Why do Christmas trees smell good?

Why do Christmas trees smell good? - Many of us have fond memories of the wonderful aroma that infuses the air when the christmas tree is brought into the house. But my thoughts about this perfume seldom extend beyond "wow, this tree smells good". What, specifically, is the characteristic scent? Why does the tree even bother making it in the first place? Do other plants make the same smells? Where can I buy a small vial of this concentrated essence so that I can invoke christmas spirit all year round!? And how is that concentrate even made? Today, Plants Are Chemists answers these questions.

Figure 1: Conifer resin. Conifer species like pines, firs, and spruces make resins to seal any wounds they sustain. These resins contain terpenes that kill organisms invading the wound and attract other organisms that might kill the invaders.

Christmas trees are conifers - Christmas trees are usually pine, spruce, or fir trees. These are all conifers, a division of land plants that bear cones and have internal water conduction systems. Though there are relatively few species of conifers, they cover vast areas in great numbers; they are earth's second largest above-ground carbon sink, after tropical forests [1]. They also provide about 45% of the world's lumber.

Conifers produce resins to protect themselves - Conifers are well known for the resins they produce (Figure 1). Maybe you've had a (frustrating?) experience with resin from one of these trees - it can be very difficult to get off of your hands. This resin is the tree's means of sealing up any wounds it sustains. It is composed mainly of a special class of plant chemicals called terpenes. Broadly speaking, terpene molecules fall into two classes: those that are small and volatile (they evaporate quickly and spread through the air), and those that are large and non-volatile. Often, the larger terpenes are toxic to insects, bacteria, or fungi that might be invading the wound. In contrast, the airborne terpenes can signal nearby organisms that might eat the invaders. Clearly, terpenes are another testament to the awesome power of the plant kingdom's skills in applied chemistry.

Resins mainly contain terpenes - The identity of the terpenes that make up conifer resins have been studied quite a bit. Common resin terpenes include limonene, terpinolene, alpha- and beta-pinene, delta-3 carene, and sabinene, just to name a few [2]. Terpenes molecules typically contain carbon atoms linked into many complicated rings (Figure 2, left). These rings are put together in a very ingenious way. First, the plant produces a carbon chain that has no rings but has one very reactive carbon that is initially covered up with phosphate to prevent the carbon from reacting (Figure 2, middle). This chain is sort of like one of those snap bracelets that you may have played with as a kid (and maybe still play with today!?) when in its flattened form. When the plant wants to make a terpene, it takes one of these chains, puts it in strong, specialized box (an enzyme) and removes the phosphate, unleashing the reactive carbon. The energy from the reactive carbon bounces all around the chain, causing immensely complex carbon rings to form (sort of like snapping the snap bracelet) (Figure 2, right). Then the terpene is released from the box. Depending on the type of rings and the kind of terpene the plant wants to make, it can use boxes (enzymes) with different characteristics.

Figure 2: Terpene synthesis. Plants put a carbon chain with an active (but restrained carbon) into a box (enzyme), then let the reactive carbon free, causing the energy from the reactive carbon to bend the chain into rings and form a terpene (shown above the dotted line). This is analogous to a snap bracelet (shown below the dotted line): in its restrained, linear form, the energy can be released causing the bracelet to bend into rings.

Figure 3: The Christmas tree smell. The molecules shown here, alpha-pinene, beta-pinene, and bornyl acetate, are the compounds that, together, are the Christmas tree smell.

The christmas tree smell is three terpenes that can be isolated with steam distillation - Using this technique conifer trees make the major aroma molecules that are associated with the classic christmas tree smell: alpha- and beta-pinene and bornyl acetate (Figure 3) [3]. For this reason these molecules, or more accurately their aroma, are very popular. These molecules are quite volatile - they evaporate at relatively low temperatures. This characteristic makes them easy to isolate - the plant is boiled in water, releasing the terpenes into the steam, then the steam is collected and concentrated into an essential oil. A quick internet search will turn up dozens of outlets where one can buy pine or douglas fir essential oils, as well as many other "Christmas blend" essential oils also containing orange, cinnamon, and other spices. Whether from a concentrated oil or from a real christmas tree, if you experience that Christmas tree smell this year, hopefully you'll be reminded a little bit about terpene chemistry and the amazing defense mechanisms of conifer trees. Happy holidays.

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[1] https://www.ipcc.ch/pdf/assessment-report/ar4/wg3/ar4-wg3-chapter9.pdf
[2] Clark, Erin L., et al. "Comparison of lodgepole and jack pine resin chemistry: implications for range expansion by the mountain pine beetle, Dendroctonus ponderosae (Coleoptera: Curculionidae)." PeerJ 2 (2014): e240.
[3] (2011), Festive fragrances. Chemistry & Industry, 75: 17–20. doi:10.1002/cind.7524_8.x

Why do chilis bother making themselves spicy anyway?

Why are chilis spicy? - Spicy food holds a special place in many hearts - some people love it and some hate it. We know that chili peppers are used to make spicy food, but why are chilis spicy in the first place? What's in it for the chili plant? Shouldn't those plants be directing all their energy into surviving or reproducing or something?

Mammals threaten chili pepper seeds - Imagine you're a chili plant: you've just spent all year packaging up your children in stored energy and nutrients (seeds). You've even wrapped your kids up in a tasty, shiny red treat (the meaty part of the pepper), to try and entice a bird to eat your pepper, carry it off far away, and poop your children out into a great new environment to start their lives (I know - plant parents have weird hopes for their children). Despite all this, some mammal suddenly comes by and gobbles up your children, using its molar teeth to grind them into a lifeless pulp, and poops them out less than a mile away to rot. What a nightmare!

Spicy capsaicin protects chili seeds from mammals - Fortunately, as a member of the Capsicum plant family, you have a secret weapon against the murderous mammals - you are a master of organic chemistry! You find (after millions of years of experimentation) that by combining a simple fatty acid and a compound similar to vanilla you can create a remarkable molecule that humans call capsaicin (Fig. 1). By coating your children in this compound you successfully make them extremely unpallatable to marauding mammals. Birds, who are unaffected by capsaicin, are then free to eat your peppers [1]. Since birds don't have the same grinding teeth mammals do, your children pass through the bird intact, and are dispersed widely over the globe to colonize new areas.

Figure 1: Capsicum plants, seeds, and capsaicin. Species in the pepper plant family (example in top left), product fruit with seeds (top right), that contain spicy capsaicin (bottom) to protect the seeds against mammals.

Humans (bizarre mammals) cultivate the chili's capsaicin spice - To humans, capsaicin has a spicy taste that many enjoy. Capsicum species were originally domesticated in Central America over 6000 years ago. After european explorers ventured to the Americas, these plants and their peppers were brought all over the world as part of the Columbian Exchange. Now, cultures all over the globe use spicy peppers in their cooking. Since then, humans have been breeding pepper plants that make incredibly hot peppers, and have developed the Scoville heat unit as a scale to describe how hot different peppers are.

Medicinal properties of capsaicin - Some claim to experience euphoria after eating capsaicin [4], supposedly related to a release of endorphins [5], though I suspect that this is folklore and varies greatly between different people. More widely experienced, and more recent, is the use of capsaicin as a pain-reliever. It is currently approved for topical application in the treatment of backache, arthritis, and sprains.

Chilis are used as a deterrent in agriculture - In large doses, capsaicin creates a burning sensation wherever it touches mammals; it is the primary active ingredient in pepper spray and bear mace. In some parts of Africa and Asia, farmers' crops are in danger of being eaten by elephants, which can step over high fences that keep out other herbivores. Elephants have large and sensitive noses, so farmers can keep them away by planting chili pepper plants in a defensive ring around other crops, and the chilis the barrier plants produce can be sold at market.

Chilis can be used as deterrents in home gardens - Chili seeds can also be added to bird seed to deter squirrels and rodents [2], and applied around flowers and other ornamentals to deter deer and rabbits [3]. Some gardeners report no deterring effects when using certain varieties of paprika (100% ground chilis) or chili powder (ground chilis + cumin and other spices). This is probably because paprika is often made with pimento peppers - chilis that are 4x less spicy than jalapenos, and barely register on the Scoville heat scale. A gardener looking to deter pests should search out a paprika with a punch!

Capsaicin is one of many valuable chemicals that plants have developed over millions of years- Capsicum species and the unique chemical called capsaicin they produce have applications in cooking, medicine, agriculture, and home gardening; all of which make the pepper plant a valuable economic commodity. Indeed, over 10 million acres of chili peppers are cultivated world wide. The plant kingdom is full of species, both known and undiscovered, that synthesize chemical compounds of enormous importance to human life. So, it is important that plant scientists work towards discovering and understanding these natural chemicals to improve both our quality of life and our agricultural practices.

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[1] Tewksbury, J. J.; Nabhan, G. P. (2001). "Seed dispersal. Directed deterrence by capsaicin in chilies". Nature. 412 (6845): 403–404.
[2] Jensen, P. G.; Curtis, P. D.; Dunn, J. A.; Austic, R. E.; Richmond, M. E. (2003). "Field evaluation of capsaicin as a rodent aversion agent for poultry feed". Pest Management Science. 59 (9): 1007–1015.
[3] "R.E.D. Facts for Capsaicin" (PDF). United States Environmental Protection Agency.
[4] Gorman J (20 September 2010). "A Perk of Our Evolution: Pleasure in Pain of Chilies". New York Times.
[5] Rollyson WD, et al. (2014). "Bioavailability of capsaicin and its implications for drug delivery". J Control Release. 196: 96–105.
[6] Fattori, V; Hohmann, M. S.; Rossaneis, A. C.; Pinho-Ribeiro, F. A.; Verri, W. A. (2016). "Capsaicin: Current Understanding of Its Mechanisms and Therapy of Pain and Other Pre-Clinical and Clinical Uses". Molecules. 21 (7): 844.

The James Bond of Plant Poisons

Bond, Poisoned by a Plant - Remember when Le Chiffre poisoned James Bond in Casino Royale? The poison makes James’ heart beat erratically and he stumbles to his car. Then the brains back at MI6 identify the toxin as “Digitalis”, and an antitoxin and defibrillator save Bond in time for him to win another round of poker. The poison has only a few minutes on screen, but the scene is exciting! Big surprise: we have a plant to thank for this excitement – and much more!

Figure 1: Digitalis (Foxglove) flowers. While foxglove plants synthesize some chemicals that turn their flowers beautiful colors to attract insects, they also synthesize chemicals that can be used as deadly poisons. How have humans used these toxins?

Foxgloves make digoxin, a deadly poison - As nature’s master chemists, plants synthesize a huge variety of chemicals that carry out specific parts of the plant’s will. Species in the Digitalis genus (Foxgloves) fill their flowers with chemicals that impart beautiful white, yellow, blue, and purple flowers and attract insect pollinators (Figure 1). However, at the same time, these plants synthesize other chemicals that pack a deadly punch. All parts of the plant contain digoxin, a chemical compound that if eaten in large quantities disables heart rate regulation and can lead to weakness or collapse, seizures, and even death.

Plants combine benign chemicals to make glycoside toxins - Foxglove plants make digoxin by binding together benign compounds: sugars and steroids. Binding sugars to steroids is a common strategy many plants use to make poisonous chemicals. These two-part chemicals are called glycosides. For example, plants in the Scilloideae and Apocynaceae families also bind sugars to steroids to make the glycoside poisons Scilliroside and Ouabain (Figure 2).

Figure 2: Examples of poisonous glycosides made in some plant families. These poisonous glycosides are all made by binding together sugar (in blue) and a steroid (in purple). The plant family in which each glycoside is found is shown to the right of the chemical structure. For example, the top glycoside called digoxin is found in the Digitalis family, shown in the top photo.

Historical uses of glycoside poisons - Humans noticed and started taking advantage of the effects, or bioactivities, of glycosides hundreds of years ago. The Eastern Africans used chemical Ouabain to poison arrowheads for hunting and warfare as early as the 3rd century BC [1], and the chemical Scilliroside is used as a rodenticide [2]. The interesting cardiac effects of the digoxin prompted its investigation as early as 1785 [3], with one main question: if administered in minute quantities, could digoxin be used to help individuals with heart problems?

Though toxic, glycosides are medicinal in small doses! - After much study scientists and doctors have found that digoxin can be used to treat atrial fibrillation, atrial flutter, and sometimes heart failure – and digoxin has been added to the World Health Organization’s List of Essential Medicines. For this chemical to help with heart conditions, it needs to be present in the blood at a concentration between 0.8 – 2.0 nanograms per milliliter (ng/ml) of blood [4]. Unfortunately, if digoxin levels increase above 2.0 ng/ml, the compound quickly becomes toxic and can revert from medicine to poison. So, to prevent poisonous digoxin levels maybe Bond should think about taking smaller sips of drinks offered to him by enemies.

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[1] Neuwinger, Hans Dieter. African Ethanobotany: Poisons And Drugs. Weinheim: Chapman & Hall, 1996.
[2] el Bahri, L.; Djegham, M.; Makhlouf, M. (2000). "Urginea maritima L. (Squill): A Poisonous Plant of North Africa". Veterinary and Human Toxicology. 42 (2): 108–110.
[3] Withering, William (1785). An Account of the Foxglove and some of its Medical Uses With Practical Remarks on Dropsy and Other Diseases.
[4] Richard C. Dart. Medical Toxicology. Lippincott Williams & Wilkins, 2004

How to seduce a bee (as an orchid)

Figure 1: Plant and pollinator. Insects spread plants' genetic material (pollen) from flower to flower in exchange for a sugar (nectar) reward.

Flowing plant reproduction - Imagine that you are a plant, rooted in the ground, and it's that time of year when you want to spread your genetic material to other plants to reproduce. You can't walk around to find a mate, so what do you do? Some plants solve this problem by allowing wind to carry their pollen in vast plumes across the landscape, hoping that it will reach the flowers of other members of the same species. However, many other plant species have stumbled upon a fortuitous partnership with bees, butterflies, and other insects that directly carry plant pollen from one flower to another.

Flowering plants work with pollinators to reproduce - Pollen-spreading insects (pollinators) make plant reproduction very efficient. Plants that recruit pollinators have been very successful - they are now the most common plants on earth. Some relationships between flowering plants and pollinators are specialized, so that only certain pollinators may spread the pollen of a certain plant species. Other relationships are general, so a single plant species is compatible with many pollinator species. In either case, plants often provide a sugary reward (nectar) for insects transferring pollen from one plant to another, which many insects and even some birds use as a primary resource (Figure 1).

Figure 2: Some flowers attract male bees to be pollinators by looking and smelling like female bees. Instead of providing a nectar reward to attract pollinators, some flowers develop to look like female bees. These flowers also release bee sex pheromones into the air to attract nearby male bees. By moving from one deceptive flower to another, male bees transfer the plants' pollen. Photo used with the permission of Carlos Enrique Hermosilla (https://www.flickr.com/photos/26925527@N07/)

Some flowers attract pollinators with pheromones - Some plant species are able to get bees to collect and spread their pollen without providing a reward. In the orchid family, a plant family with a tremendous number of species, some species produce flowers that look like female bees. These flowers entice male bees to land, pick up pollen while they wiggle around, and then carry it away while in search of a mate with a heartbeat (Figure 2). In doing this, male bees transfer pollen from one flower to another. To encourage male pollinators to land on these deceptive flowers, these plants scent the surrounding air with aphrodisiacs - bee sex pheromones that draw the male bees in with great effect. This is where chemistry comes into play!

Figure 3: Scents produced by flowers can attract specific pollinator species. The flowers of different plant species can produce different mixtures of scents, in some cases insect sex pheromones, to attract pollinators (top panels). Depending on the specific pheromones produced, different insect species may be attracted (middle and lower panels).

Deceptive pheromones are species-specific - There is a great variety of chemical compounds that plants use as deceptive pheromones with which to attract bees, and in many cases these chemical compounds are species specific [1]. For example, the orchid Ophrys exaltata attracts the bee Colletes cunicularius by producing a specific type of chemical called a 7-alkene. In contrast, Ophrys sphegodes attracts Andrena nigroaenea with 9-, 11-, and 12-alkenes (Figure 3). Even though these pheromones are extremely similar (they only differ in the position of their carbon-carbon double bond), adding the pheromones of one orchid species to the flower of the other creates a mixture that neither bee species is attracted to [2]. Overall, the exact identity and proportions of the different chemical compounds that flowers produce to attract pollinators is important [4].

Deceiving pollinators may encourage speciation- Let's perform a thought experiment - if an O. sphegodes plant acquired a mutation that caused it to only produce two of its three alkenes, we know it would probably not attract either C. cunicularius or A. nigroaenea. But, if there were another insect species nearby that was attracted by the two alkenes alone, then the mutant plant could still survive. Over time, the offspring of the mutant plant could become a population of individuals all carrying the mutation and producing only two alkenes. Perhaps the flowers of these mutants would also change shape or color over generations so as to more effectively attract the new insect pollinators. Eventually, this could lead these mutants to become an entirely different species!

So, in addition to being crucial for the reproduction of some flowering plant species, scientists think that specificity in plant-pollinator relationships is a substantial driving force in speciation, perhaps occurring via a process similar to that described above [5, 6]. Flowers make fragrances to do more than just smell nice!

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[1] Bjorn Bohman, Gavin R Flematti, Russell A Barrow, Eran Pichersky and Rod Peakall. Pollination by sexual deception — it takes chemistry to work. Current Opinion in Plant Biology 2016, 32:37–46
[2] Xu S, Schluter PM, Grossniklaus U, Schiestl FP: The genetic basis of pollinator adaptation in a sexually deceptive orchid. PLOS Genet 2012:8.

[3] Bohman B, Philips RD, Menz MHM, Berntsson BW, Flematti GR, Barrow RA, Dixon KW, Peakall R. Discovery of pyrazines as pollinator sex pheromones and orchid semiochemicals: implications for the evolution of sexual deception. New Phytol 2014, 203:939-952.
[4] Bohman B, Karton A, Dixon RCM, Barrow RA, Peakall R. Parapheromones for thynnine wasps. J Chem Ecol 2016, 42:17-23.
[5] Peakall R, Whitehead MR: Floral odour chemistry defines species boundaries and underpins strong reproductive isolation in sexually deceptive orchids. Ann Bot 2014, 113:341-355.
[6] Xu S, Schluter PM, Grossniklaus U, Schiestl FP: The genetic basis of pollinator adaptation in a sexually deceptive orchid. PLOS Genet 2012:8.

Blue Hydrangeas – How and WHY!?

Figure 1: Yellowing tomato leaves. Leaves turning yellow or brown can be caused by numerous stresses on the plant.

This story is about hydrangeas – I promise! – but it begins with tomatoes. This week the leaves of the tomato plants in our garden began to turn yellow (Figure 1). Why does this happen? Leaves are normally green because of chlorophyll, the light-collecting compound that is highly abundant in most plant cells. So, though yellow leaves might be caused by many factors, it could mean the plant is losing its ability to produce chlorophyll.

Chlorophyll is made of carbon, hydrogen, oxygen, nitrogen, and magnesium (Figure 2). It seems unlikely that our tomatoes are suffering from a shortage of carbon, hydrogen, or oxygen, since they get these from the CO₂ in the air and water we give them. Therefore it might be that a nitrogen or magnesium shortage is leading to leaf yellowing, and that our tomatoes need these elements added to their soil.

Figure 2: Chemical structure of the chlorophyll molecule. The chlorophyll molecule has three principal components, the carbon chain (bottom left), the ring structure (right), and the magnesium metal atom (in green). Magnesium deficiency can lead to decreased production of chlorophyll in plant cells.

The elements nitrogen and magnesium are among six nutrients that plants need in substantial amounts. These macronutrients are the elements nitrogen (N) and phosphorous (P), and the metal elements potassium (K), calcium (Ca), sulfur (S), and magnesium (Mg). These are all essential for plant health and are acquired through roots. Other elements are needed in only very small amounts. These micronutrients are boron (B), chlorine (Cl), and the metals manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), and nickel (Ni).

Not all metals are required for plant health, and some are even detrimental. For example, aluminum, the most abundant metal in the earth's crust, can inhibit root absorption of magnesium, the metal in chlorophyll, causing the plant to experience magnesium deficiency. In soils with low acidity, called basic soils, aluminum is bound to the soil and does not interfere with the plant. However, in acidic soils aluminum is released from the soil and gets stuck in root channels that normally absorb magnesium, inhibiting the absorption of Mg. In this way aluminum is toxic to many plants that grow in acidic soils.

Figure 3: Color change in Hydrangea macrophylla. Top: delphinidin in its normal state leads to blue colored hydrangea petals. Bottom: delphinidin bound with aluminum (a delphinidin-aluminum complex) leads to petals with red or pink color.

Some plant species have evolved mechanisms to reduce the toxicity of aluminum. Citric acid is released from the roots and binds to the aluminum, allowing the aluminum to be absorbed into the roots and transported up into areas of the plant where it will not interfere with the absorption of magnesium [1]. Many of us are probably familiar with one species of aluminum-tolerant plant – Hydrangeas (Hydrangea macrophylla in particular). This species has flowers that are normally red or pink because they contain a phytochemical called delphinidin (Figure 3). When there is aluminum present in the soil the Hydrangea releases citric acid to bind the aluminum and then absorbs the bound aluminum and transports it to the flowers for storage. Here the aluminum interacts with the red delphinidin pigment and transforms it into a blue pigment, and causing the flowers to turn blue [2]. Thus, when Hydrangea macrophylla is exposed to acidic, aluminum-rich soils, its flowers turn from red to blue!

It is remarkable that ingredients as simple as aluminum and acid can cause such a dramatic change in flower color. It is even more amazing that this phenomenon is no accident but is a clever strategy used by the plant to prevent aluminum from interfering with magnesium uptake. Maybe someday tomato plants will also develop a similar mechanism and prevent aluminum from inhibiting magnesium uptake, allowing them to thrive in a wider diversity of soils.

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[1] Jian Feng Ma, Syuntaro Hiradate, Kyosuke Nomoto, Takasi Iwashita, and Hideaki Matsumotol (1997). Interna1 Detoxification Mechanism of AI in Hydrangea, ldentification of AI Form in the Leaves. Plant Physiology 113: 1033-1039.
[2] Henry D. Schreiber, Amy M. Swink, Taylor D. Godsey (2010). The chemical mechanism for Al3+ complexing with delphinidin: A model for the bluing of hydrangea sepals. Journal of Inorganic Biochemistry 104: 732–739.

Are apple seeds poisonous? – plant cyanogenic compounds

Figure 1: Apple and seeds. Seeds of the common apple contain the ingredients for producing hydrogen cyanide as a mechanism to deter other organisms from eating the seeds.

Plants are anchored in the ground and cannot run away when attacked, but they do not meekly allow themselves to be eaten! In many cases, plants that do not want to be eaten have developed complex chemical mechanisms that deter, distract, and even poison attackers. Many different plant poisons have been identified, but one of them is hydrogen cyanide - a poison that inhibits respiration. So far, more than 3000 species of plants have been identified that are capable of generating hydrogen cyanide to punish organisms that eat their leaves. The common apple is among these plants (Figure 1) [1]. These plants produce cyanide-containing molecules called cyanogenic compounds, for example, mandelonitrile (Figure 2).

Cyanogenic compounds (cyanogens) are inactive and generally not poisonous, so plants can produce and store them in special compartments inside their cells without causing themselves harm. In nearby but separate compartments, plants also store a special enzyme capable of transforming the inactive cyanogen into toxic hydrogen cyanide. Then, when insects or other animals bite down on the leaves of the plant, the compartments' walls are crushed, mixing the cyanogen with the enzyme and releasing toxic hydrogen cyanide right inside the attacker's mouth!

Figure 2: Enzymatic activation of cyanogens. Mandelonitrile (a specific cyanogen) can be enzymatically converted into benzaldehyde (a side product) and active hydrogen cyanide.

Plants produce and store cyanogens in many forms. Oftentimes they are stored linked with sugar molecules like glucose, in which case they are called cyanogenic glycosides. These are found in the seeds of many fruits in the rose family, like apples, plums, apricots, and peaches, but only in relatively small amounts. In small amounts, these seeds are not harmful to humans - so don't worry about eating a few apple seeds!

Cassava roots, a staple food in many African, Asian, and Latin American countries [2], contain substantial amounts of cyanogenic glycosides (Figure 3) [3]. Accordingly, cassava roots need to be processed before consumption. One method of doing this is to grind the cassava, exposing the activating enzyme to the cyanogenic glycosides, spreading the paste in a thin layer to allow it to dry. Once released, active hydrogen cyanide has a low boiling point, so much of it evaporates quickly and the cassava becomes safer to consume.

Figure 3: Cassava roots contain cyanogens that must be removed prior to human consumption. Grinding the roots and allowing active hydrogen cyanide to evaporate makes cassava safer to eat. This image is in the public domain.

Cyanogens are currently being tested as alternatives to synthetic pesticides for specific applications. For example, approximately 25% of a wheat crop is lost to pests (rats, mice, insects, etc.) over the course of the agricultural process, with substantial losses occurring when the seeds are being stored prior to planting [4]. Synthetic pesticides are often applied to minimize these losses, but but we are learning that pesticides often have detrimental effects on nearby water sources and can be difficult to control when washed away by rain.

In response to the shortcomings of synthetic pesticides, materials chemists, inspired by cyanogens in nature, have invented a new coating that can applied to wheat seeds in storage. The seeds are coated with an enzyme, then a thin barrier, then a cyanogen [5]. When the barrier is ruptured by pests, the enzyme and cyanogen mix, releasing hydrogen cyanide. Such seeds are less prone to attack during the pre-planting phase of the agricultural process. While this application is still in developmental phases and is undergoing testing to determine its environmental and ecological effects, it is a step towards the protection of crops with naturally occurring, biodegradable materials.

Plant cyanogens are just one example of naturally occurring compounds that are useful to humans. The "2016 State of the World's Plants" report by the Royal Kew Gardens indicates that over 30,000 species of plants are useful to man. For example, over 5,000 feed us, 17,000 provide us with medicine, and 1,600 provide us with sources of fuel. Unstudied and unknown species hold chemical secrets that promise to aid humans in the future - these warrant study and exploration.

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[1] Poulton, J.E., 1990. Cyanogenesis in plants. Plant Physiology, 94(2), pp.401-405.
[2] Food and Agriculture Organization. http://www.fao.org/ag/agp/agpc/gcds/
[3] Banea-Mayambu JP, Tylleskar T, Gitebo N, Matadi N, Gebre-Medhin M, Rosling H. (1997). Geographical and seasonal association between linamarin and cyanide exposure from cassava and the upper motor neurone disease konzo in former Zaire. Trop Med Int Health 2(12):1143-51]
[4] Encyclopedia of Pest Management, ed. D. Pimentel, CRC Press, 2002.
[5] Jonas G. Halter, Weida D. Chen, Nora Hild, Carlos A. Mora, Philipp R. Stoessel, Fabian M. Koehler, Robert N. Grass and Wendelin J. Stark (2014). Induced cyanogenesis from hydroxynitrile lyase and mandelonitrile on wheat with polylactic acid multilayer-coating produces self-defending seeds. J. Mater. Chem. A, 2: 853