BIOLOGICAL ASPECTS OF CONSTRUCTING VOLATILE ORGANIC-COMPOUND EMISSIONINVENTORIES

The emission of volatile organic compounds (VOCs) from vegetation is s ubject to numerous biological controls. Past inventories have relied h eavily on empirical models which are limited in their ability to simul ate the response of organisms to short- and long-term changes in their growth environment. In this review we consider the principal biochemi cal, physiological and ecological controls over VOC emission with spec ific reference to how such controls can be included in ecosystem-level inventories. A distinction is made between longer-term biological con trols over basal VOC emission rates (rates determined under a standard set of environmental conditions) and instantaneous biological and env ironmental controls over instantaneous VOC emission rates (rates deter mined at the prevailing, instantaneous set of environmental conditions ). Emphasis is placed on the emission of isoprene and monoterpenes. Is oprene emission occurs essentially without a leaf reservoir and is tig htly linked to instantaneous photosynthetic metabolism and the activit y of isoprene synthase, the enzyme that underlies isoprene production. At present, there are still large uncertainties about which of these controls dominates isoprene emission rate. Ecosystem-level inventories of isoprene emission would be best handled through consideration of ( 1) the early season induction of isoprene emission, (2) seasonal and s patial variability in light, nitrogen and water availability and their influences on the basal emission rate, and (3) the influence of insta ntaneous changes in light and temperature on the basal emission rate. Monoterpene emission occurs from a large leaf reservoir, is uncoupled from instantaneous controls over biosynthesis, and is likely linked to whole-plant carbon allocation patterns. Because of the well-defined r ole of monoterpenes as herbivore deterrents and their linkage to plant carbon balance, there is promise for ecosystem-level inventories base d on biological resource allocation models and evolutionary cost-benef it models. Biological sources for several other VOCs have been identif ied, including methanol, methylbutenol, hexenol, acetone, and formic a nd acetic acids. However, the controls over these emissions have yet t o be determined, and there is no current basis for mechanistic invento ry development. From the studies reviewed here we conclude that the in corporation of mechanistic biological controls in future VOC inventori es will improve their capacity to predict emissions across complex eco logical gradients.