In order to investigate the mechanisms in two Jerusalem artichoke (
Cd is one of biotoxic metal elements, which has strong chemical activity and long-term toxicity and is relatively mobile in plants [
Existing methods of cleaning up Cd-contaminated soils are expensive, such as mechanical removal and chemical engineering [
Plant resistance to metal toxicity stress includes avoidance and tolerance [
We have previously reported that two Jerusalem artichoke genotypes, NY2 and NY5, when grown in Cd contaminated soils did not suffer from Cd toxicity, even though Cd concentration not only in roots but also in leaves and stems exceeded 100 mg kg−1 dry weight [
Two Jerusalem artichoke (
The tests were carried out in a greenhouse at Nanjing Agricultural University (N32° 2′ 6.25′′, E18° 50′ 23.47′′), Nanjing, China. The average temperature throughout the test period was between 26.6 ± 4.4°C (daytime) and 22.0 ± 2.4°C (night), and the relative humidity was 61.5 ± 1.3% (daytime) and 68.0 ± 1.9% (night). Tuber slices with buds were germinated on sand moistened with 1/2 Hoagland nutrient solution in an incubator. The nutrient solution was replaced every second day. At trefoil stage, young plants were transplanted into porcelain pots. About one week later, Cd treatments were imposed (0, 2.5, 5.0, or 10 mg L−1 as CdCl2
Roots were washed in deionized water, and then shoots and roots were separated, weighed, and used for sequential extraction to determine chemical forms of Cd [
Extractants and relevant extracted chemical forms of Cd.
Extract ion reagent | Code | Predominant forms of extracted Cd |
---|---|---|
80% v/v ethanol | FE | Cd-nitrate, Cd-chloride, Cd-amino acid complexes |
Deionized water | FW | Soluble Cd-organic acid complexes, Cd(H2PO4)2 |
1 M NaCl | FNaCl | Cd-pectates, Cd-polypeptide, or Cd-protein complexes |
2% v/v acetic acid | FAcet | Sparingly soluble CdHPO4, Cd3(PO)2, and/or other Cd-phosphate complexes |
0. 6 M HCl | FHCl | Cd-oxalate |
The extracts were digested with a concentrated acid mixture of HNO3–HClO4 (3 : 1 v/v) and heated at 160°C for 5 h. After cooling, the extracts were diluted, filtered, and made up to 25 mL with 5% v/v HNO3. The Cd concentration in the extract was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, IRIS Intrepid II XSP; Thermo Electron Company, USA). The analyses were carried out in triplicate.
FE, FW, FNaCl, FAcet, and FHCl show the amounts of the Cd-containing fractions extracted by ethanol, water, NaCl, acetic acid, and HCl, respectively.
All statistical tests were performed using SPSS 13.0. Two-way ANOVA was used to determine the significance of genotype and Cd treatment effects on Cd forms. Mean treatment differences were separated by the least significant difference (LSD0.05) test if
Even though the two Jerusalem artichoke genotypes showed good tolerance to Cd toxicity, NY2 showed some wilting in the high-Cd treatments. Compared to control, the Cd treatment (2.5 and 10 mg kg−1) decreased leaf, stem, root, and total biomass for both NY2 and NY5 (Table
Effects of different cadmium treatments on the fresh biomass of two
Genotype | Cd supply (mg L−1) | Leaves (g plant−1) | Stem (g plant−1) | Root (g plant−1) | Whole plant (g) |
---|---|---|---|---|---|
NY2 | 0 | 24.3a | 10.7ab | 23.3a | 58.2a |
2.5 | 24.6a | 11.1ab | 21.1a | 56.8a | |
5 | 28.5a | 13.0a | 28.4a | 69.9a | |
10 | 15.4b | 8.3b | 11.1b | 34.9b | |
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NY5 | 0 | 29.8a | 13.0a | 25.8a | 68.5a |
2.5 | 29.1a | 12.8a | 17.1b | 58.9a | |
5 | 30.1a | 13.9a | 20.9ab | 64.9a | |
10 | 27.5a | 12.7a | 16.9b | 57.1a |
Different letters within a column indicate the significant differences among the treatments (
As can be seen from Table
Fractionation of Cd in roots of two Jerusalem artichoke genotypes.
Genotype | Cd supply (mg L−1) | FE ( |
FW ( |
FNaCl ( |
FAcet ( |
FHCl ( |
---|---|---|---|---|---|---|
NY2 | 0 | 11a | 8.4a | 11a | 12a | 8.3a |
2.5 | 41c | 48c | 223a | 127b | 35c | |
5 | 38b | 103b | 655a | 127b | 54b | |
10 | 62b | 150b | 1675a | 329b | 91b | |
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NY5 | 0 | 3.7c | 4.9bc | 13a | 7.9b | 7.8b |
2.5 | 51c | 36c | 304a | 113b | 33c | |
5 | 55b | 83b | 469a | 259ab | 63b | |
10 | 80b | 117b | 1280a | 387b | 245b |
Different letters within a row indicate significant differences among the fractions (
There were some differences of the five main chemical forms in roots between NY2 and NY5. The Fw ratio was higher in NY2 than that in NY5. Water extracts the soluble Cd organic acid complex, Cd (H2PO4)2, which is poisonous and tends to cause harm to plants. The FAcet ratio of high accumulator was higher than low accumulator, while the Fw ratio was lower. Ethylic acid extracts unsoluble CdHPO4, Cd3(PO)2, or Cd-phosphate complexes, which may be better for Cd accumulation in high accumulator.
We can see from Table
Cd chemical forms of stems in two Jerusalem artichoke genotypes.
Genotype | Cd supply (mg L−1) | FR ( |
FW ( |
FNaCl ( |
FAcet ( |
FHCl ( |
---|---|---|---|---|---|---|
NY2 | 0 | 7.0a | 5.8a | 6.6a | 8.7a | 6.4a |
2.5 | 17c | 14c | 126a | 66b | 35b | |
5 | 18b | 28b | 278a | 48b | 34b | |
10 | 22b | 43b | 315a | 91b | 57b | |
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NY5 | 0 | 2.9c | 5.3b | 7.4a | 6.5ab | 6.4ab |
2.5 | 13c | 13c | 99a | 54b | 31bc | |
5 | 16a | 17a | 195a | 92a | 47a | |
10 | 32c | 49bc | 393a | 135b | 131b |
Different letters within a row indicate the significant differences among the forms (
The distribution ratios were raised with increased Cd supply in both NY2 and NY5 (Table
Cd chemical forms of leaves in two Jerusalem artichoke genotypes.
Genotype | Cd supply (mg L−1) | FR ( |
FW ( |
FNaCl ( |
FAcet ( |
FHCl ( |
---|---|---|---|---|---|---|
NY2 | 0 | 8.7a | 5.2a | 5.9a | 9.4a | 8.9a |
2.5 | 13c | 6.9c | 20c | 70a | 47b | |
5 | 8.3b | 12b | 17b | 44a | 42a | |
10 | 15bc | 9c | 20b | 56a | 45a | |
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NY5 | 0 | 1.4c | 5.3b | 7.3a | 6.5ab | 7.5a |
2.5 | 7.4b | 6.6b | 14b | 45a | 47a | |
5 | 10b | 8.9b | 14b | 61a | 78a | |
10 | 15b | 15b | 41b | 129a | 112a |
Different letters within a row indicate the significant differences among the forms (
The FW ratio was the lowest in leaf instead of the FR ratio in NY2. Ethanol extracts Cd-nitrate, Cd-chloride, and Cd-amino acid. The lowest FNaCl ratio might be beneficial for protecting leaves because NaCl extracts mainly Cd-pectates, Cd-polypeptide, or Cd-protein.
Cd concentration showed the same order (root > stem > leaf > tuber) and Cd accumulation in plant components showed the order of stem > leaves > roots for both NY2 and NY5 in previous work [
The transporting of Cd from roots to shoots and accumulated in shoots is a very complicated process. There are many studies on it [
Cd in leaf cells mainly comes from the water translocation from vascular bundle to leaf tissue which indicates that transpiration plays an important role in heavy mental accumulation [
In summary, Cd toxicity and tolerance mechanism are most complex. Different plants, even different strains of the same plant or different ecological types, may show diverse Cd tolerance ability and mechanism. According to the previous study, compared with NY2, genotype NY5 may be a better candidate for phytoremediation of and biofuel production on Cd-contaminated soils. The present study implied that in high accumulator, namely, NY5, the complex of insoluble phosphate tends to be shaped more easily which is much better for Cd accumulation. Besides, translocation from plasma to vacuole after combination with protein may be one of the main mechanisms in Cd-accumulator Jerusalem artichoke genotypes.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Xiaohua Long and Ni Ni contributed to the paper equally.
The authors are grateful for the financial support of National Natural Science Foundation of China (no. 31201692), the National Key Projects of Scientific and Technical Support Programs funded by the Ministry of Science and Technology of China (no. 2011BAD13B09), the Project of a Special Fund for Public Welfare Industrial (Agriculture) Research of China (no. 200903001-5), the Ministry of Science and Technology of Jiangsu Province (no. BE2011368), and Fundamental Research Funds for Central Universities (no. Y0201100249).