Characteristics of cadmium accumulation and isotope fractionation in higher plants

Identification of anthropogenic source of Pb and Cd within two tropical seagrass species in South China: Insight from Pb and Cd isotopes

Seagrass beds are susceptible to deterioration and heavy metals represent a crucial impact factor. The accumulation of heavy metal in two tropical seagrass species were studied in South China in this study and multiple methods were used to identify the heavy metal sources. E. acoroides (Enhalus acoroides) and T. hemperichii (Thalassia hemperichii) belong to the genus of Enhalus and Thalassia in the Hydrocharitaceae family, respectively. Heavy metal concentrations in the two seagrasses followed the order of Cr > Zn > Cu > Ni > As > Pb > Co > Cd based on the whole plant, and their bioconcentration factors were 31.8 ± 29.3 (Cr), 5.7 ± 1.3 (Zn), 7.0 ± 3.8 (Cu), 3.0 ± 1.9 (Ni), 1.2 ± 0.3 (As), 1.7 ± 0.9 (Pb), 9.1 ± 11.1 (Co) and 2.8 ± 0.6 (Cd), indicating the intense enrichment in Co and Cr within the two seagrasses. The two seagrasses were prone to accumulate all the listed heavy metals (except for As in E. acoroides), especially Co (BCFs of 1124) and Cr (BCFs of 2689) in the aboveground parts, and the belowground parts of both seagrasses also accumulated most metals (BCFs of 27) excluding Co and Pb. The Pb isotopic ratios (mean 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb values of 38.2054, 15.5000 and 18.3240, respectively) and Cd isotopic compositions (δ114/110Cd values ranging from -0.09‰ to 0.58‰) within seagrasses indicated the anthropogenic sources of Pb and Cd including coal combustion, traffic emissions and agricultural activities. This study described the absorption characteristics of E. acoroides and T. hemperichii to some heavy metals, and further demonstrated the successful utilization of Pb and Cd isotopes as discerning markers to trace anthropogenic origins of heavy metals (mainly Pb and Cd) in seagrasses. Pb and Cd isotopes can mutually verify and be helpful to understand more information in pollution sources and improve the reliability of conclusion deduced from concentrations or a single isotope.

Influence of arbuscular mycorrhizal fungi on bioaccumulation and bioavailability of As and Cd: A meta-analysis

Increasing industrial activity has led to a growing risk of arsenic (As) and cadmium (Cd) accumulations and biomagnifications in plants and humans. Arbuscular mycorrhizal fungi (AMF) have been extensively studied as a soil amendment owing to their capability to reduce the accumulation of As and Cd in plant tissues. However, a quantitative and data-based consensus has yet to be reached on the effect of AMF on As and Cd bioaccumulation and bioavailability. Here, a meta-analysis was conducted to quantitatively evaluate the impact of AMF using 1430 individual observations from 194 articles. The results showed that AMF inoculation caused a decrease in shoot and root As and Cd accumulation compared to control, and the reduction rates were affected by experimental duration, P fertilizer, AMF species, plant family, plant lifecycle, and soil properties. Intermediate experimental duration (lasting 56-112 days) and no P fertilizer favored AMF to reduce the shoot As and root Cd accumulation. Compared to other plant families, the reduction in As and Cd accumulation in legumes was the greatest, following AMF inoculation. The soils with alkaline, high organic carbon (OC), and low available phosphorus (AP) appeared to be more favorable for AMF to reduce As accumulation in plant tissues, while soils with low AP were more conducive to reducing the Cd accumulation in plant tissues. In addition, AMF inoculation increased pH (1.92%), OC (6.27%), easily-extractable glomalin-related soil protein (EE-GRSP) (29.36%), and total glomalin-related soil protein (T-GRSP) (29.99%), and reduced bioavailable As (0.52%) and Cd (2.35%) in soils compared to control. Overall, the meta-analysis provides valuable guidelines for the optimal use of AMF in different plant-soil systems.

Characteristics of cadmium translocation and isotope fractionation in Ricinus communis seedlings: Effects from split/cut-root and limited nutrients

Studying cadmium (Cd) transport in plants will improve the current understanding of Cd tolerance mechanisms. Due to the influence of analytical techniques, the application of Cd isotopes in plants is still in its early stages. Therefore, the relationships between Cd isotope fractionation and Cd translocation in plants remain unclear. In this study, we cultured Ricinus communis in hydroponic solutions during split/cut-root experiments and limited and infinite nutrient experiments. To understand the Cd transport process, the Cd²⁺ and other ion concentrations in different tissues (i.e., roots, stems, and leaves) and nutrient solutions, Cd isotope composition and the soluble protein in tissues were measured. The results showed that although significant effects were evident in the top leaves, the principal roots had less pronounced effects on Cd²⁺ translocation in the stems. Moreover, Cd underwent homolateral transport before it was translocated from the principal roots to the leaves on the side without Cd. It was apparent that the stems were responsible for translocating Cd²⁺ in plants. In addition, the continuous supply of high Cd²⁺ concentrations inhibited the growth of the top leaves, while in low Cd²⁺ concentrations, it was gradually transferred to the top leaves. Moreover, the tissues of R. communis were enriched with lighter Cd isotopes compared with the solutions. The clear differences between the Cd isotope fractionation of leaves under infinite and limited nutrient experiments may be attributed to plant growth and Cd uptake rates. This study provides important information for understanding Cd²⁺ translocation in R. communis and furthers our understanding of its tolerance and hyperaccumulation.

Effects of Lead (Pb) and Benzo [a] Pyrene (B[a]P) and their Combined Exposure on Element Accumulation in Ryegrass (Lolium perenne L.)

It was observed in this work that application of Pb and B[a]P co-exposure significantly (p < 0.05) reduced Pb content in ryegrass leaves and roots. The effect of Pb dominated the change of N, P, K, Cu, and Cr content in leaves and roots of ryegrass under joint stress of Pb and B[a]P. Principal component analysis showed that the foliar spraying of 400 μmol L^-1 Pb and 80 μmol L^-1 B[a]P had the best effect on improving the mineral element absorption under combined pollution. Ryegrass has strong resistance and certain Pb and B[a]P absorptive capacities, and can resist combined contamination by transferring N, P, K, Zn, Cu, and Cr contents between the overground and the root. These results highlight the potential capacity of ryegrass for use in the phytoremediation of soils contaminated by Pb and B[a]P.

Leaf application of 24-epibrassinolide mitigates cadmium toxicity in young Eucalyptus urophylla plants by modulating leaf anatomy and gas exchange

Cadmium (Cd2+) soil pollution is a global environmental problem caused by the high toxicity of Cd. 24-Epibrassinolide (EBR) is a biodegradable plant steroid involved in response modulation to biotic and abiotic stresses. The aim of this study was to evaluate if the leaf-application of EBR improves the gas exchange and possible repercussions on leaf anatomy in young Eucalyptus urophylla plants exposed to Cd toxicity. The experiment involved six treatments, which included three Cd concentrations (0, 450, and 900 μM) and two EBR concentrations (0 and 100 nM, described as - EBR and + EBR, respectively). Plants exposed to Cd toxicity suffered decreases in leaf anatomical and gas exchange parameters. However, the plants treated with EBR + 900 μM Cd showed an increase of 46%, 40%, and 54% in the net photosynthetic rate, water-use efficiency, and instantaneous carboxylation efficiency, respectively. The EBR application-induced improvements in gas exchange parameters, causing beneficial effects on the photosynthetic apparatus, mainly the effective quantum yield of photosystem II (PSII) photochemistry and electron transport rate. Furthermore, this steroid mitigated the effect of Cd toxicity on leaf anatomical variables, more specifically palisade and spongy parenchyma, which are intrinsically related to stomatal density, and stimulated the net photosynthetic rate of plants.

Cadmium isotopic fractionation in lead-zinc smelting process and signatures in fluvial sediments

Cadmium (Cd) is a toxic heavy metal pollutant. Various industrial activities, especially metal smelting, are the main sources of Cd pollution. Cd isotopes have exhibited the ability to be excellent source tracers and can be used to assess the pollution contributions from different sources. Herein, in a typical lead-zinc smelter, Shaoguan, China, significant Cd isotopic fractionation was found during the high temperature smelting process and followed a Rayleigh distillation model. The heavier Cd isotopes were concentrated in the slag, while the lighter Cd isotopes were concentrated in the dust. In the downstream sediment profile of the smelter, sediments have extremely high Cd concentrations that far exceed the Chinese background sediment, indicating severe pollution levels. The ε^114/110Cd of the sediment core, ranged from - 0.62 ± 0.5-1.73 ± 0.5, are found between slag (ε^114/110Cd=10.42) and dust (ε^114/110Cd=-5.68). The binary mixture model suggests that 88-93% of the Cd in sediment profile was derived from the slag, and 7-12% from the deposition of dust. The findings demonstrate the great potential to apply Cd isotopes as a new geochemical tool to distinguish anthropogenic sources and quantify the contribution from various sources in the environment.

Source attributions of Cadmium contamination in rice grains by Cadmium isotope composition analysis: A field study

Cd contamination in rice grains has become a topic of great concern because of the high health risks associated with the long-term consumption of Cd-contaminated rice. Identification of Cd sources in rice grains by scientific methods is important for controlling heavy metal pollution and protecting human health. Here, the Cd concentrations and Cd isotopic compositions of rice plants (root, stem, leaf, and grain) and topsoil, and possible pollution sources (agricultural fertilizers, industrial dust, and automobile exhaust) were analyzed using an instrument of inductively coupled plasma mass spectrometry (ICP-MS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The results showed variations in the Cd isotopes of different components of rice plants and the fractionation coefficient of rice grains relative to topsoil (Δ^114/110Cd_{rice grains-topsoil} = 0.25‰). The contributions of pollution sources to rice grains were realized by combining the Cd isotopic composition with an isotopic mixing model (Isosource). The analysis showed that all three possible pollution sources contributed to the Cd in the rice grains in the field, the average Cd contribution of industrial dust, agricultural fertilizers and automobile exhaust was 87%, 9%, and 4%, respectively. Our study provides a feasible method for the identification of pollution sources of Cd in rice grains at the field scale and demonstrates that Cd isotopic composition is one of the powerful tools to trace the pollution sources of Cd in crops.

Physiological responses, tolerance efficiency, and phytoextraction potential of Hylotelephium spectabile (Boreau) H. Ohba under Cd stress in hydroponic condition

A sand hydroponic experiment with different concentrations of 0, 5, 10, 20, 40 mg L^-1 Cd was used to study the growth and physiological response of Hylotelephium spectabile (Boreau) H. Ohba. and its phytoextraction potential for Cd. The results showed that total plant biomass under 5 mg L^-1 Cd treatment was slightly affected. The content of malondialdehyde (MDA) in leaf exposed to Cd was higher, and the POD and CAT activity exhibited a positive response to the low level of Cd addition (5 mg·L^-1). The photosynthesis pigments were slightly inhibited, and the ultrastructure of chloroplast remained intact after treatment with 10 mg L^-1 Cd. The maximum leaf Cd content (603 mg·kg^-1) was found in 5 mg L^-1 Cd treatment, then decreased with the Cd level increased. The maximum Cd content in the shoots far exceeds the threshold level (100 mg kg^-1) for a Cd-hyperaccumulator plant with the value of translocation factor (TF_shoot/root) for Cd reaching up to 5.62. In conclusion, H. spectabile showed normal growth and physiological response and high shoot Cd accumulation under 5 mg L^-1 Cd stress, which made it to be a good candidate for phytoextraction of low-level Cd polluted environment.

Tropospheric ozone and cadmium do not have interactive effects on growth, photosynthesis and mineral nutrients of Catalpa ovata seedlings in the urban areas of Northeast China

Heavy metal contamination and tropospheric ozone (O₃) pollution often co-occur in heavy industrial urban areas, adversely affecting urban plant health. Little is known about the characteristics of growth, physiological metabolism, bioaccumulation of cadmium (Cd) and mineral nutrients in urban trees under the combination of soil Cd contamination and elevated O₃ exposure. In this study, one-year-old street tree Catalpa ovata G. Don seedlings were exposed to Cd contaminated soil (0, 100, 500 mg/kg soil) with 40 µg/m³ O₃ (ambient air) and 120 µg/m³ O₃ (elevated O₃ exposure) for 4 weeks. The results revealed that 500 mg/kg soil Cd addition alone decreased net photosynthetic rate, stomatal conductance, peroxidase activity and increased abscisic acid content and oxidative injury in the leaves of C. ovata. Furthermore, Cd soil contamination decreased leaf, stem, root and total biomass and affected Cd, Mg, Fe, and Zn contents in leaves (P < 0.01), but it did not affect Mg, Fe and Zn contents in roots. O₃ exposure did not affect growth, net photosynthetic rate, Cd accumulation and mineral nutrient contents of C. ovata. No interactive effect between Cd and O₃ was found on growth, oxidative injury, photosynthetic rate, and the contents of Cd, Mg, Fe and Zn in plant tissues (P > 0.05). Our findings suggest that C. ovata is an appropriate tree species for urban greening and afforestation in heavy industrial urban areas with high O₃ pollution in Northeast China. To ensure successful afforestation in heavy industrial areas, the long-term and large scale studies are needed to advance our understanding of the combined effects from extreme climate conditions and multi-pollutant exposure on the metabolism of mature urban trees.

Physiological stress responses, mineral element uptake and phytoremediation potential of Morus alba L. in cadmium-contaminated soil

Fast growing woody plants are proposed for potential application for phytoremediation of contaminated soil. In this study, the plant growth, physiological responses, mineral element uptake, and phytoremediation potential of the woody plant Morus alba L. were studied in different levels of Cd-contaminated soil through dynamic sampling (30, 60, 120, and 180 d). The results indicated that M. alba L. had strong physiological coordination, tolerance and detoxification capacity in response to Cd in contaminated soil. Compared with the control, the photosynthetic pigment content in M. alba L. leaves was significantly suppressed during initial cultivation (30-60 d) and the malonaldehyde (MDA) content and electrolyte leakage (EL) were increased from 30 to 120 d of cultivation. Furthermore, the uptake of Cu, Mn, and Zn in plant tissues was imbalanced throughout cultivation (30-180 d) under 55 mg·kg^-1 Cd stress. However, the chlorophyll a, chlorophyll b, carotenoid, soluble protein, and soluble sugar contents and the peroxidase (POD) and ascorbate peroxidase (APX) activities in plant leaves, as well as the uptake of macronutrients (K, Ca, and Mg) in plant stems and leaves were maintained at normal levels. Furthermore, the catalase (CAT) activities in plant leaves and the Ca and Mg contents in plant roots were significantly (p < 0.05) enhanced in response to Cd stress after 180 d of cultivation. Furthermore, the biomass of M. alba L. was significantly increased with cultivation time in Cd-contaminated soil. Therefore, normal photosynthesis, antioxidant protection, and macronutrient regulation contribute to M. alba L. with high tolerance to Cd. Moreover, the uptake and total extraction amount of Cd in aboveground M. alba L. were significantly (p < 0.05) increased with both the plant growth period and soil Cd level, and the maximum amount of Cd reached up to 340.5 μg·plant^-1. Thus, M. alba L. can be regarded as a potential candidate for phytoremediation in Cd-contaminated sites.

Cadmium absorption and translocation of amaranth (Amaranthus mangostanus L.) affected by iron deficiency

Amaranth (Amaranthus mangostanus L.) has superior capability for accumulating cadmium (Cd) and has the potential to be used for phytoremediation of Cd contaminated soils. Iron (Fe) is chemically similar to Cd and may mediate Cd-induced physiological or metabolic impacts in plants. The purpose was to investigate the model of time-dependent and concentration-dependent kinetics of Cd absorption under Fe deficiency, understanding the physiological mechanism of Cd absorption in amaranth roots. The kinetic characteristics of Cd uptake by amaranth grown in Cd enriched nutritional solution with or without Fe addition and with methanol-chloroform, carbonyl cyanide 3-chlorophenylhydrazone (CCCP), and lanthanum chloride (LaCl₃) were compared using ¹⁰⁹Cd²⁺ isotope labeling technique. The results showed that Cd uptake was time-dependent and about 90-93% of uptake occurred during the first 150 min. The kinetics of Cd uptake showed that two stages were involved. The saturation stage fitted the Michaelis-Menten model when concentrations of Cd were lower than 12.71 μmol/L and then the absorption of Cd by roots was increased linearly during the second stage. Only linear absorption was observed with methanol-chloroform treatment while the metabolic inhibitor CCCP inhibited only the saturation absorption process, and the Ca channel inhibitor LaCl₃ partially inhibited the two stages of absorption. These results indicated that the root absorption of ¹⁰⁹Cd²⁺ was enhanced under Fe deficiency which induced more Fe transporters in the root cell membrane, and the Ca channel, apoplastic and symplastic pathways enhanced the Cd absorption in roots.