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Vol. 13 No. 1 - January 2007

VOC Emission by Plants: Significance and Implications

By: C.K. Varshney

It is widely known that green plants play a critical role in global carbon cycle by sequestering carbon dioxide from the atmosphere and converting it into organic compounds by using solar energy through the process of photosynthesis and releasing oxygen as a by product. In addition to oxygen plants from their green leaves have been shown to emit a number of complex organic compounds collectively called volatile organic compounds (VOCs).  VOCs are a complex mixture of carbon; hydrogen compounds containing chemical species (excluding elemental carbon, carbon monoxide, and carbon dioxide), which are volatile at normal temperature and pressure. In precise terms VOCs are those organic compounds whose vapor pressure range from 0.13 kPa to 101.3 kPa at 293K. VOCs also include oxygenated, halogenated and sulphur containing hydrocarbons. VOCs are basically grouped into methane and non-methane hydrocarbons (NMHCs). VOCs are emitted both from anthropogenic and natural sources. The important anthropogenic sources on NMVOCs include, fossil fuel combustion, processing of organic chemicals and organic wastes. From anthropogenic sources globally 103 Tg NMVOCs are emitted yearly. The author and his co-workers at Jawaharlal Nehru University (JNU) have estimated that from India about 8 million tonnes of NMVOCs are emitted per annum.

The natural sources of VOCs include terrestrial plants and marine organisms. Biogenic VOC emission predominantly occurs in tropics (23 S 23 N) with small amounts emitted in the northern mid-latitudes. Almost about 99 per cent of the total biogenic NMVOCs are emitted from terrestrial sources including forests, grasslands, shrub-lands and croplands. Biogenic NMVOCs comprise isoprene, monoterpene, alkane, alkene, carbonyls, alcohols, acids, esters, ethers and aromatic hydrocarbons. Global emission inventories show isoprenoids (isoprene and monoterpene as the most dominant biogenic volatile organic compounds (BVOC). Isoprene is one of the most abundant NMVOCs emitted from vegetation. Alcohols and carbonyls follow the isoprenoids as the predominant groups. Isoprene is predominantly emitted from deciduous  (hardwood/ broad leaf trees such as Oak, poplar, aspens and willows. Isoprene is not stored in plants and emitted in sunlight during photosynthesis. Monoterpenes are emitted from coniferous (softwood) trees such as pines, cedars and firs.  Monoterpenes can be stored in plants; hence, they are emitted both during day and night. There are several species such as spruce and eucalyptus, which emit both isoprene and monoterpenes. Information on VOC emission by plants from old world tropics is sadly lacking except for the studies carried out at JNU New Delhi, by the author and his coworkers who have measured foliar emission of volatile organic compounds (VOC) from common Indian plant species. Fifty-one local tree species were screened, out of which 36 species were found to emit VOC (4 high emitter; 28 moderate emitter; and 4 low-emitter), while in the remaining 15 species no VOC emission was detected or the levels of emission were below detection limit (BDL). VOC emission was found to vary from one species to another and a marked seasonal and diurnal variation was observed. The minimum and maximum VOC emission values were < 0.1 and 87 microgg(-1) dry leaf h(-1) in Ficus infectoria and Lantana camara respectively. Out of the 51 plant species studied, 13 species were reported to be VOC emitters for the first time. Among the nine tree species (which were selected for detailed study), the highest average hourly emission (9.69+/-8.39 microgg(-1) dry leaf) was observed in Eucalyptus species and the minimum in Syzygium jambolanum (1.89+/-2.48 microgg (-1) dry leaf).

Tropical and sub tropical regions are regarded as dominant source of biogenic volatile organic compounds emission (BVOC). However, measurement studies from these regions are limited and largely confined to South Africa and Amazonia. Consequently, global BVOC estimates are mainly based on modeling studies. BVOC emission estimates are altogether lacking for any region of South and South-East Asia. The Author and his students have made an attempt to estimate isoprene emission from the forest of the State of Haryana (India). Isoprene emission capacity individual plant species was found to vary from below detection limit (BDL) to 12.01 mg Cm− 2 h− 1. Maximum emission capacity (12.01 mg Cm− 2 h− 1) was observed in case of Dalbergia sissoo. The area average isoprene emission capacity for the Haryana forest was found to be 19.98 mg Cm− 2 h− 1, which is significantly (2.4 times) higher than the reported isoprene emission value of 8.2 mg Cm− 2 h− 1 for the Kalahari woodland of Africa.

NMVOC emission from vegetation is influenced by many factors, including ambient temperature sunlight CO2 concentration, genetics, leaf development, and phenological events. NMVOCs emission rates are species specific, which vary by as much as four orders of magnitude depending upon plant species. Variation in biomass density and physiological status of vegetation may also affect emission.  Accordingly, NMVOC emission from vegetation is sensitive to land cover changes (plant species composition and dominance) and environmental conditions.

The functional role of NMVOC emission from plants is not much understood. The isoprene emission has been implicated in a variety of roles including protection of photosynthetic apparatus from sudden temperature fluctuations, as flowering hormone, as an antioxidant or simply as an overflow to get rid of excess of carbon. Plants emit monoterpenes under specific circumstances for various purposes. Some of the monoterpenes released from plants have an allelopathic function i.e. control of seed germination and growth of other species to avoid competition. Monoterpenes are also known to act as defense compound against pathogens and herbivores. It has been reported that corn seedlings, which do not release terpinoids under normal growing conditions, provide a fascinating example of insect defense by monoterpenes. Corn seedlings emit monoterpenes when attacked by certain caterpillar, reacting specifically to the insect’s saliva. This emission signal attracts wasp, which deposits its eggs into the caterpillars. Thus, the corn seedlings defend their population against their predators.

Atmospheric residence time of VOCs is short – from few hours to months- hence, their direct impact on radiative forcing is relatively small, but their indirect impact due to their involvement in atmospheric photochemistry i.e. production of ozone in presence of NOx and light and production of secondary aerosols is quite significant. Biogenic volatile compounds add to the photochemical smog in the same way as emissions from human sources - there is no discrimination. Oxidation of NMVOC in the atmosphere (a) produces large quantities of CO; organic acids; hydrogen peroxide; oxygenated hydrocarbons, and secondary organic aerosols (b) increase of lifetime of methane in the atmosphere. VOCs play an important role in determining tropospheric chemistry, aerosol burden and oxidizing capacity of the atmosphere, global carbon cycle, and global climate.

In recent years studies on VOC emission by plants are receiving increasing attention on more than one account. Foliar emission of volatile organic carbon compounds from leaf directly into the atmosphere represents an interesting “atmosphere — leaf — atmosphere” sub-loop of   global carbon circulation. An assessment of the magnitude and ecological significance of this atmosphere — leaf — atmosphere” sub-loop of   carbon cycle for plants and for the atmospheric chemistry represent a fascinating area of study. Secondly, because of the dramatic increase in atmospheric CO2 concentration and impending global climate change, plant metabolism is expected to alter seriously. These changes are likely to induce unpredictable alterations in biogenic emission of non-methane hydrocarbons (NMHCs), oxygenated hydrocarbons (OxHCs) and halocarbons. It is important to ascertain the effect of elevated CO2 and temperature regime on future biogenic emissions. Such information is essential for developing reliable model simulations of carbon dynamics of plants in relation to global climate change as well as for generating scenarios of future air quality.

C.K. Varshney is Former Dean and Professor, School of Environmental Sciences, Jawaharlal Nehru, University, New Delhi –110067. email: ckvarshney@hotmail.com

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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