Evaluation of arsenic remediation, morphological and biochemical response by Vetiveria zizanoides L. plants grown on artificially arsenic contaminated soil: A field study
Introduction
Arsenic (As) is a metalloid, considered non-essential for plants and other organisms and natural biogeochemical processes are primarily responsible for its worldwide contamination (Upadhyay et al., 2019; Shaji et al., 2020). However, various anthropogenic sources like mining, coal combustion, tanning processes, wood preservation, use of insecticides and semi-conductor industry are also significantly contributing to it (Mondal et al., 2006).
Due to toxic and carcinogenic nature of As, its contamination in soil poses a great environmental, agricultural, and health threat. Practice of usage of As laden groundwater for irrigation and agricultural activities, use of agrochemicals, and disposal of mining waste have increased its level in agricultural soils (Roychowdhury et al., 2002; Awasthi et al., 2017). The problem is even worse when As enters into the food chain and affects flora and fauna on a larger scale. In rice fields irrigated with As-rich groundwater, high As concentrations (even in seeds) and yield losses have been reported indicating the severity of the problem (Bhattacharya et al., 2010). As accumulation in crops and in turn, their consumption poses a serious risk to the health of approximately 200 million people across the globe directly or indirectly (WHO, 2012).
To ensure food safety and healthy soils, remediation and restoration of these As-contaminated soils is of great importance. Compared to conventional (physicochemical) methods, phytoremediation is a promising approach as it has in-situ capability, conserves soil structure and lacks secondary pollution (Yadav et al., 2018). Phytoremediation of As-contaminated soils may provide an eco-friendly, cost-effective and efficient alternative (Kertulis-Tartar et al., 2006).
Success of phytoremediation greatly depends on selection of appropriate plant species (Singh and Fulzele, 2021). Vetiveria zizanioides L. Nash is a fast-growing perennial grass of poaceae family having large biomass and massive root system. This plant is reported to be tolerant to various abiotic stresses like drought, salinity and water stagnation and is also being used to control soil erosion (Almeida et al., 2019; Oshunsanya et al., 2019). It also remediates heavy metals from wastewater, iron ore, red mud sludge and pesticide contaminated soils (Roongtanakiat et al., 2009; Datta et al., 2011; Banerjee et al., 2016; Gautam and Agrawal, 2017). However, the reports of its efficiency to remediate As are scanty. We have reported that in vitro grown plants of V. zizanoides can tolerate low to moderate level of As in hydroponic conditions and can accumulate As in their tissues (Singh et al., 2017). The information related to potential of fully grown V. zizanoides plants for As phytoremediation from contaminated soils having moderate to high levels is not available. Mean soil and sediment As concentrations range from 5 to 3000 mg kg−1, with the higher levels occurring in areas of contamination (WHO, 2012). However, acceptable limit for As in agricultural soils is 20 mg kg−1 as given by the European Community (Rahman et al., 2007). Therefore, the problem of soil contamination having varied levels of As needs attention and to be addressed urgently.
To the best of our knowledge, this is the first field study that explores the potential of V. zizanoides plants for As remediation from artificially contaminated soils (having moderate to high As levels) and also records the response of exposed plants with respect to growth and biochemical changes.
Despite being a non-redox metalloid, As induces generation of reactive oxygen species (ROS) through its intra conversion of ionic forms (Mylona et al., 1998) and oxidative stress is the main process for As toxicity in plants (Requejo and Tena, 2005). In plants, metal induced lipid peroxidation alters membrane structure and also modifies their enzymatic and transport activities (Maleki et al., 2017). Membrane permeability alterations lead to water stress, which further results in increase in proline levels (Hayat et al., 2012). Accumulation of proline is considered as a stress indicator including heavy metals having important protective roles (Zouari et al., 2016).
To cope with As induced oxidative stress, plants have a network of various antioxidants, enzymatic as well as non-enzymatic (Verbruggen et al., 2009; Awasthi et al., 2017). Antioxidant enzymes ‘i.e.’ superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol peroxidase (GPX) and catalase (CAT) play crucial role in protecting plants from oxidative stress and cell injury (Singh et al., 2017). SOD dismutates superoxide radical to oxygen and H2O2, while CAT and peroxidases take part in the reduction of H2O2 (Sharma and Dietz, 2009).
Besides antioxidative enzymes, various non- enzymatic antioxidants ie. cysteine, ascorbic acid and thiols also take part in providing defense to plants by scavenging free radicals. In photosynthetic systems, carotenoids also play important role in antioxidant defense (Maoka, 2020). GSH (glutathione) is an important low molecular weight thiol in plants and it acts as a reductant in the process of As(V) reduction and converted to GSSG (oxidixed glutathione) in this detoxification process (Bleeker et al., 2006).
In this backdrop, this study was designed to investigate efficiency of V. zizanoides plants to accumulate and tolerate As from artificially contaminated plots having varied (moderate to high) concentrations of As. Plant's antioxidative response to As stress is also studied here. This study will help in understanding the detoxification response of V. zizanoides plants grown on As contaminated soil and designing further phytoremediation strategies for real contaminated sites.
Section snippets
Description of experimental set up
Present study was conducted in the experimental field of Bhabha Atomic Research Centre (BARC, 19.0760° N, 72.8777° E), Mumbai, India. For the experiment, six plots of 2 × 1 × 1 m (l × b × h) size were dug, according to the plantation requirement having 45 × 45 cm spacing between plants in each pot (Fig. 1). Thick plastic sheets (175 micron) were spread in the dug plots to prevent any loss of As levels during irrigation and also to prevent leaching of As to the ground water and nearby areas.
Results and discussion
The results of the present study to assess the feasibility of V. zizanoides plants for As removal from artificially contaminated soil are being presented here. At each exposure, As removal enhanced with increase in duration and maximum accumulation was found after 90 d (Fig.2A). As level in agricultural soils of West Bengal, India have been reported in the range of approx. 50 mg kg−1 (Srivastava et al., 2014; Upadhyay et al., 2019). In the present work, at 50 mg kg−1As exposure, plants could
Conclusion
In the present study, plants of V. zizanoides were found as efficient As accumulator when grown on As contaminated soil. The growth response as well as biochemical analysis revealed that the plants have not experienced metalloid induced toxicity and were found tolerant to As exposure. It is also demonstrated that antioxidant system was stimulated in plant parts and a strong positive correlation has been observed in comparison with As accumulation. V. zizanoides plants growing at 50 mg kg−1 As
Credit author statement
SS designed the experiments, carried out biochemical analysis and prepared manuscript draft. HM performed the field experiments. PS reviewed the manuscript. All authors have read and approved the manuscript.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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