Plant Chemistry and Defense
The magnificent extent to which plants go to stockpile carbon dioxide into sugars through photosynthesis, and subsequently alter a carbon backbone to form a diverse phytochemical concoction, is beautifully described through a quote from the American poet Gary Snyder when he wrote that:
“Plants are all chemists, tirelessly assembling the molecules of the world”
These “molecules of the world” have contributed extensively towards how humans live today. For instance, volatile organic compounds (VOCs) are an important group of chemical which plants produce to communicate with their surrounding environment (Maffei 2010). Given VOCs possess a relatively low boiling point, a large number of molecules can enter a gaseous state at room temperature, which in turn enables VOCs to travel long distances with very little effort (i.e. the smell of perfume).
The ability of VOCs to travel long-distances, has been exploited in nature by plants for purposes of balancing plant-animal, plant-microbe, and plant-plant interactions by attracting pollinators and insects, deterring infestations from pests, or warning surrounding neighbours of immediate dangers (Maffei 2010). For instance, VOCs produced by Big Sagebrush (Artemisia tridentata Nutt) are believed to mediate communication between individuals in the wild, and it is believed that the chemical cues are altruistically working to warn nearby neighbours of stressors such as insect herbivory (Karban 2010). This process is often referred to as volatile communication.
Big Sagebrush (bottom left) is endemic to North America grasslands also known as Sagebrush steppe ecosystems
VOCs in Big Sagebrush Smoke
To date the exact mechanism governing volatile communication in Big Sagebrush is unknown, although it is believed that volatile cues from monoterpene and sesquiterpene phytochemicals as well as plant growth regulators such as methyl jasmonate may play an important role. In recent years, Sagebrush ecosystems have declined and been fragmented by various environmental pressures including wild fires (Davies et al. 2011), interestingly there is some evidence which suggests that smoke derived from Sagebrush can influence germination patterns (Blank and Young 1998), which raises an interesting question:
Is smoke derived from Sagebrush signaling the surrounding environment through chemical cues?
What exactly happens to a plant’s phytochemistry during burning largely remains a mystery to date. However, research suggests that burning of incense or other plant materials can lead to the release of many unique chemicals which are not normally present, including oxygenated phytochemicals (Chuang et al. 2011). Furthermore, phytochemicals which are not typically volatile at room temperature (not a VOC) can enter the gaseous state during burning, due to increased temperature (Yamada and Yatagai 2007). Once in a gaseous state, these chemicals are free to move around and can interact with the surrounding environment through agonist or antagonist effects upon specific receptors located externally and internally in insects, plants, humans, etc.
To what extent can VOCs present in Big Sagebrush communicate and interact with the surrounding environment is still up for debate. However, recently it was found that smoke extracts from Big Sagebrush can interfere with cholinergic signaling (Turi et al. 2014), which in turn may explain the traditional use of leaves as smudges or fumigants by First Nations and Native American peoples (Moerman 2009).
Traditional use of Artemisia Smoke
The traditional use of smoke derived from the genus Artemisia is widespread. In parts of China, species of Artemisia are burned or used to ignite incense made from Juniperus squamata Buch. and Cupressus funebris (Staub et al., 2011). Similarly, in parts of India and Nepal, Artemisia species are burned as incense (Ahuja et al., 2011; Bhattacharyya, 1991; Shah and Joshi, 1971) or are used ritualistically as offerings to local deities (Shah, 2013). In Pakistan, Artemisia scoparioa Waldst & Kit and Artemisa herba-alba Asso are burned to treat burns and muscle aches respectively (Hayat et al., 2009).
Although smoke derived from Artemisia species has been exploited for thousands of years for medicinal, spiritual or ritualistic purposes, understanding towards the therapeutic potential of smoke derived from species is limited. Still there are several studies which suggests that smoke from Artemisia can modulate one’s neurochemistry upon inhaling. For example, in one study, mice were exposed to smoke derived from Artemisia vulgaris for 15 or 30 minutes each day, and the researchers reported increased levels of cerebral serotonin, dopamine and norepinephrine (Xu et al. 2013).
Interestingly, other biological activity has also been reported from smoke derived from Artemisia species. For instance, when exposed to smoke derived from Artemisia princeps, human breast cancer cells underwent apoptosis, possibly due to the presence of specific phytochemicals which cause the mitochondrial membrane to depolarize and down-regulate the expression of the anti-apoptitic protein BCL-2 (Sarath et al., 2007). Additionally, antimicrobial activity has been observed against Bacillus cereus, Klebsiella pneumonia and Cryptococcus neoformans from smoke extracts collected from Artemisia afra Jacq. Ex Willd. (Braithwaite et al., 2008).
Glandular trichomes produce oils containing a plethora of chemicals. These complex oily mixtures can be used therapeutically as essential oils (i.e. Lavender or Peppermint), incense or smoke (i.e. cannabis). Image: Public Domain, Wikimedia Commons
Concluding Remarks
The traditional use of plant derived smoke is well documented within many cultures as air purifiers, febrifuges, disinfectants, ear and eye remedies, hallucinogens or as gastrointestinal, genito-urinary, dermatological, neurological, and oral aids (Mohagheghzadeh et al., 2006); and it is estimated at least 1, 460 plant species globally have been exploited for their smoke Pennacchio et al. (2010). Despite the obvious importance of plant derived smokes, there is still much to learn about how the phytochemicals found within the smoke mediate responses within humans and nature as only a select few plants (i.e. Tobacco and Cannabis) having received attention. Given the widespread use of Big Sagebrush as well as other North American Artemisia species as smudges during ceremony, it is very likely that much remains to be discovered in smoke with respect to its ability of mediate plant-plant, plant-animal, and plant-human interactions, and should be examined in greater detail in the future.
