To Smell a Rose…

This May, will you be celebrating Mom with a fragrant bunch of plant genitals?

by Jenny Morber

On warm nights after a day spent in the thrill and tedium of running a household, caretaking children, and trying to maintain a career, I like to go for a run. After a good day, I turn left as the sidewalk crests the hill leading from my home, up a longer slope that will take me past houses whose occupants I know – men and women, children, dogs barking in back yards, cats on the prowl. On good days I take a chance at seeing someone I recognize, someone to wave to and maybe share some words.

On difficult days I turn right. A right turn takes me down to a bridge over a shady creek and then back up again through more houses. Suburbia has been so maligned that it feels in the mouth like a dirty word, but I find great beauty in the lush greenness of where I live. I enjoy chance encounters with wildlife and notice small changes in the houses spaced closely enough to feel connected but not so close that I cringe for them when the baby wakes screaming in the middle of the night.

I am running, yes, to clear my head and feel my heart beat, but when I turn right, I am seeking neither company nor solitude. I am seeking scent. The scent of petals and pistils and stamens – the plant genitalia humans curiously gift to one another to show sentiment. I cross the bridge, push uphill for another quarter mile, and I am there. A plain blue house sits overlooking a rather miraculous suburban garden. I admire the lavender, sunflowers, hostas, and ferns, but I am there for the roses. Rows and rows of fragrant roses bloom in peach, deep red, white, pink, and yellow. And each one smells different.

Photo by Bev Lloyd-Roberts LRPS

Photo by Bev Lloyd-Roberts LRPS

Because I stop to smell them, I knew that each variety carries a different scent. What I didn’t know until recently was that, incredibly, there are so many factors that influence my perception of that scent that with each sniff I am creating a unique experience unlikely to be repeated by myself or anyone else, ever. Rose fragrance is like fine wine. It really is a shame that we humans can’t smell better.

To start, there are thousands of varieties of fragrance-producing roses. And despite recent breeding programs for cut roses that nearly eliminated scent in store-produced varieties, rose fragrance continues to be developed and prized by the perfume industry, an appreciation that dates back at least to the time of the Ancient Egyptians, and likely much earlier.

Scientists have identified more than 500 compounds that roses release to produce fragrance, with each individual releasing its own unique combination. Darker and deeper colored roses tend to produce more of the chemicals typically associated with the sweet scent of ‘rose.’ Orange and yellow varieties produce more citrus-smelling compounds. Longer carbon chains smell different from shorter chains, and the structure and addition of other atoms, including oxygen, nitrogen, and – in the case of putrid-smelling plants – sulfur matter as well.

Roses release their scent molecules from the petal’s epidermal cells – both the top and second layers, researchers have found. And despite furious activity over the last twenty years or so to figure it out, we are still largely in the dark regarding the genetics, biochemistry, and mechanics of how exactly it all happens.

photo by Carlos Koblischek

photo by Carlos Koblischek

Part of the reason we still struggle to understand these mechanisms is that scent production is so complex. Sylvie Baudino, who runs a research team studying how roses make scent at a university in France, continues to be surprised by “the diversity of enzymes and pathways, and the fact that a same molecule can be made very differently by different plants,” she says. “The plasticity of the biochemical pathways is also amazing: an enzymatic step can be easily gained and lost, by mutation.” There is not one simple way to make one simple molecule, so tracing both the genetic history of fragrance and how roses create it today are frustratingly difficult. “If we start to know better how plants make their scent, then maybe we will be able to understand why some cultivars (for example, the roses bred for the cut flower market) have lost their scent. I think this is an interesting question,” Baudino says.

Cutting even fragrant flowers can be problematic. If I were to secretly snip one of my neighbor’s roses to hoard the scent for myself, the stem would smell very different from its living counterpart. As described by an article in a 1990 edition of Pure and Applied Chemistry, “when the umbilical cord connecting the fruit or flower to the plant is severed, the excised plant part may then be thought of as being, in effect, biologically dead, and the aroma changes perceptibly.” A cut flower is but an olfactory ghost of the living specimen.

And that’s not even the half of it. The combination and volume of a rose’s scent compounds also vary by time of day (more fragrant in the morning), weather (stronger when it is sunny and just before a storm), the age of the bloom (the bud and the open flower smell different, and scent dissipates with age), climate, soil, and the time of year.  Much like us, roses react to cycles of light and darkness as well as time of day, releasing scent according to a complex biological rhythm, almost as if the plant wakes, exhales its fragrance for a while, and then goes to sleep. (The floral industry is currently working to create more flowers that release scent at the end of the day, when many people return from work.)

Then there is the variable of me. Even if I took a friend to smell the same exact flower at the same exact time, differences between our olfactory receptors, how our brains process them, and the memories we associate with scent would create an entirely different experience. Our eyes add another layer of subjectivity since studies show that our sense of smell is colored by, well, color.

photo by Bart Hickman

photo by Bart Hickman

But why do we care about these smells anyway? Presumably, humans are not a rose’s preferred pollinator. Some researchers have posited that humans used to be important enough pollinators for fruits, eating them outside and then defecating the seeds. Humans needed the food, and the plants needed humans, so goes the thinking, so as we evolved, we maintained our appreciation for the scents of ripe fruit or the plants continued to produce the scents that we enjoy, or both. Flower scent, these researchers argue, contains many of the same aromatic compounds found emanating from fruit, making the allure of flowers a happy coincidence. Still, if this were true, wouldn’t raspberry and papaya be our favorite smells, rather than the long-reigning rose and jasmine?

Other scientists argue that floral associations are learned. Children, after all, are equal opportunity samplers, enjoying the novelty of thyme, grass, coffee, and feces equally at first. (Mine especially enjoys the scent of slugs.) But over time, they learn the smells of things that are supposed to be pleasant, and those supposed to be icky, and they internalize these lessons. Studies on other cultures seem to back this idea up. Humans’ attraction to and dislike of smells vary across time and culture. People become immune to noxious smells and culturally adopt and abandon others. To some, a fragrant runny cheese is gag-inducing. To others, it signals a delightful lunch. The infamously putrid smell of durian fruit utterly delights many who grew up eating it.

Our favorite scents, as perfumers well know, tend to be those that evoke pleasant memories. If your mother was a source of safety and happiness, and she wore scents of lemon and jasmine, it’s a good bet that you enjoy those fragrances too.

But what about my quest to find fragrance at the end of a bad day? Is it simply that I am looking for an escape and a pleasant experience, or do the roses actually act on my neurochemistry? The science underlying aromatherapy lags behind its popularity, but evidence is mounting that we may have underestimated the importance of our sniffer. A 2007 double-blind study on 56 male volunteers found that lemon improved the subjects’ self-reported measures of mood compared to water. Lavender, however, did not show these effects, and neither scent did much to alleviate pain.

The study on lemon appears to support an earlier finding that another citrus scent – this time orange – reduces anxiety in female patients awaiting dental procedures. And while mood can be subjective, researchers continue to assert that scent and disease should be linked in both treatment and diagnosis.   Two researchers in a 2006 article in Autoimmunity Reviews suggest: “There may be something unique about the olfactory system that is inextricably related to immunological function. In addition, accumulating evidence confirms the existence of olfactory dysfunction in brain disease, much of which appears at early stages including multiple sclerosis, Alzheimer’s disease, Parkinson’s disease, schizophrenia, and depression.”

photo by Andrea Kratzenberg

photo by Andrea Kratzenberg

It’s not just our noses that do the smelling. The cells that line our gut, kidneys, heart, blood vessels, and more seem to contain olfactory receptors. These receptors respond to scents released by our own resident gut bacteria to help regulate blood pressure. And just last year, researchers announced that a synthetic sandalwood fragrance encourages wound healing in human skin cells.

Though there is little evidence that those roses on my route – rather than just the exercise – make me feel better at the end of a bad day, I don’t think I am too foolish for believing so. Now knowing how unique and variable each scent is, and the incredible combination of nature and nurture that work to create it, I appreciate those moments even more. And the next time I feel like giving someone a bouquet of flowers, maybe instead of presenting a dying scentless bouquet, I will consider a fragrant, potentially health-boosting live specimen instead.

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Tara Haelle

About Tara Haelle

Tara Haelle, health editor of DoubleXScience, is a photographer, former high school teacher, current adjunct journalism professor (Bradley University), aspiring children's book writer, avid scuba diver, former triathlete, sometimes yogi, and eternally curious journalist who primarily specializes in health and science reporting. She was once a world traveler, eating strange insects, climbing ancient ruins and swimming with sharks, but that was before she became a mom (though she knows those days beckon again soon). She also blogs about health and science for parents at Red Wine & Apple Sauce and is a senior editor of mental health at dailyRx News. She is most passionate about reporting on vaccines, marine biology, mental health, parenting and prenatal and children's health, but she also dreams of a day when she can revamp the entire U.S. educational system to improve reading instruction and science literacy.