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" From Guenther (1999) and references therein. h Other VOCs include all volatile organic compounds other than methane, isoprene, and monoterpenes.

" From Guenther (1999) and references therein. h Other VOCs include all volatile organic compounds other than methane, isoprene, and monoterpenes.

muth, 1998; Fall, 1999). These influences of light and temperature are believed to be major factors in the variable emissions of isoprene measured in field studies (e.g., Lamb et al., 1996; Guenther et al., 1996b; Geron et al., 1997; Owen et al., 1997; Guenther and Hills, 1998; Drewitt et al., 1998).

This dependence on light levels and temperature is believed to be due to the mechanism of production of isoprene in the plant, which involves the enzyme isoprene synthetase and dimethylallyl diphosphate (DMAPP) as a precursor to isoprene (e.g., see Silver and Fall, 1995; and Monson et al., 1995). Either the enzyme, the formation of DMAPP, or both may be light sensitive (Wildermuth and Fall, 1996). The temperature effect has been attributed to effects on the enzyme, increasing its activity initially and then leading to irreversible denaturation (and/or possibly membrane damage) (Fall and Wildermuth, 1998).

Conifers tend to be sources of a variety of larger terpenoids, a major component of which are the C10 monoterpenes (Table 6.24). The structures of some of these are shown in Fig. 6.22. The mechanisms of the formation of these hydrocarbons in plants are closely linked, so that emissions of more than one monoter-pene often occur together. For example, geranyl a

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