Cadmium toxicity in cowpea plant: Effect of foliar intervention of nano-TiO2 on tissue Cd bioaccumulation, stress enzymes and potential dietary health risk
Introduction
The rapid pace of industrialization and increased release of agrochemicals into the soil environment has resulted in wide-spread concerns about the likelihood of build-up of heavy metals (HMs) in agricultural soil (Wong et al., 2007). The contamination of agricultural soil with toxic HMs has become a menace throughout the world, and most importantly in developing countries due to increased agricultural activities to meet the teeming population demands. Ingestion of HMs through the food chain has been found to be the primary source of human exposure to toxic HMs in agrarian area (Liu et al., 2007). Cadmium is one of the most dangerous HMs because of its high mobility in living organisms and the small concentration at which its effects manifests (Benavides et al., 2005). It is considered a major factor in yield limitation of plants due to its high accumulation and toxicity to crop plants (Rizwan et al., 2017). Although Cd is not essential for plant growth, its bioaccumulation index in tissues of plants greatly exceeds all other HMs (Kabata-Pendias and Pendias, 1992). Cadmium may accumulate to chronic level in human body for many years, so intake of foods having high Cd content may portend potential human health risk (Jackson and Alloway, 1992).
Nanotechnology is a recent field with diverse applications in many areas of study. The advantageous use of nanoparticles (NPs) in agriculture have been proved (López-Vargas et al., 2018). Nanotechnology is being used in the application of nanomaterials such as nanofertilizers, NPs, or nanopesticides for genetic improvement, nutrient management, treatment of plant diseases, and to aid plant growth (De la Rosa et al., 2017; Rizwan et al., 2017; Munir et al., 2018). These nanomaterials can speed up seed germination, or influence plants tolerability of both biotic and abiotic stresses, promoting efficient management of nutrients, improving plant growth while lessening the environmental impact compared to the conventional methods (Genady et al., 2017).
Recently, the idea of nanoremediation using metal-based NPs as environmentally-friendly and cost-effective amendments for the remediation of metal-contaminated soils is becoming popular gradually (Adrees et al., 2015; Abbas et al., 2017; Rehman et al., 2017). Several studies have documented the potential of NPs exposure to plants in promoting growth and at the same time, reduce metal concentrations in plant biomasses when compared to control (Wang et al., 2012a, b; Konate et al., 2017; Venkatachalam et al., 2017; Rizwan et al., 2019). In addition, silicon NPs was also found to alleviate Cr toxicity in pea seedling (Tripathi et al., 2015) while the exogenous application of Fe3O4 NPs also promoted the growth of wheat and decreased metal uptake (Konate et al., 2017). Titanium dioxide has also drawn considerable research attention because of its chemical reliability, affordability, ease of access and its non-toxicity potential. Titanium dioxide (TiO2) nanoparticles has proven promising as efficient supply of plants’ nutrients for improved biomass production due to its ability to promote nitrogen assimilation, photo-reduction activity of photosystem II and removal of reactive oxygen species in plants (Raliya et al., 2015). Studies have also shown that combined application of sol SiO2 (nano-SiO2) and TiO2 (nano-TiO2) could increase nitrate reductase in soybean, promote water retention potential and enhance antioxidant system (Lu et al., 2002). Titanium dioxide applied through roots or leaves at lower concentrations has been reported to enhance crop yield by stimulating enzyme activities, improve chlorophyll contents and photosynthesis, increase nutrient uptake and heighten stress tolerance (Gao et al., 2013; Antisari et al., 2014; Ji et al., 2017).
Cowpea (Vigna unguiculata (L.) Walp.) is a legume grown extensively in the sub-Saharan Africa, and also comprise major sources of proteineous diets of Southern Nigerian people. The seeds provide valuable dietary protein of about 25 % to humans and animals (Bressani, 1985) while the leaves are important nutritious vegetable to humans (IPM CRSP, 2000). Sequel to the above, this present study aims to evaluate the potential of foliar-applied nano-TiO2 as remediation intervention for Cd toxicity in cowpea plants by assessing its effects on Cd bioaccumulation, stress enzymes, micronutrients of seeds and dietary health risk in the plant.
Section snippets
Experimental set-up
The experiment was carried out in the screen house of the Botanical garden of University of Ilorin, Ilorin, Nigeria between March and June 2019. Seeds of cowpea (Vigna unguiculata- IT07K-292-10) used in this study were c from collected from the International Institute of Tropical Agriculture, Ibadan, Nigeria. The soil used in this experiment was loamy soil that was collected from a fallow land at the Research farm of the University. The physico-chemical characteristics of the test soil are
Effect on chlorophylls and Cd accumulation
The application of sole Cd at 10 mg/kg soil did not alter the content of chlorophyll a (Chl-a) in the leaves in relation to control (Table 1). Similarly, foliar-applied nano-TiO2 (nTiO2) at 100 and 200 mg/l, respectively did not alter the contents of Chl-a. But as expected, Chl-b and T-Chl. contents were significantly reduced (p < 0.05) by 24 % and 12 %, respectively upon Cd exposure to sole Cd. This was consistent with some other reports in literature showing decrease in the pigment contents
Conclusion
The study showed that foliar application of nano-TiO2 as nanoremediation strategy reduced Cd uptake and its transport in cowpea plant under moderate Cd stress. The chlorophyll contents (Chl-b and T-Chl.) of cowpea leaves were considerably improved after nano-TiO2 intervention in the stressed-cowpea plants. Likewise, nano-TiO2 intervention promoted enzymes activity (CAT and SOD) responsible for eliminating ROS and furthermore, reduced MDA contents of root and leaf tissues after intervention.
Funding
None
Credit author statement
Clement O. Ogunkunle conceptualized the study and designed the methodology of the experiment
Deborah A. Odulaja carried out the experiment, monitor the growth stages and collected data
Funmilola O. Akande was involved in the data analysis and drafting of manuscript
Mayank Varun analyzed the data statistically and interpreted the results
Vinita Vishwakarma provided the nanoparticles and carried out the characterization
Paul O. Fatoba supervised the overall study and review the manuscript draft before
Declaration of Competing Interest
The authors declare no competing interest in this study
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