The garden smells we love are plant protectors

By Lauren Lewis

If you visit the San Francisco Botanical Garden on a hot day like we’ve been having recently, your nose may well have a more interesting experience than it would on a normal foggy day. That’s because most plants are constantly sending out odiferous volatile compounds (essentially chemicals), and warm air allows those volatiles to move around more and intensify. Some are even synthesized specifically to protect a plant from heat damage, so a hot day triggers greater release of those volatiles than a cool day. Our noses know many leaf volatiles, particularly from herbs like sage, rosemary, basil, and we certainly know the volatiles that give flowers their sweet scents, but plant volatiles have complexity that goes far beyond our olfactory system.

img_5022.jpgA simple but accurate way of thinking about plant volatiles is that they help solve a plant’s challenge of being stationary, and therefore limited in its ability to escape dangers or attract assistance. Plants release these self-made chemical compounds into the surrounding environment for a huge number of benefits to the plant. Some volatiles provide direct defense for the plant, like the isoprenes that help an oak quickly return to a normal photosynthetic rate after exposure to high heat. Or the volatiles released by a leaf that is being eaten, that temporarily deter herbivores from continuing to eat the leaf. These are essentially plant-produced pest deterrents.

Other volatiles allow for more complex forms of protection for the plant. Some allow a plant to summon another species: a leaf being chewed by an insect releases a volatile that attracts predatory insects, who arrive to eat the herbivorous insect. A longer-term use of volatiles is a phenomenon called allelopathy, when a plant releases compounds that prevent the growth of other plants around it, therefore reducing competition for resources. A well-known example is the eucalyptus tree; examine the ground in a eucalyptus grove and you’ll notice that not much else is growing there.

Many of these volatiles that plants synthesize to protect themselves also have benefits for humans, which is why the sense of smell is an important element of horticultural therapy, or healing gardens. Researchers have shown benefits from multi-sensory garden therapy for patients with mental illness and dementia. Unfortunately, little work has been done to isolate the effects of scents in the garden, but research has shown benefits from essential oils, which are plant volatiles concentrated into liquid form. One study of two common garden oils, rosemary and lavender, showed that both had positive effects on mood, and that exposure to rosemary (typically thought to be a stimulating oil as compared to sedating lavender) temporarily improved memory. Follow our Instagram this month as we explore various scented volatiles and their possible human health benefits.

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