Physiological and Growth Responses to Pollutant-Induced Biochemical Changes in Plants: a Review
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Pollution, 6(4): 827-848, Autumn 2020 DOI: 10.22059/poll.2020.303151.821 Print ISSN: 2383-451X Online ISSN: 2383-4501 Web Page: https://jpoll.ut.ac.ir, Email: [email protected] Physiological and Growth Responses to Pollutant-Induced Biochemical Changes in Plants: A Review Mulenga, C.1,3*, Clarke, C.2, Meincken, M.1 1. Department of Forest and Wood Science, Stellenbosch University, Bag X1 Matieland 7602, Stellenbosch, South Africa 2. Department of Soil Science, Stellenbosch University, Bag X1 Matieland 7602, Stellenbosch, South Africa 3. Department of Biomaterials Science and Technology, Copperbelt University, P. O. Box 21692, Kitwe, Zambia Received: 19.05.2020 Accepted: 01.07.2020 ABSTRACT: Industrial activities compromise the ambient air quality at a local, regional and global level through gaseous and dust emissions. This study reviews uptake mechanisms and the associated phytotoxicity of pollutants in plants, focusing on heavy metals and SO2. It further describes detoxification mechanisms and the resultant biochemical and physiological changes in plants. Finally, the morpho-physiological and growth responses to stress-induced biochemical changes are discussed. Heavy metals and SO2 enter the plant tissue through the stomata, cuticular layers, lenticels and root hairs. In 2- 2- the plant cells, SO2 converts to SO3 or SO4 ions upon reacting with water molecules, 2- which in excess are toxic to plants. However, the detoxification process of SO3 increases the production of reactive oxygen species (ROS). ROS are toxic to plants and damages biomolecules such as lipids, proteins, carbohydrates and DNA. On the other hand, heavy metals, such as Cu and Fe catalyse the Fenton/Haber-Weiss reactions, breaking down H2O2 into OH•. Additionally, Pb and Zn inhibit the activities of ROS-detoxifying enzymes, while other heavy metals bind to cellular layers making them rigid, thereby reducing cell division. Therefore, pollutant toxicity in plants affects biochemical parameters damaging organic molecules and limiting cambial activity. Damaged biomolecules inhibit the plant's capacity to carry out physiological functions, such as photosynthesis, stomatal functions, transpiration and respiration while impaired cambial activity reduces cell division and elongation resulting in reduced plant growth and productivity. Keywords: Heavy metals, SO2, biomolecule damage, physiological functions, cambial activity. INTRODUCTION oxides (NOx), Carbon dioxide (CO2) and Industrial pollution sources produce a wide Carbon monoxide (CO). Comparatively, the range of pollutants in a combined form composition of dust emissions may include including gases and dust emissions heavy metals, such as Zinc (Zn), Manganese depending on the industrial processes of a (Mn), Copper (Cu), Iron (Fe), Nickel (Ni), particular site. Gaseous pollutants may Mercury (Hg), Cadmium (Cd), Chromium account for Sulphur dioxide (SO2), Nitric (Cr), Lead (Pb) and Aluminium (Al). Most pollutants emanating from industrial * Corresponding Author, Email: [email protected] processes are aerodynamic (Ncube et al., 827 Mulenga, C. et al. 2014) and are therefore dispersed by wind polluted environments. Seyyednejad & and the constituents deposited across the Koochak (2013) argued that plant response nearby landscape. Gaseous pollutants and to environmental toxicities depends on particulate matter become available for factors, such as plant species and age, the uptake by plants via the leaf stomata and pollutant type and exposure conditions, such cuticles, bark lenticels and root hair. as duration, levels and season. The observed Heavy metals, such as Mn, B, Fe, Mo, changes in morphological and physiological Ni, Cu and Zn are required in trace amounts characteristics suggest the activation of as plant micronutrients (Etienne et al., 2018; strategic adaptation mechanisms in Tripathi et al., 2015), while non-essential mitigating the detrimental effects of toxicants elements like Pb, Hg, Al, Cr, and Cd are in a stressful environment (Leghari & Zaidi, toxic to plant life even in traces 2013). (Emamverdian et al., 2015). Conversely, The phytotoxicity of SO2 and heavy research has shown that even those required metals in plants can be traced from in traces may become toxic to plants at biochemical reactions at the cellular level. elevated levels (Emamverdian et al., 2015; However, most reviews focus on the growth Kulshrestha & Saxena, 2016). According to and yield changes as a response to pollution Tripathi et al. (2015), Fe in excess of 2000 stress. This manuscript delineates the plant mg/kg in plants is considered toxic, while morpho-physiological and growth alterations Cu, Zn, Mn and B toxicity threshold stands in response to pollutant-induced biochemical at 25 mg/kg, 120 mg/kg, 200 mg/kg and 80 stress. It first reviews uptake mechanisms mg/kg, respectively. Further, non-essential and the associated phytotoxicity of pollutants elements exhibit phytotoxicity at such low in plants focusing on heavy metals and SO2. concentrations as 3 mg/kg (Al), 10 mg/kg Subsequently, enzymatic and non-enzymatic (Cd), 28 mg/kg (Pb) and 100 µg/kg (Cr) attempts at pollutant detoxification and the (Amari et al., 2017; Shanker et al., 2005). resultant changes in biochemical and Similarly, at low concentration SO2 physiological reactions are described and stimulates physiological processes and finally, the morpho-physiological and growth growth of plants growing in sulphur (S) responses to stress-induced biochemical deficient soil where sulphate is metabolised changes are discussed. to meet the demand for S as a nutrient (Khan & Khan, 2011). On the other hand, Uptake Mechanisms and Phytotoxicity 2- 2- of Sulphur Dioxide and Heavy Metals excess sulphite (SO3 ) or sulphate (SO4 ) ions resulting from increased uptake of SO2 Sources and Dispersion of by plants are toxic and affect plant growth Environmental Pollutants and productivity (Brychkova et al., 2007). There are two primary sources of Studies show that exposure to air pollutants in the environment, namely pollution alters the biochemical natural and anthropogenic sources. (Seyyednejad et al., 2013; Wang et al., 2009; Putrefying organic matter, volcanic Woodward & Bennett, 2005), morphological eruptions and solar action on seawater (Ahmed et al., 2016; Leghari & Zaidi, 2013; constitute the natural sources (Cullis & Pourkhabbaz et al., 2010; Salam et al., 2016; Hirschler, 1980; Singh et al., 2012), while Saleem et al., 2019) and physiological processes, such as smelting, refining and (Gupta & Sarkar, 2016; Sen et al., 2017; fossil fuel combustion account for human- Thakar & Mishra, 2010; Pourrut et al., 2011) induced sources (Gheorghe & Ion, 2011). characteristics of plants. Ultimately, the Figure 1 shows the most common sources pollutant-induced changes affect the growth of environmental pollutants, such as SO2, and productivity of plants growing in air NOX, VOCs, heavy metals, CO and CO2. 828 Pollution, 6(4): 827-848, Autumn 2020 Fig. 1. Common sources of natural and anthropogenically-induced environmental pollutants 1 Environmental pollutants such as SO2 singlet oxygen atoms- O2 (Das & and heavy metals emanating from Roychoudhury, 2014) another family industrial activities are aerodynamic and member of the toxic ROS, which also dispersed by the wind across a given includes hydroxyl radicals (OH•). Singlet landscape (Ncube et al., 2014). Therefore, oxygen causes severe damage to proteins, weather conditions and topography play an lipids, pigments and nucleic acids (Dogra essential role in dispersing these pollutants & Kim, 2020) putting photosystems and across landscapes around fixed pollutant photosynthetic machinery into jeopardy sources (Cassiani et al., 2013). (Das & Roychoudhury, 2014). On the other •– hand, O2 transforms into the more toxic 1 • Uptake and Toxicity of Sulphur Dioxide and reactive O2 and OH , which causes in Plants lipid peroxidation (Halliwell, 2006). SO2 enters the plant tissue either through H2O2 has two roles, depending on its leaves or roots (Lee et al., 2017). In leaves, intercellular concentration. It regulates the it penetrates through the stomata regulated signal for physiological processes such as by the guard cells. It further diffuses into 2- photorespiration, photosynthesis and the mesophyll cells and converts to SO3 2- senescence (Das & Roychoudhury, 2014; or SO4 ions upon reacting with water Tanou et al. 2009) at low concentration. At molecules (Khan & Khan, 2011). The high intercellular concentration, H2O2 internal (mesophyll) resistance to SO2 is oxidises methionine (-SCH3) and cysteine low, because it is highly soluble and (-SH) residues and inactivated Calvin cycle dissolves rapidly in the cell sap. Slow 2- enzymes iron superoxide dismutase (Fe- conversion of SO2 to SO3 leads to their 2- SOD) and Cu/Zn-SOD by oxidising their oxidation to SO4 and consequently thiol groups. Bienert et al. (2007) noted utilisation by plants (Brychkova et al., that the longer half-life of H2O2 enables it 2007; Friend, 1973; Khan & Khan, 2011). 2- 2- to traverse long distances and across cell Excess SO 4 or SO 3 is toxic to plant life, membranes through aquaporins covering although the latter is about 30 times more significant lengths within the cell and toxic (Thomas et al., 1943). cause oxidative damage. They further According to Omasa et al. (2008), the 2- 2- argued that H2O2 is responsible for detoxification process of SO3 to SO4 programmed cell death and 50% loss of increases the production of toxic reactive enzymatic activities at a high cellular