E9 Chemical Exchange

Topic

Chemical exchange serves as the plant's communication interface with its ecosystem and occurs in multiple directions simultaneously: the plant emits chemical compounds into the air and soil while also absorbing compounds from the surrounding air. Each of these directions of chemical exchange fulfills a distinct function for the plant.

One direction of chemical exchange involves the emission of volatile organic compounds—such as isoprene and various monoterpenes and sesquiterpenes—into the air. The plant releases these compounds in response to thermal stress (when leaf temperature deviates from its optimal range) and herbivory (when a herbivore begins feeding on its tissues). The more intense the thermal stress or herbivory the plant faces, the greater the quantity of volatile organic compounds it releases into the air.

Another direction of chemical exchange is the release of root exudates into the soil, a strategy the plant employs for both nutrition and defense. Through these exudates, the root attracts soil microorganisms that facilitate nutrient access while simultaneously repelling or inhibiting soil organisms that could cause harm.

A third direction of chemical exchange involves allelopathic signaling between neighboring plants. A plant releases chemical compounds into the air or soil that, upon reaching a neighbor, alter that neighbor's germination, growth, or physiology—either by inhibiting it to reduce competition for shared resources or by benefiting it when the signal serves as a warning of a shared threat.

The fourth direction of chemical exchange runs counter to the others: instead of emitting substances, the plant absorbs atmospheric pollutants—such as ozone or sulfur dioxide—from the surrounding air. Notably, the very pathway used to emit volatile organic compounds into the air is, in part, the same route that allows atmospheric pollutants to enter the leaf tissue. Collectively, the emission of volatile organic compounds, the release of root exudates, allelopathic signaling, and the uptake of atmospheric pollutants constitute the four modes of chemical exchange that transform the plant into an active—rather than merely passive—participant in the communication interface between the plant and its ecosystem; through these very pathways, the plant responds to heat stress and herbivory, nourishes and defends its roots, communicates with neighbors, and is exposed to atmospheric pollutants in the surrounding air.

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