Study on Accumulation of Cd by Amaranthus mangostanus

Menglu GUO

Abstract [Objectives] The effects of three exogenous phytohormones on the uptake and accumulation of Cd in the seedlings of Amaranthus mangostanus L. were investigated.

[Methods] Three effective plant hormones， auxin （IAA）， gibberellin （GA） and salicylic acid （SA） were selected and formulated into three concentrations， respectively， and were studied through pot experiments.

[Results] The application of IAA， GA and SA increased the accumulation of Cd in each part of A. mangostanus. The absorption capacities of Cd in the roots， stems and leaves of A. mangostanus were related to the types and levels of hormones， and the overall trend was GA>SA>IAA. The leaves had significantly higher Cd translocation and bioaccumulation capacities than the stems. Under the treatment of 500 mg/L low concentration of gibberellin， the translocation and bioaccumulation factors reached the highest.

[Conclusions] The results indicated that the absorptive， accumulative and transferred capacities of Cd of the amaranth plants after using phytohormones were obviously higher than the control.

Key words Phytohormones; Cd; Phytoremediation

Received： March 5， 2022  Accepted： May 8， 2022

Menglu GUO （1986-）， female， P. R. China， lecturer， devoted to research about botany.

*Corresponding author. E-mail： 549802453@qq.com.

At present， Cd has been classified as the first carcinogen by the International Agency on Cancer Research[1]. Cd is one of the most toxic and polluting heavy metals. In places where cadmium mineralization occurs and the soil is polluted by industry， the cadmium content in the soil will be hundreds of times higher than the background value， and even as high as thousands of times in some cases[2]. Cd has a strong mobility and is easily absorbed by plants and accumulates in the body. If the concentration is too high， it will affect the growth， development and even yield of plants. Meanwhile， Cd will reduce the integrity of the biological corneal endothelium from the accumulated plants through the food chain， impair the metabolism of biological bone minerals， and induce prostate cancer and testicular tumors in animals. Furthermore， Cd is carcinogenic to humans and animals， so it is urgent to remove this heavy metal. Zhang et al.[3] reported in 2016 that the addition of phytohormone-auxin （IAA） and abscisic acid （ABA） could alleviate the excessive accumulation of reactive oxygen species （ROS）， which in turn alleviated Cd-induced oxidative stress. Mohd Israr et al.[4] sprayed IAA， GA and EDTA on maize plants respectively to study and improve the growth of maize in lead-rich soil. The results showed that the combined application of IAA or GA3 and EDTA could significantly increase the lead content of maize plants. Wei et al.[5] took farmland weeds as the research object and found for the first time that the weed nightshade is a Cd hyperaccumulator. Wang et al.[6] studied the remediation efficiency of Brassica juncea varieties on cadmium-contaminated soil and proved that the cadmium content of B. juncea was significantly lower than that of rape Chuanyou. Chen et al.[7] studied the enrichment effect of Houttuynia cordata on soil Cd， and the results showed that cadmium played a selective role in that Cd-tolerant microorganisms could survive and the plant roots could absorb Cd more easily under the action of microorganisms. The phytoremediation technology can remove organic matter， nuclides， heavy metals and other single and composite pollution in soil. It is a new method for efficient and economical treatment of heavy metals and radioactive pollutants. Considering the dual needs of restoration efficiency and ecological environmental protection， phytohormones have been paid more and more attention by experts， scholars and the general public， because plant regulation of hormone levels in the body is an important way for plants to adapt to nuclide and heavy metal stress[8]. In this study， three effective plant hormones， auxin （IAA）， gibberellin （GA）， and salicylic acid （SA）， were selected.

Materials and Methods

Experimental materials

The test plant in this experiment was Amaranthus mangostanus L.， and the selection was based on its characteristics of strong resistance， easy growth， drought tolerance， humidity tolerance， high temperature tolerance， and few diseases and pests. Studies have shown that A. mangostanus has a large biomass and a strong ability to enrich nuclides， and is suitable for growth in various regions.

Experimental methods

Plant cultivation and experimental treatment

The seedlings were raised in the field， and transplanted after the cotyledons had grown. For Cd， Cd（NO3）·4H2O was used as the Cd source， which was added at the concentration of 20 mg/kg， and mixed into the soil in the form of water solution. A. mangostanus seedlings were transferred to Cd-added soil for pot planting， with two plants per pot. Three hormones （IAA， GA， SA） were formulated into three treatment concentrations （10， 100， and 1 000 mg/L）， respectively， and corresponding concentrations of aqueous solutions were sprayed on plant surface： CK： 0 mg/L， IAA1： 10 mg/L， IAA2： 100 mg/L， IAA3： 1 000 mg/L， GA1： 10 mg/L， GA2： 100 mg/L， GA3： 1 000 mg/L， SA1： 10 mg/L， SA2： 100 mg/L， and SA3： 1 000 mg/L. Plus the control （CK） with clear water， a total of 10 treatments were set up in three replicates， each of which included 6 plants.

During the experiment， the seedlings were cultivated in the open air and watered irregularly according to the lack of water in the pot， so that the water holding capacity in the soil was maintained at about 70% to 80%.

Determination of growth index of A. mangostanus

The harvested A. mangostanus was washed with deionized water， drained， and the roots， stems and leaves were separated， then fixed at 105 ℃ for 30 min， and dried at 70 ℃ to constant weight （about 48 h）. The various parts were weighed for their dry weights， respectively.

Determination of Cd content

After grinding the dried plant samples， a 0.2 g of treated sample was accurately weighed and added into a conical flask， which was added with 10 ml of mixed acid （nitric acid∶perchloric acid at a volume ratio of 3∶1） and covered overnight. The sample solution was poured into a Kjeldahl flask and digested on an electric furnace until white smoke was emitted and the digestive solution was colorless and transparent. The solution was made up to 100 ml with 0.5 mol/L nitric acid. The Cd contents of plant samples were determined by flame atomic absorption spectrometry， and the testing instrument was AA700 flame atomic absorption spectrometer of American PE company.

Data processing and analysis

Microsoft Excel 2013 software was used for data processing， and DPSv7.05 software was used for one-way ANOVA and Duncans multiple comparisons at the 5% level.

Results and Analysis

Enrichment of Cd in A. mangostanus

Fig. 1 was obtained by measuring the Cd contents in the above-ground part and roots of A. mangostanus treated with phytohormones. As can be seen from Fig. 1， the Cd content in the above-ground part after three hormone treatments was significantly higher than that in the roots， and the overall trend of the effects of the hormones was GA>SA>IAA. Compared with the CK， the Cd contents of the three hormone treatments were significantly higher than that of the CK， and the Cd absorption capacity of the above-ground part differed with the types and levels of hormones. Under the GA treatment， there were significant differences among the three levels. Under the treatment of 500 mg/L GA， the Cd content in the above-ground part reached the maximum value of 1.309 mg/g in all treatments， which was 1.27 times that of the CK， and the enrichment of Cd in the above-ground part showed a downward trend with the concentration of GA increasing.

Characteristics of Cd translocation in A. mangostanus

Fig. 2 was obtained by statistical analysis of the Cd translocation factors （the ratio of the Cd content in each part to that in the roots） of the stems， leaves and above-ground part of A. mangostanus treated with phytohormones. It can be seen from the figure that the translocation factors under various treatment were all greater than 1. Compared with the CK， the three plant hormone treatments all increased the Cd translocation factor， and the overall trend of the three hormone treatments was GA>SA>IAA， and the Cd translocation capacity of leaves was significantly higher than that of stems. The leaf translocation factors were all greater than 10， and the variation range was 15.26-25.12. Under the treatment of gibberellin GA1， the TF value reached a maximum of 26.06， which was 58.32% higher than that of the CK.

It can be seen from Fig. 3 that the enrichment factors of Cd in the above-ground part under various treatments were greater than 5. Compared with the CK， the three plant hormone treatments all increased the enrichment factor of Cd in the above-ground part， and the overall trend of the treatment effects of the three hormones was GA>SA>IAA. The minimum value of root enrichment factor appeared in IAA3， and the maximum value appeared under GA1 treatment. Under the auxin treatment， the enrichment factor showed a trend of first increasing and then decreasing with the increase of auxin concentration， and the highest value was 61.05， which was lower than 7.21% of the maximum value. The enrichment factor reached the maximum value under 100 mg/L SA for the SA treatment. Under gibberellin treatment， the enrichment factor was negatively correlated with the concentration of GA， namely GA1>GA2>GA3. Under the treatment of 500 mg/L gibberellin， the enrichment factor reached the highest， and the aboveground enrichment capacity reached the maximum. 100 mg/L salicylic acid and auxin and 500 mg/L gibberellin treatments significantly improved the enrichment capacity of the above-ground part.

Discussion and Conclusions

This study proved that the application of auxin， gibberellin and salicylic acid increased the accumulation of Cd in each part of A. mangostanus. The absorption capacities of Cd in the roots， stems and leaves of A. mangostanus were related to the types and levels of hormones， and the overall trend was GA>SA>IAA. The absorption of Cd in each part reached the highest under 500 mg/L GA treatment. As plant growth regulators， they generally exhibit the effect of "low promotion and high inhibition"， that is， a too-high concentration is not conducive to the enrichment of heavy metals.

Compared with the CK， the three phytohormone treatments all increased the Cd translocation and bioaccumulation factors. The overall trend of the three hormone treatments was GA>SA>IAA， and the leaves had significantly higher Cd translocation and bioaccumulation capacities than the stems. Under the treatment of 500 mg/L low concentration of gibberellin， the translocation and bioaccumulation factors reached the highest. Zhao et al. studied Cd2+ stress and found that the addition of growth hormones could significantly increase the enrichment of Cd2+ in rice roots. It shows that the application of plant hormones can increase the absorption of Cd by plants， which is consistent with the research results of this study.

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