Some plants emit isoprenes into the air
The hemiterpene isoprene is formed from dimethyallyl-PP upon the release of pyrophosphate by an isoprene synthase, which is present in many plants (Fig. 17.6). Isoprene is volatile (boiling point 33°C) and leaks from the plant in gaseous form. Trees, such as oak, willow, planes, and poplar, emit isoprene during the day at temperatures of 30°C to 40°C. At such high temperatures, as much as 5% of the photosynthetically fixed carbon in oak leaves can be emitted as isoprene. Isoprene emissions of up to 20% of the total photoassimilate have been observed for the kudzu vine (Pueraria lobota), a climbing plant that is grown in Asia for fodder. Together with monoterpenes and other compounds, isoprene emission is responsible for the blue haze that can be observed over forests during hot weather.
Isoprene is produced in the chloroplasts from dimethylallyl pyrophos-phate, which is formed via the MEP synthase pathway (Fig. 17.3). Isoprene synthase is induced when leaves are exposed to high temperatures. The physiological function of isoprene formation is still a matter of debate. There are indications that low amounts of isoprene stabilize photosynthetic membranes against high temperature damage. The global isoprene emission by plants is considerable. It is estimated to be about as high as the global methane emission. But, in contrast to methane, isoprene decomposes in the atmosphere rather rapidly.
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