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Ozone Sensitivity Of Indian Plant Species
By: Elina Oksanen*,
Vivek Pandey**
Sari Kontunen-Soppela*,
Sarita Keski-Saari*
Anthropogenic
activities are altering the composition of the atmosphere, resulting
in increased carbon dioxide (CO2)
and ozone (O3),
elevating temperature and increasing water deficits in many
agricultural areas. These changes impose difficulties for plant and
crop growth in many parts of the world. Sustainable and equitable
global food security is dependent on the selection of crop plants
with increased resistance to abiotic stresses, causing often
oxidative stress to plants.
Oxidative
stress occurs when plants are exposed to stress conditions that
induce changes in oxygen (O2)
metabolism in plant tissues. During oxidative stress reactive oxygen
species (ROS), such as hydrogen peroxide (H2O2),
are formed within plant tissues. Accumulation of ROS during severe
stress may result in irreversible damage, loss in physiological
competence and eventually cell death. However, under moderate or mild
stress, ROS formation may induce defence activation, promoting plant
adaptation to stress conditions.
Oxidative stress in
general is expected to increase during the climate change due to more
frequent occurrence of extreme temperatures, soil drought or
salinity, increasing UV-radiation, ozone concentrations, nutrient
imbalance and high light stress. Combined action of these factors may
significantly alter plant growth and development.
Tropospheric
ozone
Tropospheric
ozone is globally the most important gaseous oxidative stressor in
many areas. Ozone is a natural constituent in the atmosphere being
present in the stratosphere and throughout the troposphere.
Stratospheric ozone is "good ozone", providing protection
from UV radiation, while tropospheric ozone is a greenhouse gas and a
harmful pollutant on the earth’s surface, called also as "bad
ozone". Ozone is formed in the atmosphere by sunlight driven
chemical reactions between nitrogen oxides (NOx) and volatile organic
compounds (VOCs). These ozone precursors (compounds
that participate in the chemical reaction that produces another
compound) may be of natural origin or may be emitted as a consequence
of human activities.
As with eutrophication
and acidification, attempts have been
made
to estimate nature’s “tolerance level” to ozone exposure.
In
the case of gaseous substances these tolerance limits are expressed
as
critical levels. The critical level for ozone is calculated as
accumulated ozone exposure over a given threshold value. The
threshold has been set at 40 parts per billion (ppb). For sensitive
crops the critical level is set at 3000 ppb hours daytime during a
three-month growing season (May–July) in Europe. This exposure is
believed to result in crop losses of about 5 per cent.
In
large areas of the industrialised and developing world, ground-level
or tropospheric ozone is one of the most pervasive of the global air
pollutants, having adverse effects on human health, food production,
the capacity of forests to store carbon, and the environment even at
current ambient concentrations of 35-40 ppb.
Ozone
concentrations have doubled within the last century and continue to
rise globally at an annual rate of 0.5-2.5%. During this century,
economic growth and increasing global population will drive the
processes that lead to increasing emissions of ozone precursors such
as NOx compounds especially in new hot
spots
arising in rapidly developing Asia, Central Africa and South America.
Increasing demand for energy, transport, food and non-food crops and
other resources will generally enhance the precursor emissions from
human activity. In Asia, increases in power generation and traffic
volumes are the main reasons for larger NOx emissions. The effects of
climate change on future ozone concentrations in 2050 will be
regionally variable; ozone will tend to increase in already polluted
environments (due to dense population and high emissions) and
decrease in clean environments (due to improved technologies in
industry and traffic leading to reduced emissions). In addition to
anthropogenic sources, ozone precursor emissions are increasing from
natural sources such as lightning, soils, wetlands and vegetation,
particularly due to warming climate and during heatwave events. The
average lifetime of ozone is approximately three weeks, and therefore
it can be transported long distances, e.g. from North America to Asia
and from Asia to Europe and vice
versa.
This, in combination with the potential for ozone to be produced from
its precursors for a long time after they have been emitted, makes
ozone a global problem, where active
international cooperation is needed.
Impacts
of ozone on
plants
In crop plants, major
concerns of ozone are related to impaired primary production, smaller
leaf area and decreased photosynthesis, resulting in late grain
filling and losses in production and yield. To cope with elevated
ozone concentration, plants activate several chemical defence
processes (e.g. production of volatile organic compounds and phenolic
compounds), which results in enhanced levels of antioxidative
capacity. Investments to defence processes reduces allocation of
resources to growth and production of reproductive organs. Plants
try to avoid harmful ozone by reducing the entry of ozone to leaves,
by smaller leaf area or by structural modifications (e.g. thicker
leaves).
Much
research has been done to understand the phytotoxic action of
tropospheric ozone, which is ultimately leading to impaired carbon
fixation and accelerated aging of leaves. However, ozone experiments
have been mainly conduced with those European and North American crop
species and forest trees, which are important for human nutrition and
welfare, and for mitigating the climate change as carbon sinks. So far,
ozone sensitivity of Indian crop species is poorly known,
although the yield losses and socio-economic impacts of ozone
have
serious implications in India.
In
addition to ozone problem, oxidative
stress in general is expected to increase during the climate change
due to more frequent occurrence of extreme temperatures, soil drought
or salinity, increasing UV-radiation, nutrient imbalance, and high
light stress. Therefore, it is very important to use all available
means to diminish oxidative stress of vegetation.
Limited
information from
India
According
to Air Pollution Crop Effect Network (APCEN) assessment
(http://www.sei.se/apcen/),
in south Asia, particularly India and Pakistan, evidences of high
concentrations of ozone have been reported. However, present ozone
monitoring stations in the Indian region, concentrating on
metropolitan areas, are not sufficient to document the extent of
ozone problem in India. Some recent studies have indicated that
surface
ozone levels are much above critical levels at most places in the
Indian region.
They also showed that there is a substantial temporal and spatial
variation in ozone concentrations across the region due to
meteorological conditions and anthropogenic emissions of precursor
gases. Highest ozone concentrations have been measured in March, at
the same time with the main growing season.
Simulated
percentage loss in gross
primary productivity (GDP) due to ozone in India is expected to be
roughly 20-30% by 2100. But, species-level information is very
limited. It is known that there are clear differences in ozone
tolerance among the Indian wheat and rice cultivars. However, only
few cultivars have been tested so far.
Ozone
stress has caused reduction in biomass and yield of wheat, rice,
mung, spinach, and Indian (yellow) mustard. Increasing ozone has also
been reported to result in delayed flowering, impaired flower
production, increased flower abortion and impaired seed quality and
seed germination in several crop species. Therefore, we
urgently need more detailed information about the differences in
ozone sensitivity of Indian plant species and different varieties of
species.
New
collaboration between
Finland and India
A new collaboration has
been established between University of Eastern Finland (UEF) and
National Botanical Research Institute (CSIR-NBRI), Lucknow. It is
funded by the Academy of Finland and The Finnish Government.
The main purpose of
this project is to screen the general ozone sensitivity and tolerance
of the most important Indian crop and tree species and varieties so
that we can eventually help the farmers to select the most suitable
varieties. In parallel, we collect ozone concentration data from the
main agricultural and forested regions of India.
Both crop and tree
species will be selected for ozone sensitivity screening. Rice, wheat
and pea are the most important crop plants grown in the region (Uttar
Pradesh), and different varieties used by local farmers in this area
will be tested. From tree species, teak that is valuable for timber
will be selected. All these plants constitute a major part of
livelihood of small and marginal farmers.
For ozone exposures and
validation of results different and complementary methodological
approaches will be utilized. Laboratory experiments with elevated
ozone concentrations will be mainly conducted in Finland. Field
studies will be conducted in Lucknow, NBRI, which has two campus
sites with cultivation fields, differing in background ozone
concentrations. To evaluate the effect of current ambient ozone
concentrations, the exposure plants will be grown in these two field
plots, where ozone concentrations will be monitored regularly. One
of the ultimate aims is to set up a free-air ozone fumigation (FACE)
system in NBRI,
Lucknow, where the most realistic ozone responses of Indian species
could be studied (Photo 1).
The exposure plants will be studied for example for growth, visible
injuries (Photo 2) photosynthesis efficiency, grain quality,
anatomical properties, antioxidants and changes in chemical
composition.
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Photo
1: Free-air ozone fumigation site in University of Eastern Finland,
Kuopio campus.
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Photo
2: Typical ozone injuries in birch (Betula
pendula) leaves
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Expected
outcomes of this
ozone project
This multidisciplinary
project will produce (1) basic information about the ozone
sensitivity/tolerance of the most important Indian crop and tree
species and differences between varieties about (2) the tolerance
mechanisms that are necessary for plant breeding actions to increase
ecological tolerance of Indian plants. In addition, (3) the data will
be collected from prevailing ozone concentrations across India, and
(4) make synthesis of plant sensitivity and ozone data information to
form a reliable ozone risk assessment paper from India, that is
useful for forestry and agriculture managers and planners, and
ultimately farmers. (5) Finally, these studies will contribute to
greenhouse gas mitigation through forestry and land-use actions. In
addition to high scientific value, the results of this project would
have large economic and socio-economic impact through agricultural
improvements in food production in this highly populated developing
country needing to improve its food security.
*University
of Eastern Finland, Department of Biology, Joensuu, Finland,
E-mail: [email protected]
**Plant
Physiology Lab, National Botanical Research Institute (CSIR-NBRI),
Lucknow, India E-mail: [email protected]
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