Toxic Effects Of Cadmium On Glycine Max Plants And Differences Of Cadmium Tolerance Of Various Glycine Max Varieties

Cadmium contamination in soils is a serious environmental threat in China. Theplants with highly efficient Cd uptake and high tolerance have successfully used in Cdremediation in soils. The main objectives of this dissertation were: (â…°) to study theeffects of different concentration of Cadmium (Cd) on the growth, the amount ofphytohormones and the stress physiological indexes of Glycine max plants; (â…±) toinvestigate the distribution and transportation of Cd in Glycine max organic and theeffects of Cd on uptake of nitrogen. (N), phosphorous (P), potassium (K) and Zinc(Zn); (â…²) to study the fractions of Cd in different soils and their bioavailability; (â…³) toevaluate the toxic effects of Cd on Glycine max plants and the differentiation ofvarious Glycine max cultivars to Cd tolerances; (â…´) to investigate the influences ofexogenous hormone, naphthalene acetic acid (NAA) and micronutrient zinc (Zn), onCd toxicity. The results showed that:1. The nutrient hydroponic experiments showed that Cd stress with lowconcentrations (0.25 mg L^-1) for a short period (5 days) slightly increased the heightsand biomass of Glycine max plants without significant statistical difference and it alsostimulated the synthesis of indole-3-acetic acid (IAA), glibberellic acid (GA₃) andzeatin in Glycine max seedlings. However, the heights and biomass of Glycine maxplants thereafter significantly decreased with increasing Cd concentrations andelongating Cd stress period, and the synthesis of above three hormones decreased. Inaddition, all Cd concentrations increased the synthesis of abscisic acid (ABA) inGlycine max seedlings, and this increase enhanced with increasing Cd concentrations.2. The pot experiments showed that Cd stress depressed the synthesis of IAA andzeatin in Glycine max leaves and this depress increased with increasing Cdconcentrations. However, a stimulated synthesis of GA₃ was found at low Cdconcentrations, while a depressed effect was exhibited at high Cd concentrations.Zeatin contents in Glycine max leaves at pod stage were lower than those at seedlingstage. As compared with the seedling stage, higher IAA and GA₃ contents were foundat pod stage, which could maintain a certain growth potential for Glycine max plants.The synthesis of ABA was distinctly stimulated by Cd stress. Moreover, thissimulative effect enhanced with increasing Cd concentrations and elongating Cdstress period. 3. Cadmium concentrations in roots were the highest among different Glycinemax plant parts. Cd uptake by roots was mainly distributed in stems and leaves, butminor Cd in pods and seeds. Except the control treatment, the ratios of Cdconcentrations in roots, stems, leaves, pods and seeds of Glycine max plants for all Cdtreatments were 1:0.48-0.81:0.48-0.80:0.09-0.34:0.09-0.40 in the tested red soil,and 1:0.25-0.75:0.22-0.69:0.08-0.33:0.06-0.22 in the tested alluvial soil.Exchangeable Cd with water and bio-available Cd in the red soil were higher those inthe alluvial soil. Therefore, the critical concentration of Cd toxicity for Glycine maxplants in the red soil was lower than that in the alluvial soil, which indicated thatalluvial soils possesses of greater Cd environmental capacity compared to red soils.4. Interactive effects of N, P, K, Zn contents on Cd contents in Glycine max seedswere investigated in the present study. Phosphorus and Cd showed a synergistic effectat low Cd concentrations (≤0.50 mg L^-1), and an antagonistic effect at increased Cdconcentrations (＞0.50 mg L^-1). Nitrogen and Cd exhibited obviously an antagonisticeffect. Moreover, K and Cd exhibited an antagonistic effect at low Cd concentrationsin Glycine max seeds, but a significant synergistic effect in Glyeine max roots. Zincand Cd exhibited consistently a synergistic effect for all Cd treatments in Glycine maxseeds, but a significant antagonistic effect in Glycine max leaves.5. In nutrient hydroponic experiments, low Cd concentrations stimulated rootactivity of Glycine max plants, enhanced Cd uptake by Glycine max roots and Cdtransportation to the stems and leaves. In reverse, high Cd concentrations decreasedthe root activity, Cd uptake and transportation, but increased Cd accumulation in theroots. When the Cd concentration was lower than or equal to 1.00 mg L^-1, there weresignificantly positive relationships between the root activities (5 days) and the Cdcontents in Glycine max leaves and roots with correlation coefficients (n=4) of 0.996and 0.979, respectively. When the Cd concentration was higher than or equal to 2.50mg L^-1, significantly negative relationships between the root activities (5 days) andthe Cd contents in Glycine max leaves and roots were obtained. Their correlationcoefficients (n=3) were-0.995 and -0.993, respectively.6. Treated with various Cd concentrations in the tested red soil and alluvial soil inpot experiments, the contents of malondialdehyde (MDA) and proline (PRO) ofGlycine max leaves at seedling and pod stages increased with increasing Cdconcentrations until 2.5 mg kg^-1 of external Cd in the soils. Thereafter, these contentsdecreased with increasing Cd concentrations, but were still higher than that in the control treatment. In the red soil, peroxidase (POD) activities in Glycine max leavesincreased with increasing Cd concentrations with a maximum value at 2.5 mg Cd kg^-1soil, but it declined thereafter. In the alluvial soil, when Cd was lower than or equal to0.5 mg kg^-1, Cd treatments showed a lower POD activity of Glycine max leaves thanthe control. However, when Cd concentration was higher than the above value, PODactivity increased with further increasing Cd concentrations, and it was significantlyhigher than that in the control. The results suggest that although Cd stress resulted inthe increased MDA contents in Glycine max leaves, Glycine max plants still exhibitedcertain suitability to Cd stress due to increase of POD activity and ABA and PROcontents.7. The results of the principal component analysis showed that decreased rates ofchlorophyll in Glycine max leaves, root activity, increment rates of POD activity andCd contents in the stems, leaves and roots of Glycine max plants due to Cd stress werewell response to Cd tolerance capacity of different Glycine max varieties, which couldbe used to select Glycine max varieties with high Cd tolerance. The extractive Cd withNaC1 solution was the main Cd form in Glycine max leaves and roots, accounting for87.6 % and 88.4 % of total extractive Cd contents, respectively. Different Cd forms inGlycine max plant leaves and roots were in the following sequence: F_NaCl＞F_HAc＞F_H2O＞F_ethanol. Capacity of Cd tolerance for Glycine max plants was not onlyrelated to Cd contents in the plants, but also to Cd forms. Those Glycine max varietieswith low contents of ethanol and water extractive Cd showed higher Cd tolerance.8. NAA application increased nitratase activities in Glycine max leaves, butdecreased amounts of free PRO and MDA. Lipid peroxidation of cell membrane andprotein decomposition were also alleviated by NAA application. Furthermore, Znapplication decreased MDA and PRO contents in Glycine max seedlings, whichindicated that Zn could alleviate Cd toxicity to Glycine max plants.