Benzene, also called phenyl hydride, is a volatile organic compound (VOC) that occurs naturally from volcanic eruptions and forest fires, but also is a man-made chemical used to make plastics, resins, dyes, detergents and petroleum-based fabric fibers. It usually is a colorless or pale yellow liquid at room temperature and emits a sweet odor. When in the soil, benzene evaporates readily into the air, and in water it floats on the surface. Benzene is one gas monitored to determine air quality in urban areas.
Trace amounts of benzene included in the watering of plants results in increased root formation. This results in overall plant growth stimulation, according to GTZ Corporation of Germany. It is only slightly soluble in water, reinforcing the fact that minute amounts of the VOC is appropriate for this effect on plants. Interestingly, the National Aeronautic and Space Administration (NASA) found that many house plants effectively absorb and remove benzene that occurs in typical interior home or office spaces. This is why house plants may be considered "air cleaners."
In higher concentrations in the air, soil or in water, benzene can inhibit photosynthesis or block normal plant cell division and therefore stunt growth. Excessive exposure to the gaseous form can suffocate plants as the gas displaces the needed carbon dioxide that plants use to metabolize. In some plant species, if the liquid form of benzene is on foliage and evaporates, a "burn" may occur as the change in state causes a drop in temperature on leaf surfaces, partially killing tissue or leading to foliage dessication.
Ultimately, excessive exposure to benzene kills the plant. The exposure can be lethal solely through soil-, air- or water-borne means, or a combination thereof depending on duration and concentration of benzene.