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Vol. 17 No. 2 - April 2011

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.

Photo 1: Free-air ozone fumigation site in University of Eastern Finland, Kuopio campus.

Photo 2: Typical ozone injuries in birch (Betula pendula) leaves

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: elina.oksanen@uef.fi
**Plant Physiology Lab, National Botanical Research Institute (CSIR-NBRI), Lucknow, India E-mail: v.pandey@nbri.res.in

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

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