Characteristics of cadmium accumulation and tolerance in apple plants grown in different soils

Abstract

Cadmium (Cd) is a nonessential element and highly toxic to apple tree. However, Cd accumulation, translocation and tolerance in apple trees planted in different soils remain unknown. To investigate soil Cd bioavailability, plant Cd accumulation, physiological changes as well as gene expression patterns in apple trees grown in five different soils, 'Hanfu' apple seedlings were planted in orchard soils collected from Maliangou village (ML), Desheng village (DS), Xishan village (XS), Kaoshantun village (KS) and Qianertaizi village (QT), and subjected to 500 μM CdCl₂ for 70 d. Results showed that soils of ML and XS had higher content of organic matter (OM), clay and silt, and cation exchange capacity (CEC) but lower sand content than the other soils, thereby reduced Cd bioavailability, which could be reflected by lower concentrations and proportions of acid-soluble Cd but higher concentrations and proportions of reducible and oxidizable Cd. The plants grown in soils of ML and XS had relatively lower Cd accumulation levels and bio-concentration factors than those grown in the other soils. Excess Cd reduced plant biomass, root architecture, and chlorophyll content in all plants but to relatively lesser degree in those grown in soils of ML and XS. The plants grown in soils of ML, XS and QT had comparatively lower reactive oxygen species (ROS) content, less membrane lipid peroxidation, and higher antioxidant content and enzyme activity than those grown in soils of DS and KS. Transcript levels of genes regulating Cd uptake, transport and detoxification such as HA11, VHA4, ZIP6, IRT1, NAS1, MT2, MHX, MTP1, ABCC1, HMA4 and PCR2 displayed significant differences in roots of plants grown in different soils. These results indicate that soil types affect Cd accumulation and tolerance in apple plants, and plants grown in soils with higher OM content, CEC, clay and silt content and lower sand content suffer less Cd toxicity.

Keywords: apple; cadmium; gene expression; oxidative stress; soil physicochemical property.

Figure 1

Cd concentrations of acid-soluble (A), reducible (B), oxidizable (C), and residual (D) forms in soils sampled from five apple orchards subjected to 500 μM CdCl₂ for 70 d. Data are means ± SE (n = 3). Different letters on bars indicate significant differences between soils. ML, Maliangou village in Liaoyang city; DS, Desheng village in Panjin city; XS, Xishan village in Chaoyang city; KS, Kaoshantun village in Xinmin city; QT, Qianertaizi village in Xinmin city.

Figure 2

Proportions of various Cd fractions in soils sampled from five apple orchards subjected to 500 μM CdCl₂ for 70 d. Data are means ± SE (n = 3). ML, Maliangou village in Liaoyang city; DS, Desheng village in Panjin city; XS, Xishan village in Chaoyang city; KS, Kaoshantun village in Xinmin city; QT, Qianertaizi village in Xinmin city.

Figure 3

Cd concentrations in roots (A), stems (B), and leaves (C) of ‘Hanfu’ apple plants grown in soils sampled from five apple orchards subjected to 0 (CK) or 500 μM CdCl₂ (Cd) for 70 d. Data are means ± SE (n = 3). Different letters on bars indicate significant differences between treatments. P-values indicated for ANOVA of Cd treatment (Cd), soil (S), and their interaction (Cd × S). ^**** P < 0.0001. ML, Maliangou village in Liaoyang city; DS, Desheng village in Panjin city; XS, Xishan village in Chaoyang city; KS, Kaoshantun village in Xinmin city; QT, Qianertaizi village in Xinmin city.

Figure 4

Total plant Cd content (A) and bio-concentration factor (BCF, B) of roots and aerial parts of ‘Hanfu’ apple plants grown in soils sampled from five apple orchards subjected to 500 μM CdCl₂ for 70 d. Data are means ± SE (n = 3). Different letters on bars indicate significant differences between soils. ML, Maliangou village in Liaoyang city. DS, Desheng village in Panjin city; XS, Xishan village in Chaoyang city; KS, Kaoshantun village in Xinmin city; QT, Qianertaizi village in Xinmin city.

Figure 5

Superoxide anion ( ${\text{O}}_2^{·-}$ , A1-A3), hydrogen peroxide (H₂O₂, B1-B3) and malondialdehyde (MDA, C1-C3) in roots, stems and leaves of ‘Hanfu’ apple plants grown in soils sampled from five apple orchards subjected 0 (CK) or 500 μM CdCl₂ (Cd) for 70 d. Data are means ± SE (n = 3). Different letters on bars indicate significant differences between treatments. P-values indicated for ANOVA of Cd treatment (Cd), soil (S), and their interaction (Cd × S). *P < 0.05. **P < 0.01. ***P < 0.001. ****P < 0.0001. ns, not significant. ML, Maliangou village in Liaoyang city; DS, Desheng village in Panjin city; XS, Xishan village in Chaoyang city; KS, Kaoshantun village in Xinmin city; QT, Qianertaizi village in Xinmin city.

Figure 6

Free proline (A1-A3), ascorbate (ASC) (B1-B3), total thiols (T-SH) (C1-C3) and soluble phenolics (D1-D3) in the roots (A1-D1), stems (A2-D2) and leaves (A3-D3) of ‘Hanfu’ apple plants grown in the soils sampled from five apple orchards subjected to 0 (CK) or 500 μM CdCl₂ (Cd) for 70 d. Data indicate means ± SE (n = 3). Different letters on the bars indicate significant differences between the treatments. P-values indicated for ANOVA of Cd treatment (Cd), soil (S) and their interaction (Cd × S). *P < 0.05. **P < 0.01. ***P <0.001. ****P < 0.0001. ns, not significant. ML, Maliangou village in Liaoyang city; DS, Desheng village in Panjin city; XS, Xishan village in Chaoyang city; KS, Kaoshantun cillage in Xinmin city; QT, Qianertaizi village in Xinmin city.

Figure 7

Principal component analysis (PCA) of plant growth related parameters, photosynthetic pigments, Cd concentrations, ROS and antioxidant levels in roots, stems, and leaves of apple plants grown in soils sampled from five apple orchards subjected to 0 (CK) or 500 μM CdCl₂ (Cd) for 70 d. Triangles and circles represent 0 (CK) and 500 μM CdCl₂ (Cd) treatments, respectively. Different colors represent different soils. Red, Maliangou village in Liaoyang city. Blue, Desheng village in Panjin city. Yellow, Xishan village in Chaoyang city. Green, Kaoshantun village in Xinmin city. Purple, Qianertaizi village in Xinmin city. PCA loadings presented in Table S5 .

Figure 8

Relative expression levels of genes encoding proteins regulating Cd absorption (A–D) detoxification (E–I) and transport (J, K) in roots of ‘Hanfu’ apple plants grown in soils sampled from five apple orchards subjected to 0 (CK) or 500 μM CdCl₂ (Cd) for 70 d. Data are means ± SE (n = 3). Different letters on the bars indicate significant differences between the treatments. For each gene, expression level was normalized to the reference gene β-Actin and the expression level was set to 1 in the roots of ‘Hanfu’ apple plants grown in soil of ML subjected to 0 μM CdCl₂. Corresponding fold changes in other treatments were then calculated accordingly. P-values indicated for ANOVA of Cd treatment (Cd), soil (S) and their interaction (Cd × S). **P < 0.01. ***P < 0.001. ****P < 0.0001. ns, not significant. ML, Maliangou village in Liaoyang city; DS, Desheng village in Panjin city; XS, Xishan village in Chaoyang city; KS, Kaoshantun village in Xinmin city; QT, Qianertaizi village in Xinmin city.