Marianne Ophardt

Garden Tips: The science of petunia genetics

Creating pure petunia lines and then crossbreading them can create plants uniform in color, size and habit.
Creating pure petunia lines and then crossbreading them can create plants uniform in color, size and habit. Courtesy Pixabay

I broke with tradition this year and did not praise the best and brightest of the new petunias. However, I am still in awe of how well today’s petunias, especially my favorite Wave petunias, perform in our summer heat.

Petunias, like the Waves and the Supertunias, are F1 hybrids that have resulted from painstaking, tried-and-true plant breeding techniques, not from gene modification.

What is an F1 hybrid? Breeders select a petunia that has a characteristic that they would like to have in a new variety. They then grow this petunia and repeatedly cross it with itself, taking the seed from the parents and re-sowing them after each cross until they get a “pure line” of inbred plants with the desired characteristic. The plants in this pure line are genetically uniform and all exactly alike. The breeders keep this “pure line” isolated from all other petunias so it will not be “contaminated” with other petunia genes. They also create other pure inbred lines with different desirable characteristics.

The next step is crossing two pure inbred lines in hopes that the offspring will be uniform and have both of the selected characteristics. The offspring are called F1 (first filial) hybrids. Of course, the combination of the two does not always result in an improved new variety. Also, there are usually more than just two characteristics that breeders want in a new variety, requiring additional breeding efforts.

The F1 hybrids produced by inbreeding and the crossing of two pure lines are very uniform in plant habit, size, and color. They also typically have increased vigor and higher yields than their parents. When you buy the same F1 hybrid flower or vegetable variety from one year to the next, you can depend on it being the same. That is because the same pure parent lines are maintained to produce the seed for that specific F1 hybrid variety. However, if you save the seeds from F1 hybrids and plant them, most of the offspring will be quite different than their F1 parents because the genes have become mixed.

Native petunia species were first discovered in South America by Spanish explorers. They did not perceive them as useful or pretty, so they did not send specimens back to Spain. However, later explorers did collect different petunia species and sent them home to Europe. In the late 1800s, breeders worked on crossing different petunia species to get petunias with larger flowers of different colors. Their efforts resulted in petunias that were called Petunia x hybrida. However, technically this was only a simple cross between two species, not a true hybrid.

The first true single-flowered multiflora F1 hybrid petunia, ‘Silver Medal,’ was introduced in 1949 by Charles Weddle. It was followed in 1952 by ‘Ballerina,’ an F1 hybrid grandiflora petunia. The superior garden performance of F1 hybrid petunias meant the downfall of the earlier “hybrids.” In 1983, the Ball Seed Company set a petunia milestone by introducing floribunda petunias and then PanAmerican Seed introduced the first spreading petunia, ‘Purple Wave,’ in 1995. If you want to try some of those older “hybrid” petunias, check out Baker Creek Heirloom Seeds (rareseeds.com).

This year I decided not to plant flowers in some of my patio containers, but the containers had a different idea. Once spring arrived, I noticed tiny volunteer petunia seedlings growing in the pots. I decided to let them grow and see what would happen.

Serendipity has ensued! My “empty” pots are now filled with a riotous mix of pretty pink, fuschia, purple, and white petunias. They are not carbon copies of their F1 hybrid parents that grew there last year. Chances are they will not have the superior performance of their parents, but I am having fun enjoying the beauty produced by petunia genetics.

Marianne C. Ophardt is a retired horticulturist for Washington State University Benton County Extension.

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