The idea that plants "talk" to each other through chemical signals in the air was first proposed scientifically by Baldwin and Schultz in 1983 (Science) and by Rhoades in 1983 (Journal of Chemical Ecology): both groups had observed that plants near individuals under herbivore attack increased their own production of chemical defenses before being attacked themselves. The idea was initially highly controversial (Science magazine refused to publish one of the original studies with the comment "plants don't talk"). Today, chemical communication between plants through volatile organic compounds (VOCs) is firmly established by research, though its ecological and adaptive significance remains debated.

Plant Volatile Organic Compounds: What They Are

Plants produce over 1,700 identified VOCs (out of estimates of 30,000–100,000 total). The main groups of VOCs with communicative function: Terpenes and terpenoids: the largest and most diverse group. They include isoprene (the most emitted VOC globally by forests, with a protective function against heat), monoterpenes (limonene, linalool, α-pinene: found in pine needles, lemon, basil), sesquiterpenes (β-caryophyllene: found in black pepper, oregano, rosemary), diterpenes. Green Leaf Volatiles (GLVs): a group of C6 compounds produced rapidly when leaves are mechanically damaged (when a leaf is cut or bitten). The characteristic smell of freshly cut grass is a mix of GLVs (cis-3-hexenol, trans-2-hexenal, hexanal). They serve as a rapid alarm signal for nearby plants. Nitrogen compounds (volatile alkaloids, amines): less widespread but important as stress signals. Methyl jasmonate and methyl salicylate: volatile hormones that diffuse through the air and activate defenses in nearby plants (methyl jasmonate: defense against herbivores; methyl salicylate: defense against fungal pathogens). The characteristic smell of herbivore-attacked plants is partly due to methyl jasmonate emission.

Aerial Communication Between Plants: Key Experiments

Farmer and Ryan Experiment (1990, PNAS): tomato plants exposed to methyl jasmonate vapors (without physical contact) increased production of protease inhibitors in their own leaves: exactly the same defensive response that occurs when the plant is directly attacked by herbivores. Communication happened through the air. Karban et al. Experiment (2000) on California sagebrush: sagebrush (Artemisia tridentata) under herbivore attack emitted VOCs that induced a defensive response in nearby sagebrush plants (but not in plants of other species). The response was stronger in nearby sagebrush plants compared to those farther away. Experiment with plant tissue in bags: tobacco plants placed in bags impermeable to VOCs did not respond to nearby plant attacks, while those with VOC-permeable bags did. Direct proof that the signal traveled through volatiles. The solidarity or parasitism experiment?: a controversial aspect: if the nearby plant responds to the alarm signal by preparing its defenses, does the emitting plant gain any advantage? The answer isn't obvious: if the nearby plant is a competitor, the "signal" warning it could reduce damage to the competitor without benefit to the emitter. Some researchers propose that VOC emission into the air is an accidental byproduct of internal signaling (volatiles "seep" through the leaf cuticle) rather than an intentional communicative signal.

VOCs and Plant-Insect Interactions: Tripartite Communication

One of the most fascinating and best-documented functions of plant VOCs is communication with insects, both herbivores and their predators. Pollinator recruitment: flowers produce specific VOC blends that attract the specific pollinators for that species. Each flower species has a distinctive "scent" that combines dozens of VOCs in precise proportions. Bees perceive scents that humans don't (in UV and olfactory infrared bands). Deceptive flowers: some orchids produce VOCs that mimic the sexual pheromone of females of specific bee or wasp species, attracting males for pseudocopulation (pollination by deception). The most evolved pollinator deceiver in the plant world. Herbivore predator recruitment (HIPVs: Herbivore-Induced Plant Volatiles): when a caterpillar eats a leaf, the plant emits a specific mix of VOCs that attracts parasitoid wasps of the caterpillar. The mix is specific to the herbivore species: the plant "recognizes" the type of attack (from the caterpillar's oral secretions) and emits the specific VOCs to attract the natural predators of that caterpillar. Dicke et al. Experiment (1990): lima bean plants attacked by predatory mites emit VOCs that attract predatory mites of herbivorous mites. An extraordinary alert and reinforcement call system. "Informant" or "cry for help"?: the VOC blend emitted by an attacked plant is different from the blend emitted under normal conditions, and parasitoids use it to find infested plants (their prey). It's a form of tripartite communication: plant → VOC → parasitoid → herbivore.

VOCs and Induced Defense: Plant Aerial Immunity

The defensive response induced by VOCs in nearby plants is studied as a form of "external immunity" or "defensive priming". Priming: the plant exposed to alarm VOCs doesn't immediately activate defenses (which cost energy), but "prepares itself" (priming) to activate them much faster if directly attacked afterward. Priming produces a faster and more intense defensive response to initial aggression. Priming costs: priming has an energy cost (maintaining response mechanisms "ready") but is lower than the cost of keeping defenses always active. An efficient preparation strategy. VOCs and defense in agriculture (push-pull approach): the Push-Pull system (ICIPE, Kenya) uses plants that emit VOCs repellent to herbivores (pushing the pest away from the main crop) and trap plants that attract parasitoids of herbivores (pulling predators into the field). Developed to combat striga (parasitic plant) and Lepidoptera larvae in African maize. Documented yield increases of 30–40% with reduced pesticide use. Applications in organic farming: spraying methyl jasmonate or methyl salicylate on crops as a "primer" of defense is studied as an alternative to pesticides for some crops. Effective under experimental conditions, more difficult in agricultural practice due to costs and logistics. Plant consortium with priming effects: some crop rotations and some plant associations (e.g., corn-bean-squash, the "Three Sisters" of the Americas) could exploit aerial chemical communication for mutual defensive priming. An emerging field of research in agroecology.

Plant VOCs and Human Health: Phytoncides and Biophilia

VOCs emitted by trees (especially coniferous forests) have documented effects on human health. Japanese research on Shinrin-yoku (forest bathing): studies by Qing Li (University of Tokyo) have shown that exposure to forest VOCs (especially terpenes: α-pinene, limonene, β-caryophyllene from conifers) during forest walks produces: increased activity of Natural Killer cells (NK cells) of the immune system (up to 50% increase measured in subjects exposed for 2–3 days), reduced cortisol (stress hormone), reduced heart rate and blood pressure, improved mood and cognitive function. Hypothesized mechanisms: inhaled terpenes enter the bloodstream through the lungs and have direct effects on the immune system and nervous system (effects on opioid receptors and cytokines). Phytostimoline (an extract of VOCs from sprouting grain) is used in dermatology to accelerate skin healing. In Japan, forest therapy (Shinrin-yoku) is a recognized medical practice with certified pathways in specific forest areas. In Italy, some regions (Trentino, Valle d'Aosta) have developed "forest therapy" programs with regional recognition. The connection between nature exposure and health (biophilia: E.O. Wilson's concept) finds a concrete biological basis in the chemical language of plants.

VOCs and Climate Change: A Feedback Cycle

Forest VOCs play a significant role in atmospheric chemistry and the carbon cycle, with implications for climate change. Isoprene and terpenes: global forests emit approximately 500 million tons of isoprene per year (the main natural VOC emission, comparable in volume to industrial methane emissions). In the atmosphere, isoprene reacts with ozone and OH radicals producing secondary organic aerosols (SOA) that contribute to cloud formation and atmospheric albedo (reflectivity). The forest making its own clouds: European pine forests emit terpenes that form SOA that nucleate water droplets, contributing to fog and cloud formation in forest areas. A positive feedback mechanism: more forest → more SOA → more clouds → more rain → more forest. Loss of forests breaks this feedback. Climate warming and VOCs: warming increases isoprene and terpene emission from trees (their production increases with temperature). This increases SOA formation and tropospheric ozone (a potent short-term greenhouse gas). A positive feedback cycle: warming → more VOCs → more ozone → more warming. Understanding this cycle is critical for accurate climate modeling.