Abstract
Nickel, a micronutrient essential for plant growth and development, has been recognized as a metallic pollutant in wastewater. The concentration of nickel ions in the water course, exceeding the maximum tolerable limit, has called for an alarming attention, due to the bioaccumulative entry in the water–plant–human food chain, leaving a burden of deteriorative effects on visible characteristics, physiological processes, and oxidative stress response in plants. In this work, the renewable utilization of nickel electroplating industrial wastewater effluent (0, 5, 10, 25, 50, and 100%) as a viable source of irrigation water was evaluated using a hydroponic cultivation system, by adopting Lablab purpureus and Brassica chinensis as the plant models, in relation to the physical growth, physiological and morphological characteristics, photosynthetic pigments, proline, and oxidative responses. The elongation of roots and shoots in L. purpureus and B. chinensis was significantly inhibited beyond 25 and 5% of industrial wastewater. The chlorophyll-a, chlorophyll-b, total chlorophyll, and carotenoid contents, accompanied by alterations in the morphologies of xylem, phloem, and distortion of stomata, were recorded in the industrial wastewater-irrigated groups, with pronounced toxicity effects detected in B. chinensis. Excessive proline accumulation was recorded in the treated plant models. Ascorbate peroxidase (APX), guaiacol peroxidase (POD), and catalase (CAT) scavenging activities were drastically altered, with a profound upregulation effect in the POD activity in L. purpureus and both POD and APX in B. chinensis, predicting the nickel-induced oxidative stress. Conclusively, the diluted industrial wastewater effluent up to the optimum concentrations of 5 and 25%, respectively, could be feasibly reused as a renewable resource for B. chinensis and L. purpureus irrigation, verified by the minimal or negligible phytotoxic implications in the plant models. The current findings have shed light on the interruption of nickel-contaminated industrial wastewater effluent irrigation practice on the physical and biochemical features of food crops and highlighted the possibility of nutrient recycling via wastewater reuse in a sustainable soilless cultivation.
Highlights
Phytotoxicity of heavy metals, mainly featured by the alterations of numerous physiological processes at the molecular or cellular level, including the inactivation of enzymes, blockage of functional groups at the metabolically important molecules, substitution or displacement of essential elements, and disruption of membrane integrity, has emerged to be a worldwide agenda among the scientific community (Khan et al, 2020; Rizvi et al, 2020; Wakeel et al, 2020)
The concentrations of copper, manganese, iron, and zinc fell within the range of 0.001–0.025, 0.001– 0.027, 0.002–0.031, and 0.007–0.028 mg/g DW, respectively, with the highest concentrations detected in the roots of the plant models
Significant morphological alterations in the leaves not limited to the wide openings of stomata, with swollen surface, were consistent with the findings reported by Rai and Mehrotra (2008) in the leaves of the chromium-treated medicinal plant Phyllanthus amarus and by Mondal et al (2013) in the leaves of cadmium-treated chickpea
Summary
Phytotoxicity of heavy metals, mainly featured by the alterations of numerous physiological processes at the molecular or cellular level, including the inactivation of enzymes, blockage of functional groups at the metabolically important molecules, substitution or displacement of essential elements, and disruption of membrane integrity, has emerged to be a worldwide agenda among the scientific community (Khan et al, 2020; Rizvi et al, 2020; Wakeel et al, 2020). The bio-accumulative entry and indiscriminate discharge of these heavy metals, in particular nickel, lead, and chromium, to the waterways, soils, and eventually to the food chain, from the anthropogenic activities of metal mining, smelting, and electroplating industries; fossil fuel burning; steel manufacture; emissions from vehicle; disposal of household, industrial, and municipal waste; fertilizer and organic manure application; and other miscellaneous sources, constitute a sharp and alarming health risk to the public health and ecosystems (Ceasar et al, 2020; Yan et al, 2020) This pollution becomes more drastic, in urban and agricultural catchments, which receive a variety of organic manures heavily loaded with different toxic metals. The most common toxicity symptoms in plants are necrosis, chlorosis, wilting, and disturbance of physiological processes, including photosynthesis, transport of photo-assimilates, mineral nutrition, and plant structure damage (Kasprzak et al, 2003; Rather et al, 2020)
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