The Effect of Lead and Zinc Chlorides on Some Physiological Traits on Growth and Total Protein in Soybean
Department of Basic Sciences, Islamic Azad University, Science and Research Branch, Tehran, Iran
* Corresponding author email: firstname.lastname@example.org
Received: 20 Sebtember 2017 Accepted: 18 January 2018
Among heavy metals, lead is a powerful pollutant that can be easily accumulated in the soil. Zinc is also a heavy metal but the minimum concentrations are required for plant growth. However, excessive amounts of these elements can become harmful to plants. In order to study the physiological traits, several concentrations of Lead (II) chloride (0, 0.5, 2.5, 4.5 and 6.5 mM) with different concentrations of Zinc chloride (5, 10, 15 and 20 mM) was conducted on soybean plant (Glycine max L.) growth biochemical parameters. Results were obtained from a factorial experiment in a completely randomized design with four replications under in vitro conditions. The results showed that the total protein contents decreased with increasing zinc and lead concentrations. This research concluded lead has a negative effect on this plant, and these plants has no tolerant to heavy metals stresses despite this metal uptake by roots and it has negative effects on plant performance.
Heavy metal bioaccumulation in the soil, water and atmosphere may be seriously hazardous to both human and animals with the contamination of food supply chain (Dong et al., 2014). Recent studies show that, lead (Pb) is a nonessential toxic heavy metal with large solubility in water that alters the morphology and physiology of plants (Kamran et al., 2015), which quickly absorb and compound into plant tissues and transfer to aerial organs where it can cumulate to high levels in contrast with mercury (Zhang et al., 2014). The most typical symptom of Pb toxicity in plants is stunting, chlorosis and necrosis in the leaves, Chlorosis occurrence by the excess level of Pb appears to be due to a direct or an indirect interaction of Pb with Fe content (Elavarthi, and Martin, 2010). lead stress can considerably cause growth reduction, inhibition of seed germination, also prevents photosynthesis activity, respiration, cell proliferation, plant water relationship and mineral nutrition, leading to poor growth and low biomass (Kamran et al., 2015). Although soybean (Glycine max L.) is a staple food in diets of many parts of the world as a source of polyunsaturated and saturated fatty acids. It is a complex mixture of triglycerides with vegetable proteins and minerals such as calcium so the cultivation of this plant has been more widely considered among other legumes. Bojinova et al. (1994) reported that soybean and other beans belong to a group of crops that strongly accumulate heavy metals. Therefore, it is more important to recognize of elements that are involved in contaminated soils in order to eliminate contamination to reduce the absorption of harmful metals, as well as that affect the reduction of lead absorption and reduce symptoms of its toxicity in plants (Arif et al., 2012). Zinc (Zn) is the second most common metallic metal in organisms and is the only metal found in all six enzymes (oxidoreductases, transferases, hydrolases, lysates, isomers, lignases) (Arif et al., 2012). Dang et al., In 2014, showed that although zinc at low concentrations can show the positive effects but toxicities will occur when the Zn concentration exceeds (Arif et al., 2012). Therefore, the objective of this study was to investigate the effect of lead and zinc application on soybean growth and yield and evaluate the morphological characteristics of this plant.
Materials and methods
The seeds of soybean plant (Glycine max L.) used in the present investigation were collected from the Agricultural Jahad Research Institute. Seeds were selected uniformly and disinfected with 5% hypochlorite for 1min. After germination, uniform seedlings were transferred into plastic pots containing sand. After seven days, they were irrigated with Hoagland solution 1/2, twenty days after planting, treatment began. Plants were treated with different concentrations of PbCl2 at five levels of 0, 0.5, 2.5, 4.5 and 6.5 mM, ZnCl2 (0, 5, 10, 15 and 20 mM) under controlled environment. For each treatment, four replications were considered, they were irrigated alternately with a nutrient. After 45 days, the plants were harvested for biochemical and physiological measurements. Analysis of variance and comparison of means were analyzed using SPSS software and Excel.
Total protein measurements
The aerial parts and fresh roots of the plants after weighing were mixed with 2 ml phosphate buffer 0.1 molar (pH 6.8) homogeneously. After homogenization, each of them was transferred to 2 ml vials. Samples were centrifuged in 15000 g for 12 minutes at 4 ° C. The surface of the extract was used to measure the total protein concentration. Protein concentrations were determined by a dye binding method (Bradford, 1976).
Fig. Changes in total protein content (mg g−1fw) in different concentrations of PbCl2 and ZnCl2 in soybean seedlings
Results and discussions
Total protein content
Results revealed that, in the treatment of PbCl2 with increasing concentrations, the total protein content of the leaf was 12.34%, 25.67%. 43.27% decreased compared to control treatment. Increasing zinc concentration also resulted in a significant decrease in protein content compared with control. The minimum amount of protein is observed in the treatment of 10 mM zinc, that decreased in 44.22% compared to the control. The results are obtained in the same effects of heavy metals such as cadmium and lead on the amount of soybean protein. The same results were shown in the Seppanen et al.,2003, Hussain et al. 2013 and Hansch and Mendel, 2009 research On the rice, maize and barely plants respectively. Also the effect of lead and zinc on the sunflower plant and the ability of absorb soil lead was considered (Agilera et al., 2013). Overall, the increase of zinc affected the protein amount in various organs of some plants, especially in their roots (Elavarthi and Martin, 2010) but, Gamalero et al (2015) have stated that the process of changes in protein content is not always the same thus depends on the species and environmental conditions. Increasing the amount of protein in high concentrations can be attributed to increased levels of antioxidant enzymes and other anti-stress proteins (Hansch and Mendel, 2009). Dang et al., (2014) showed that zinc at low concentrations can show its useful effects, but in high concentrations it has toxic impacts. The beneficial effects of this element have been reported on increasing resistance to non-biological stress (Arif et al., 2012).
The results of this study indicated that with increasing levels of lead and zinc in the soil, their concentration in different organs of soybean plant has increased, due to poisoning, protein declined and also plant growth. On the other hand, lead stress causes biochemical changes in the plant. Owing to the comparative effect of soybean plant on the two studied elements, it can be concluded that this plant is more susceptible to zinc and this element has more negative effects on the physiological and biochemical parameters of soybean plant. But due to the comparative effect of soybean plant on these two elements, it can be concluded that this plant is more sensitive to Zn, and this element has more negative effects on the physiological and biochemical parameters of soybean plant. Also, it seems that soybean plants cannot be tolerated by these elements because of their genetic and inductive ability, they were not able to tolerate the normal metabolism of cells without damage to the cells. However, such studies are likely to result in highly variable data and may take years or even decades to conclude.
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