text: Most VOCs in Earth's atmosphere are biogenic, largely emitted by plants.
  Biogenic volatile organic compounds (BVOCs) encompass VOCs emitted by plants, animals, or microorganisms, and while extremely diverse, are most commonly terpenoids, alcohols, and carbonyls (methane and carbon monoxide are generally not considered). Not counting methane, biological sources emit an estimated 760 teragrams of carbon per year in the form of VOCs. The majority of VOCs are produced by plants, the main compound being isoprene. Small amounts of VOCs are produced by animals and microbes. Many VOCs are considered secondary metabolites, which often help organisms in defense, such as plant defense against herbivory. The strong odor emitted by many plants consists of green leaf volatiles, a subset of VOCs. Although intended for nearby organisms to detect and respond to, these volatiles can be detected and communicated through wireless electronic transmission, by embedding nanosensors and infrared transmitters into the plant materials themselves.
  Emissions are affected by a variety of factors, such as temperature, which determines rates of volatilization and growth, and sunlight, which determines rates of biosynthesis. Emission occurs almost exclusively from the leaves, the stomata in particular. VOCs emitted by terrestrial forests are often oxidized by hydroxyl radicals in the atmosphere; in the absence of NO pollutants, VOC photochemistry recycles hydroxyl radicals to create a sustainable biosphere-atmosphere balance. Due to recent climate change developments, such as warming and greater UV radiation, BVOC emissions from plants are generally predicted to increase, thus upsetting the biosphere-atmosphere interaction and damaging major ecosystems. A major class of VOCs is the terpene class of compounds, such as myrcene.
  Providing a sense of scale, a forest  in area, the size of the US state of Pennsylvania, is estimated to emit  of terpenes on a typical August day during the growing season.  Researchers investigating mechanisms of induction of genes producing volatile organic compounds, and the subsequent increase in volatile terpenes, has been achieved in maize using (Z)-3-hexen-1-ol and other plant hormones.