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: [email protected]