflower breeders have been actively selecting for lines that are less susceptible to this disorder.
In the summer of 2006, we began to look for ways we can characterize and measure the problem of early petal drop in some sunflowers, and perhaps find ways to avoid it. If we can find ways of effectively reducing the petal abscission problem and increase vase life of some of the dark-colored varieties, then we might be able to persuade consumers to buy more of these varieties and increase sunflower sales overall.
Our results from previous experiments enabled us to classify sunflower varieties into categories based on their susceptibility to petal abscission. We used a petal break strength meter to measure petal drop. It records the force (in newtons) that it takes to detach a petal from the receptacle of a flower. Flowers were harvested when they just opened, within 2 hours of petals unfolding. Flowers were laid on the weighing platform of a scale, weighed down with an 8.5 lb weight, and an alligator clip attached to a petal. The clip was attached to a drill press working in reverse (arbor press). As the clip with the petal attached was pulled slowly up, the scale, attached to a computer, recorded the force exerted as the petal separated from the head. Four petals were pulled on opposite sides of the flower, and the average force per flower head was then calculated. In this trial, 17 varieties were measured and each variety had 5 replicates.
A statistical analysis of petal detachment force of 17 sunflower varieties enabled us to classify sunflower varieties into 3 major groups. The susceptible varieties with pulling force ranging from 0.90 to 1.27 newtons include ‘Strawberry Blonde’, ‘Moulin Rouge’, ‘Cherry Rose’ and ‘Procut Bicolor’. The moderately susceptible varieties with pulling force ranging from 1.37 to 1.91 newtons include ‘Procut Peach Blush’, ‘Procut White Lite’, ‘Sun4U Bicolor’, ‘Procut Red Lemon Bicolor’, ‘Orange King’, ‘Procut Apricot Lite’ and ‘Sunrich Orange’. The resistant varieties with pulling force ranging from 2.04 to 2.45 newtons include ‘Procut Early Orange’, ‘Premier Lemon’, ‘Procut Yellow Lite’), ‘Procut Yellow’ and ‘Procut Lemon’. Most of the susceptible varieties however, had dark-colored petals.
The petal break strength meter provides an accurate, reproducible way of measuring the susceptibility to petal drop. A faster way is just to brush against the flower head near the base of the petal, but this method is easily biased. Given the differences we observed in petal drop tendency among varieties, is there a relationship between that characteristic and vase life? We will be testing that relationship in experiments this summer. Also underway is a search for chemicals that could slow petal loss of susceptible varieties, either as a vase solution or a dip. Stay turned!