Abstract:

The classification, biological functions and the ecological roles in the atmospheric chemical processes of biogenic volatile organic compounds (BVOCs) emitted from plants of terrestrial ecosystems are summarized in this paper. BVOCs are grouped according to their structure and atmospheric lifetime into four categories: isoprenes, monoterpenes, other reactive BVOCs and other less reactive BVOCs. BVOCs are emitted from a diverse array of plants, and these emissions are affected by environmental factors and the chemical traits and synthesis mechanisms of different BVOCs. From a plant energetic standpoint, BVOC emissions may substantially reduce the amount of carbon that is fixed by vegetation and consequently may strongly affect plant productivity. Why plants give off such a relevant amount of resources is still a matter of debate. The most accepted opinion is the Thermotolerance Hypothesis, which states that isoprenes protect photosynthesis from damage caused by high leaf temperatures. Thylakoid membranes become leaky at moderately high temperatures, and isoprene could reside in thylakoid membranes for a time and enhance hydrophobic interactions. The second common hypothesis is that BVOCs serve as an antioxidant in leaves due to the rapid reaction of isoprenes with ozone and hydroxyl radicals. BVOCs play an important role in some ecological processes, such as deterring herbivores and attracting pollinators. More recently, isoprenes emitted by plants are thought to be associated with the enhancement of nitrogen assimilation from the atmosphere. This idea is based on the fact that early successional forest communities emit large amounts of isoprenes as a means for converting nitric oxide (NO) emitted by soils to available forms of nitrogen, such as nitrogen dioxide (NO 2), nitric acid (HNO 3), and various organic nitrates in the canopy atmosphere, which are assimilated by plants. In spite of all these hypotheses, the biological function of BVOCs still is unclear due to the lack of direct evidence. BVOCs also play an important role in atmospheric chemical processes. In the presence of NOx, BVOCs react in the atmosphere to form tropospheric ozone, an important pollutant in the atmosphere. These reactions may also cause a decrease in the concentration of the hydroxyl radical (OH) which could lead to the accumulation of methane and other greenhouse gases. A further consequence of these reactions is the formation of secondary organic aerosol particles, which has an influence on the regional climate as well as on the atmospheric environment. Compared with other areas, research on BVOCs in China is in its beginning state, but some basic work has been conducted. The work includes the identification of BVOCs emitted from some important plants and factors influencing their emission, BVOC flux at different scales and in various ecosystems, and ozone concentrations in some areas, which are thought to be related to BVOC emissions. In the future, research on BVOCs should focus on the following aspects: 1) BVOC surveys from different plant groups and their chemical and physical properties in the atmosphere; 2) The biosynthesis and metabolic mechanisms of BVOCs in plants; 3) The roles of BVOCs in plant-environment interactions; 4) Enhancement of the study of BVOCs in some unstudied regions, such as tropical Asia; and 5) Modeling of BVOCs.

Key words: Isoprene, Monoterpene, Ozone, Atmospheric chemistry, Environmental stress, Climate, Global change