Why do essential oils exist? Scientists discover plants detect insect attacks by ‘sniffing’ each other

Why do essential oils exist? Did you know that the gorgeous smelling essences we so prize in perfumes are actually a way plants communicate with each other (and defend themselves from insect attacks?) This fascinating report reveals all…

‘Plants have nowhere to run from their enemies – flying, crawling and jumping insects want to eat them alive. But plants are not defenceless. They deploy chemical toxins to deter insects. These can make the plant taste bitter, inhibit the herbivore’s digestive enzymes, disrupt their metabolism or poison them.

But they have a more subtle defence too – perfumed chemical compounds, known as volatiles, that they emit into the air to warn neighbours of danger or convey when they’re hurt. An example is the smell of cut grass, a mix of molecules called ‘green leaf volatiles’ which are released when a plant is damaged.

‘Plants are nature’s chemists. They take a few simple inorganic molecules and produce thousands of different organic molecules by just adding (energy from) sunlight,’ said Professor Matthias Erb, a plant scientist at the University of Bern, Switzerland. He investigates the volatiles that plants emit when attacked by insects for a project called PERVOL.

‘Some of these volatiles attract natural enemies of the herbivore, so, friends of the plant,’ said Prof. Erb. For example, if a caterpillar attacks a plant, these volatiles may attract parasitoid wasps or trigger defence responses in neighbouring plants. He says plants don’t help one another by signaling ‘I’m under attack’. Rather, they snoop on one another’s chemical signals to warn themselves about imminent threats.

Insects

Decoding these signals could teach us how to better protect crops against insects, according to Prof. Erb.

Insects are responsible for destroying one-fifth of the world’s total crop production each year. This is predicted to rise further for grain crops with climate change, hitting the temperate zones hardest.

‘These (plant-derived) molecules can be useful for agriculture in that they are natural protective mechanisms of plants. We could use them instead of synthetic chemicals,’ he said.

Prof. Erb works with maize, a strong emitter of volatiles. One chemical it emits is indole, which has a pleasant flowery aroma in small concentrations. Indole is not released by cutting maize. It requires the presence of a molecule in moth caterpillar saliva that activates defence responses in the plant. ‘(Healthy) maize plants do not emit indole. It is only triggered by herbivory,’ he said.

Prof. Erb and his colleagues found that when indole wafts towards the part of the plant that is not under attack, it triggers what he calls a primed state. ‘(Indole) doesn’t induce a defence response, but it prepares the plant, so that when the plant is attacked by a herbivore, it will respond quicker and stronger,’ he said.

Doing this means it can fend off its attacker more effectively, he says.

 

Why do essential oils exist?

One limitation of indole, however, is that it is also released by some flowers, such as jasmine and orange blossom. To prevent confusion, as a single volatile might be misleading, maize plants often tune into chemical mixtures to deduce attacks.

‘We have shown that indole and green leaf volatiles act synergistically to induce defences in an even stronger fashion than an individual volatile,’ said Prof. Erb.

‘Plants are nature’s chemists. They take a few simple inorganic molecules and produce thousands of different organic molecules by just adding sunlight.’

Professor Matthias Erb, University of Bern, Switzerland

 

To paint a fuller picture of plant behaviour, scientists are also exploring the impact of insect saliva on green leaf volatiles.

This is something that Dr Silke Allmann at the University of Amsterdam in the Netherlands has investigated in her work looking at how the green leaf volatiles of hurt plants is perceived by both plants and insects.

She experimented on tobacco plants by mechanically cutting them and applying water or the saliva of a tobacco hornworm caterpillar. The results surprised her: overall, the amount of green leaf volatiles did not change much, but the composition of the volatiles shifted dramatically. An enzyme in the caterpillar’s spit changed the compound, causing it to shift from a grassy to a sweet smell.

 

Why do essential oils exist?

 

She then discovered that a shift to the sweet-smelling compound attracted big-eyed bugs, which are natural enemies of the hornworm caterpillar, to the tobacco plant. This seemed puzzling to Dr Allmann, as the caterpillar’s own enzyme helped alert its presence to its enemies.

However, the sweet smell also warned adult tobacco hawk moths that a tobacco plant had already been colonised by caterpillars and steered them towards those with fewer competitors and fewer predators.

Dr Allmann is now studying this compound further as part of a project called VOLARE, and exploring practical uses.

‘A big challenge with plant volatiles is finding applications in agriculture. That is the holy grail,’ said Dr Allmann.

These chemicals can help farmers in a greener way, say the scientists.

‘You could imagine applying plant volatiles at the right moment to trigger specific reactions in a plant, for instance, resistance to herbivory,’ said Prof. ‘That would be a far more environmentally friendly strategy of boosting plant immunity or resistance to stress than applying a bioactive chemical to kill insects.’

Such natural chemicals could be released into fields under threat from pests to activate plant defences at the right moment. Insights into how plants detect warning smells could also allow breeders to develop crop varieties that are responsive to the signals.

Nose

What remains puzzling for scientists is how plants sniff out volatiles in the first place. They don’t have noses like us but can smell.

‘Our hypothesis is that volatiles enter through the stomata, small pores in leaves. We expect that there are sensors inside the leaf, perhaps proteins on the surface of cells, that the volatiles bind to,’ said Prof. Erb.

Dr Allmann is also hunting for these sensors. ‘If we found these receptors, we could find ligands (a type of molecule) that bind to them and switch them on. We could perhaps breed plants to be more or less sensitive to volatiles,’ she said. Plants could be bred that are easily triggered and could serve as sentinels to warn other plants nearby.’

The research in this article has been funded by the EU’s European Research Council. This post Plants can detect insect attacks by ‘sniffing’ each other’s aromas was originally published on Horizon: the EU Research & Innovation magazine | European Commission.

Castle Farm’s lavender harvest: our fragrant field trip

Lavender is having a moment in fragrance right now – according to a repot by the NPD group, sales of prestige lavender beauty products in the U.K. have increased by a staggering 552% from January 2019 to the end of April 2019. So you might have previously dismissed it as ‘grandma’s scent’, but get ready to leave your lavender preconceptions at the door…

When you think of a purple swathe of lavender being harvested for the perfume industry, your mind possibly strays to the fields of Provence. But did you know we have our own beautiful lavender farms right on our doorstop in the U.K?

We paid a visit with the British Essential Oil Association on a field trip to Castle Farm in Sevenoaks, Kent, to see the lavender harvest in action and – most excitingly – to see their very own distillery, and watch those tiny flowers be turned into the most lusciously fragrant essential oil: from field to bottle, in front of our very eyes (and noses)…

Starting our day with a glass of apple juice from the farm and a lavender shortbread biscuit, we ignored the gathering clouds and headed out to the fields to learn about the differing types of lavender they grow there, and how the oils from them are used.

The Alexander Family have been farming in the Darent Valley since 1892, when the enterprising James Alexander apparently brought down 17 milking cows on the train from Ayrshire in Scotland. Today, the farm is managed by William and Caroline Alexander, with involvement from each of their children, Lorna, Thomas and Crispin, and supported by the hard-working Castle Farm team.

The lavender farm was established in 1985, when William and Caroline went to market – Covent Garden as it happens – selling dried hops. By 1990, The Hop Shop (as Castle Farm is also known) was a major producer of dried flowers, growing over 70 different flowers and won many awards, including 5 consecutive Gold Medals at the RHS Chelsea Flower Show. Now the farm is a sprawling 1,100 acres, with crops of wheat, barley, rapeseed, hops, apples, pumpkins and a grass-fed herd of beef cattle. And, of course, the magnificent fields of lavender – the largest lavender farm in the U.K.

In the distillery, they had the ingenious idea of using the trucks that harvest the lavender to directly distill the essential oil from – time is of the literally essence for the highest quality yield – and so they put a lid on top of the container, in which 7 tonnes of lavender are heated to 100° in just ten minutes. From there, a huge pipe is attached to catch the and filter the steam to the tanks in a large barn.

As we stood with steam wafting around our ankles, the smell became almost overpowering – it felt like the lavender fields had been turned into a sauna, with misty swirls of mineralic steam mixing with the scent of drying hay and engine oil. A bit like being on a steam train in a dream. A truly surreal and unforgettable experience. And then, of course, we got to smell the oils themselves…

Premium Lavender Oil: An elite single origin oil – luxuriously delicate with exceptional fragrance notes for aromatherapy. Intensely pure. ‘This oil is extracted solely from our Maillette Lavender plants – and taken in the first 20 minutes of distillation only – making it the finest and most delicate of our lavender fragrances.’ With all the characteristics of a ‘high altitude Lavender Oil’ (which, we learned, has nothing whatever to do with the altitude a lavender’s grown at, and everything to do with the type of lavender and how long it’s distilled for!)

Kentish Lavender Oil: A high-grade, honeyed, gentle Lavender essential oil with antiseptic, anti-inflammatory, antibacterial and mild anesthetic properties. This oil is made from their Lavandula Angustifolia, grown and distilled on Castle Farm. Widely used in aromatherapy, perfumery and medicine, the pure oil is safe for use directly on the skin.

Kentish Lavandin Oil: Lavandula x intermedia is extracted from Castle Farm’s Grosso plant variety and has a strong, cleansing and refreshing scent with elements of camphor. Often used for room fragrancing, candles, as a moth repellant and to clear cold-sufferers’ heads, the fragrance is comforting but less nuanced than the others, which are more suitable for perfumery.

The oils are sold in the farm shop, and online, and also sold to private companies for use in the fragrance and beauty industry.

The Hop Shop – is open daily throughout the year, however the Castle Farm Lavender fields are only in flower from late June until late July. Lavender plants for sale from the shop from late April/early May, but they explain, ‘the Lavender season really gets underway in mid-June and the glorious scent and vibrant colour of the fields fill the valley and we start to hand cut fresh Lavender bunches. Harvesting for oil starts in mid July and continues until early August, but is obviously weather dependent.’

Of course, on the day they began harvesting and the morning of our field trip – the heavens opened. If the rain lasts too long, half the entire crop can be lost in under an hour. Luckily, it wasn’t too long before we could venture out from under the trees and take photos – Castle Farm had the brilliant idea of including an Instagram Field, complete with perfectly positioned bench, so people can take scented selfies to their heart’s content, without disturbing the harvesting. Genius!

And to show it really is a family affari, this photo (above) is of William and Caroline’s grandaughter, taken by their talented son.

Castle Farm began harvesting while we were there, ‘…but it takes us over a week to clear all the fields, and the distillery is now running daily. We completed harvest of the large field at the back of the valley on 22nd July, but our field with the Lavender Bench and viewing area will NOT be harvested until after this weekend (27/28 July). We are still running Lavender Tours until we harvest our very last field!’

You can book online for weekday tours, or on arrival at the farm for weekend tours.

We’re not sure if it was the supremely soothing smell of the lavender or how long we’d been traipsing through the fragrant fields – possible a mixture of both – but goodness, we slept well that night.

We were also enthused to seek out four of our favourite lavender fragrances and urge you to do likewise. Forget the aromatherapy benefits for a moment and focus instead on the incredible nuances that lavender can add to a scent…

Atkinsons Lavender On the Rocks
True to the cocktail-esque name, this one has a double-shot of lavender to tickle your fancy. From the bracingly fresh opening with geranium and basil to the honeyed hay-like dry down with almond, guaiac wood and saffron, every facet is allowed to shine.
£130 for 100ml eau de parfum
harrods.com

Creed Aberdeen Lavender
An Oriental fougére, absinthe is added to rosemary, bergamot and lemon before succumbing to a powdered, musky lavender ramped up with tuberose, iris and a dusky rose. Dark patchouli, smoky leather and cool vetiver make for a surprisingly sexy flourish.
£200 for 100ml eau de parfum
libertylondon.com

Milano Centro HIM
Luminescent citrus segues to a herbaceously dappled breeze of rosemary, lavender and basil. As it warms, you’re swathed in the musky warmth of smooth sandalwood and suavely sprinkled spicy notes of clove, cinnamon and amber atop a darkly glimmering patchouli base.

Want to try it for yourself? Find this in our Explorer Men’s Discovery Box – along with SIXTEEN other fragrances to explore!

Tom Ford Private Blend Lavender Extrême
Trust Tom to turn lavender sexy. Here it’s subverted into an electric floral ‘to be worn at maximum volume.’ Bergamot drenches the lavender with freshness at first, then it’s full-on delicious with wild-grown tonka and creamy benzoin swirled to perfection.
£220 for 50ml eau de parfum
johnlewis.com

By Suzy Nightingale