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Effects of Organic Pollutants on Photosynthesis
Rupal Singh Tomar 1, Bhupendra Singh 1, and Anjana Jajoo 1,2
1 School of Life Science, Devi Ahilya University, Indore, Madhya Pradesh, India
2 School of Biotechnology, Devi Ahilya University, Indore, Madhya Pradesh, India
1.1 Introduction to Organic Pollutants
Life on earth is powered by the process of photosynthesis. For more than billion years, life on earth has been transformed by the photosynthetic organisms. Photosynthetic organisms like cyanobacteria, algae, and plants harvest sunlight and produce oxygen and organic molecules, which are responsible for life on earth. Photosynthesis starts with the absorption of light of the visible region coming from the sun. It includes several partial processes such as splitting of water to molecular oxygen, electrons, and protons, which participate directly in the electrochemical reactions leading to phosphorylation and fixation of carbon dioxide into sugars.
Plants are sessile organisms that cannot move and thus cannot avoid exposure to fluctuating environmental conditions. Plants face several abiotic stress factors, such as water deficit (drought), excess water (flooding/water logging), extremes of temperatures (cold, chilling, frost, heat), high salt, mineral deficiency, and toxicity. Because of climate changes, it is predicted that these abiotic stresses may become more intense and frequent. Climate change, occurring either naturally or anthropogenically, poses serious challenges for agriculture all over the world.
During the last decades, environmental contamination has become one of the major problems on this planet. Anthropogenic activities have led to an abundance of soil, water, and air pollutants, factors that directly affect plants. Amongst these, environmental organic pollutants (OPs) have an immense effect on plant growth and development. OPs and their transformation products have been the most investigated environmental pollutants in last two decades. They accumulate in humans, animals, and plants as they are hydrophobic and lipid-soluble, and they biomagnify as they move up the food chain. OPs can be found everywhere on earth as they can travel great distances in both air and water. OPs have been found to cause serious disorders in mammals such as cancer and endocrine disorders. It is therefore essential to understand how these contaminants enter and move in the ecosystem and environment. Plants are capable of taking up, transforming, and accumulating environmental pollutants such as OPs. Several physiological and biochemical reactions in plants are influenced by OPs in the same way as other toxic compounds such as metals. They can change the energetic metabolism of plants and are associated with growth and development. About 90% of the OPs accumulate in the soil due to their hydrophobic nature, because of which they rapidly associate with solid particles of soil and permeate to bottom sediments. Several studies have been carried out on the uptake of OPs by plants and their toxicity to plants cells. Here we present an updated account of these studies, focusing on (i) the uptake of OPs by the plants and (ii) their harmful effects on the photosynthetic reactions.
The rapid growth in chemical and agrochemical industries has resulted in the release of a large number of new and toxic chemical compounds into the environment. These OPs are getting significant attention in environmental and engineering research. Several countries and international organizations have published lists of harmful pollutants, which are at alarming levels and should be controlled immediately. The group of organic chemicals discussed here include the pesticides, antibiotics, bisphenol A (BPA) and polycyclic aromatic hydrocarbons (PAHs) (Figure 1.1). Several internal and external factors regulate normal development and productivity of plants. External factors include natural and man-made chemicals that have detrimental effects on plants. OPs are a major threat in terrestrial as well as aquatic ecosystems. They quite easily cross the cell membrane of plant and animal cells due to their lipophilic character resulting in substantial bioconcentration. Plant roots and leaves serve as a major sink for these pollutants. Plants and bacteria are both involved in the biogeochemical cycling of OPs. Uptake of OPs depends on cell size, temperature, and their hydrophobicity (Dachs et al. 2002; Gang and Xitao 2005). The toxic effect of OPs may be a result of direct interaction, or some OPs may accumulate into the plant tissue to a toxic level and can affect the plant development at any stage (Hanano et al. 2015). OPs contaminate water, soil, and sediments and thus become a major environmental problem that needs to be rectified. In more recent years, the studies with these OPs are more focused on the characterization of toxicity response in a variety of plant and animal species. Scientists are also trying to explore some plants species to degrade or at least to detoxify (phytoremediation) these OPs to protect other organisms from the adverse effects of these compounds.
Figure 1.1 Broad classification of organic pollutants (OPs), based on their effects on plants and photosynthetic machinery.
1.2 Characteristics of the Organic Pollutants
All OPs are synthetic chemicals, many are pesticides, while some others are products or by-products of industrial processes or of incomplete combustion. They are quite persistence in the environment, and it may take a long time, up to several decades or even centuries, for their degradation. OPs have been found in tissues or environmental samples from almost all parts of the world. They are lipophilic in nature and have a tendency to remain in lipid-rich tissues. This affinity for the fat tissues suggests that most likely, OPs will accumulate, persist, and bioconcentrate, and eventually could reach toxicologically significant amounts. In nature, OPs enter the food chain and prove to be toxic to plants, animals, and human beings.
Because of their unique physicochemical characteristics, OPs are either adsorbed on atmospheric particles or exist in the vapor phase, which facilitates their transport over larger distances in the atmosphere. Very low water solubility and high affinity for lipids lead to their accumulation in the tissues (El-Shahawi et al. 2010). From the atmosphere, they can be transferred to the ground surface either by dry (e.g. flying ashes) or by wet (through rainout/washout) deposition. They are, however, easily deposited on solid particles such as ash, dust, and soil. They have fair solubility in organic fluids such as fats, oils, and liquid fuels. This implies that there will be more OP content if more solid particles and organic liquids are present in the water (Katsoyiannis and Samara 2005). Interestingly, they have also been detected in snow and ice at the North Pole, along with the animals of the North Sea. This shows that they traveled long distances to reach that location, as nobody has used them in the polar regions (Kumar et al. 2005; Katsoyiannis and Samara 2005). It is reported that from the environment, 45% of PAHs are taken up by the plants (Wagrovski and Hites 1997).
1.3 Sources of Organic Pollutants
Two possible natural sources of OPs are volcanic activity and forest fires. Several industrial sources also pave the way for their entry e.g. power stations, incinerating plants, agricultural sprays, and thermal stations. Sometimes, humans also contribute to OPs unintentionally through chemical factories, wastes from the use of obsolete oil, fly ash, cement plants, sewage sludge, products from incinerators, and burning of fossil fuels (Ying et al. 2005; Wenzel et al. 2006).
Thus, important emission sources of OPs are: combustion processes, industrial production processes, energy production emissions, and open burning process emissions. These sources account for just over half of total PAH emissions and more than one third of the total dioxin and furan emissions. Apart from these, there are agriculture sources and waste incineration emissions too. Industrial processes and product use sources account for half of the polychlorinated biphenyl (PCB) emissions.
1.4 Uptake and Accumulation of Organic Pollutants in Plants
About 80% of the land surface on earth is covered with green vegetation having a cuticle rich in lipids. Plants have a significant role in global distribution and cycling of various OPs. Plants absorb OPs either from soil (through roots) or air (through leaves) (Figure 1.2). OPs, such as PAHs, BPA and antibiotics are mainly absorbed by plants through roots because of their low volatility. The plant root generally comes in contact with OPs first and so absorption through roots is the commonest way of uptake. Plants may sequester these pollutants through several routes. It includes (i) uptake from soil, transfer to plant roots and thereafter translocation within the xylem, (ii) deposition from the atmosphere on the leaf surface or uptake through stomata and further translocation through the phloem (Simonich and Hites 1994, 1995). The route of uptake depends on the physicochemical properties of the pollutant, property of the soil, and the plant species. For lipophilic OPs, however, the main pathway of accumulation is transfer from atmosphere to plant (Ockenden et al....
