At the expense of research focused on maximizing other traits such as color, shape and long-lasting blooms after cutting, flowers have been left without much scent at all.
Natalia Dudareva, assistant professor of plant molecular biology in the Department of Horticulture at Purdue University, has found new insight into the biology of floral scents Ð insight that might result in sweeter smelling flowers, plus a bouquet of other benefits.
Dudareva said flower scents have drastically diminished over the years. “I have watched people sniff flowers in a store, unable to smell anything. I remember as a child that flowers were much more fragrant than they are now.” However, data measuring the fading of floral scents over time are lacking, she added.
Making Sense of Floral Scents
Floral scents begin as oils that are produced by the petals in most plants. Because these oils evaporate easily in warm weather, scientists call them volatile compounds. The aroma of a flower may contain as few as seven to 10 different oils, as with snapdragons or petunias, or as many as 100 different chemicals, as with orchids.
Dudareva began her work on floral scents with a visit to snapdragon growers in California.
The snapdragon was selected for the study because it is one of the few modern plants that has a strong floral scent. “Previous scientific work has been done on a California plant called Clarkia breweri, a wild plant, but I wanted to use another plant model to know if similar mechanisms were found in other plant species.”
The growers Dudareva worked with were interested in trying to develop stronger-scented snapdragons. An obstacle for this already difficult task was a typical production practice for snapdragons. “The growers usually cut the flowers when the second and third blooms begin opening (maximum production of volatiles is day five),” she said. “Because of the early cutting practices, many people have the impression that snapdragons do not smell.”
Her study “Developmental Regulation of Methyl Benzoate Biosynthesis and Emission in Snapdragon Flowers” appears in the June issue of Plant Cell. In it, she and colleagues measured the scent of each flower in development. To do so, she put each flower sample in a small bag with two openings. A filter in one of the openings cleaned incoming air. The other opening had an attached column that collected volatile compounds through a pump. The researchers collected volatile compounds for approximately 24 hours and conducted a comparative analysis by measuring the volatiles on a gas chromatograph/mass spectrometer.
Dudareva's study found that emission of the volatile ester methyl benzoate (the most abundant scent compound in snapdragons) is rhythmic, with maximum emission during the day.
Engineering An Aroma
Dudareva and colleagues also discovered that both the moth-pollinated Clarkia and the bee-pollinated snapdragon produce floral scents using similar molecular mechanisms. They found that the amount of substrate used by snapdragons is important to produce fragrant oils and that it is regulated by a separate gene.
“This suggests that there may be basic genetic regulatory mechanisms responsible for floral scent production in different plant species,” Dudareva said. “This is important for applied research. Now we know that floral scent production is regulated in a similar way in different plants and that the substrate is involved in the regulation of scent production. But it will also make the engineering of transgenic plants more complicated because before you genetically modify a plant you have to check to see if the substrate is present in the petals.”
Dudareva says the lack of substrate may be why previous attempts to genetically engineer plants to produce more scent have failed.
Because of this step forward in understanding scent production, will growers soon see a shift toward the breeding of flowers that have stronger scent? Dudareva doesn't believe this will happen for some time.
“Equipment for scent breeding is expensive, and right now the testing methodology used is a lot of work. You have to screen a huge population of flowers,” she said. “If there was a way to measure more quickly, or if we could develop and understand what happened and how to activate or put in new genes, then sure.”
The idea of developing a quick measuring tool teeters on the possibility of “designer genes” for flower scents Ð custom-tailored aromatherapy.
“We could even eventually develop new 'custom' scents for flowers that did not previously exist,” she hinted.
Dudareva is quick to reiterate, however, that this is not something that will happen tomorrow. “Only five genes have been isolated for floral scents. There could be a lot of regulated enzymes involved (that we don't know about). There are many genes involved in the regulation of floral scent.”
Researchers first need to direct the genes that regulate scent to the right places and ensure that plants have substrates for the reaction. If the plant doesn't have the right substrates, it won't know to produce the volatile compound.
“It's not easy. That's why we're trying to better understand scent production. Different species work in different ways,” Dudareva said.
“We are always searching for different scent components and plants that don't produce scent. It's good to see which don't or do attract pollinators by studying different types of plants. We're also comparing different flowering plant species emission times Ð daytime versus nighttime Ð to see if there are similarities in the mechanics of the release times between species.”
According to Dudareva, the floral industry is interested in creating flowers that are fragrant in the evening when people come home from work.
“Petunia is one flower that releases its scent in the evening,” she said. “But again, before we can make flowers more fragrant in the evening, we need to increase the scent overall.”
The Buzz about plant pollinators
Enhancing floral scent also is important to agriculture. Almost three-fourths of all crops depend on insect pollinators attracted by floral scents. Honey bees alone are responsible for pollinating one third of U.S. crops.
That is why boosting floral scents would not only make bedding plants more aesthetically pleasing; it also would improve the yield and quality of many crops. After all, plants not only use floral scents to attract pollinators, but also to repel harmful insects.
“When growers breed plants to have a longer shelf life, the plants tend to conserve energy and not produce volatiles,” she said.
Dudareva's research has found that snapdragons have developed a close relationship with the bumblebee. Snapdragons release four times more scent during the day, when the bees are active, than at night. In fact, 24-hour graphs of scent production by snapdragons and bee activity show bell-shaped curves that line up almost exactly on top of one another.
Dudareva currently is finishing an investigation of why some plants that do not release a scent have genes to produce volatile compounds. She is using the 'Potomac Pink' variety of snapdragon for this study.