Abstract
In recent years, nutrient management has gained much attention as a way to mitigate heavy metal stress. Silicon (Si) promotes plant defense responses against toxic metal stresses. In this study, we evaluated the effects of silicon (Si) on copper (Cu) toxicity in two flax genotypes (Sakha 1 and Sakha 2) as it relates to plant growth, yield attributes, total chlorophyll, nucleic acid content, enzymatic and non-enzymatic antioxidants, oxidative damage, lipid peroxidation, copper and silicon content, and fatty acid composition. The results showed that Cu (100 and 200 µM) inhibited plant growth and increased Cu accumulation in soil, roots, and shoots. Cu significantly decreased the yield attributes, total chlorophyll by 9.5% and 22% in Sakha 1 and by 22.5% and 29% in Sakha 2, and enhanced the accumulation of non-enzymatic (tocopherol), enzymatic antioxidants such as superoxide dismnutase, peroxidase, ascorbate peroxidase and catalase) and secondary metabolites (phenol and flavonoids). The DNA content significantly decreased in stressed plants with 100 and 200 µM Cu about 22% and 44%, respectively, in Sakha 1 and about 21.6% and 34.7% in Sakha 2, and RNA content also decreased by about 20% and 29%, respectively, in Sakha 1 and by about 2% and 13% in Sakha 2 compared to the control plant. Furthermore, Cu stress accelerated the generation of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2) and induced cellular oxidative injury caused by lipid peroxidation. In parallel, Cu induced a change in the composition of fatty acids, resulting in lower unsaturated fatty acid levels and increased saturated fatty acids (increased saturation/unsaturation ratio for both genotypes). Treating the flax plants with irrigation three times with Si protected the plants from Cu toxicity. Si treatment decreased the uptake and the transport of Cu to the shoots and harvested seeds and promoted plant growth, yield attributes, and antioxidant defense systems by reducing Cu accumulation, lipid peroxidation, and the generation of H2O2. In addition, the alleviation of Cu toxicity correlated with increased Si accumulation in the roots and shoots. In conclusion, Si can be used to improve the resistance of flax plants to Cu toxicity by up-regulating the antioxidant defense system such as superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX) and catalase (CAT) and decreasing the oxidative damage caused by reactive oxygen species (ROS).
Highlights
Heavy metal pollution has become a serious problem worldwide for food safety and the environment due to industrialization, globalization, smelters, foundries, and sludge [1]
Excess Cu induces the production of reactive oxygen species (ROS) that become harmful to plant tissues and cause oxidative stress leading to cell death
We found that the other four genotypes are moderately tolerant to copper stress but that the other two genotypes (Sakha 1 and Sakha 2) are sensitive to copper stress
Summary
Heavy metal pollution has become a serious problem worldwide for food safety and the environment due to industrialization, globalization, smelters, foundries, and sludge [1]. High concentrations of heavy metals in agricultural land can have adverse effects on plants, resulting in decreased crop yields and plant productivity [2]. The contamination of agricultural soil in Egypt depends on the type of pollution, land characteristics, climatic conditions, and the use of pesticides, mineral fertilizers, and reused wastewater in irrigation [3]. Copper (Cu) is an essential micro-element of plant growth and plays a vital role in several physiological and biochemical higher plant processes, such as antioxidant activity, cell wall metabolism, photosynthesis, the electron transport chain (ETC), mineral nutrient uptake, chlorophyll biosynthesis, and the production of carbohydrates, lipids, proteins, and nucleic acids [4]. Excess Cu results in intense phytotoxic reactions affecting plant growth and biomass, seed germination, DNA, photosynthesis, pigment production, the metabolism of nitrogen and proteins, and cell membrane mineral absorption [6,7]. A heavy metal overdose could suppress a plant’s antioxidant defense [10,11]
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