Variations in root morphology among 18 herbaceous species and their relationship with cadmium accumulation

Physiological and biochemical characteristics of high and low Cd accumulating Brassica napus genotypes

Phytoremediation is a widely used and cost-effective technique for in situ remediation of heavy metals. Brassica napus L. genotype with high Cd accumulation and strong Cd tolerance is an ideal candidate for phytoremediation. In this study, a hydroponic experiment was conducted to select a Brassica napus genotype with either high or low Cd accumulation from a panel of 55 genotypes. The physiological mechanisms governing Cd accumulation and Cd tolerance were then explored. BN400 and BN147 were identified as the high and low Cd accumulating genotypes, respectively. Additionally, BN400 exhibited greater tolerance to Cd stress compared to BN147. Root morphology analysis revealed that BN400 exhibited longer root length, smaller root surface area and root volume, and less root tips but bigger root diameter than BN147. Subcellular Cd distribution showed that the Cd concentrations in the cell wall and vacuole in shoot were significantly higher in BN400 than in BN147, whereas the opposite trend was observed in the roots.. Pectate/protein-integrated Cd was found to be the predominant form of Cd in both shoots and roots, with significantly higher levels in BN400 compared to BN147 in the shoot, but the opposite trend was observed in the roots. These results suggest that the long fine roots play a role in Cd accumulation. The high Cd accumulating genotype was able to retain Cd in leaf cell walls and vacuoles, and Cd was mainly present in the form of pectate/protein-integrated Cd, which contributes to its strong Cd tolerance. These findings have important implications for the screening and breeding of Brassica napus genotypes with high Cd accumulation for phytoremediation purposes.

Health risk assessment of heavy metal exposure through vegetable consumption around a phosphorus chemical plant in the Kaiyang karst area, southwestern China

The phosphorus chemical industry is an important source of heavy metals in farmland. Vegetables grown on contaminated soil potentially impose adverse effects on human health. In this study, the pollution status and health risks of heavy metals in vegetables around a phosphorus chemical plant in Kaiyang County, Guizhou Province, southwestern China, were assessed, and the low-accumulation vegetables were screened by bioaccumulation factor (BAF) and cluster analysis. Results showed the average concentrations of Hg, As, Cd, Pb, Cr, Mn, Co, and Zn in vegetables were 0.015, 0.728 0.382, 0.227, 0.850, 27.227, 0.525, and 6.438 mg/kg, respectively. The single-factor pollution index showed that Cd was moderately polluted, and Cr, Hg, As, and Pb were slightly polluted. The Nemerow pollution index showed that the overall heavy metal pollution was classified as moderately polluted. The accumulation of heavy metals in different vegetables varied greatly, and chard, crown daisy, chayote, pumpkin, eggplant, white radish, sweet potato, carrot, and potato were selected as the low-accumulator vegetables. The consumption of all vegetables except chayote poses both carcinogenic and noncarcinogenic risks; among them, the consumption of sweet potato leaves has the highest health risks. The local population needs to adjust plantation structure and change dietary habits, and government should strengthen the management of phosphorus chemical plant pollution.

Response strategies of fine root morphology of Cupressus funebris to the different soil environment

Understanding fine root morphology is crucial to uncover water and nutrient acquisition and transposition of fine roots. However, there is still a lack of knowledge regarding how the soil environment affects the fine root morphology of various root orders in the stable forest ecosystem. Therefore, this experiment assessed the response strategies of fine root morphology (first- to fifth -order fine roots) in four different soil environments. The results showed that fine root morphology was related to soil environment, and there were significant differences in specific root length (SRL), specific surface area (SRA), diameter (D), and root tissue density (RTD) of first- and second -order fine roots. Soil total nitrogen (TN), alkaline nitrogen (AN) and available phosphorus (AP) were positively correlated with SRL and SRA and negatively correlated with D and RTD. Soil moisture (SW) was positively correlated with the D and RTD of first- and second-order fine roots and negatively correlated with the SRL and SRA. Soil temperature (ST), organic carbon (OC), soil bulk density (SBD) and soil porosity (SP) were not significantly correlated with the D, SRL, SRA, and RTD of the first- and second -order fine roots. AN was positively correlated with SRL and SRA and negatively correlated with both D and RTD in the first- and second -order fine roots, and the correlation coefficient was very significant. Therefore, we finally concluded that soil AN was the most critical factor affecting root D, SRL, SRA and RTD of fine roots, and mainly affected the morphology of first- and second -order fine roots. In conclusion, our research provides support for understanding the relationship between fine root morphology and soil environment, and indicates that soil nutrient gradient forms good root morphology at intraspecific scale.

The regulatory role of abscisic acid on cadmium uptake, accumulation and translocation in plants

To date, Cd contamination of cropland and crops is receiving more and more attention around the world. As a plant hormone, abscisic acid (ABA) plays an important role in Cd stress response, but its effect on plant Cd uptake and translocation varies among plant species. In some species, such as Arabidopsis thaliana, Oryza sativa, Brassica chinensis, Populus euphratica, Lactuca sativa, and Solanum lycopersicum, ABA inhibits Cd uptake and translocation, while in other species, such as Solanum photeinocarpum and Boehmeria nivea, ABA severs the opposite effect. Interestingly, differences in the methods and concentrations of ABA addition also triggered the opposite result of Cd uptake and translocation in Sedum alfredii. The regulatory mechanism of ABA involved in Cd uptake and accumulation in plants is still not well-established. Therefore, we summarized the latest studies on the ABA synthesis pathway and comparatively analyzed the physiological and molecular mechanisms related to ABA uptake, translocation, and detoxification of Cd in plants at different ABA concentrations or among different species. We believe that the control of Cd uptake and accumulation in plant tissues can be achieved by the appropriate ABA application methods and concentrations in plants.

Root characteristics critical for cadmium tolerance and reduced accumulation in wheat (Triticum aestivum L.)

Root radial transport is important for cadmium (Cd) absorption and root-shoot translocation. However, the relationship between root structural characteristics and radial transport of Cd in wheat is still unclear. Six wheat cultivars with different Cd tolerance and accumulation characteristics were used to investigate the roles of root phenotype, microstructure, and apoplastic and symplastic pathways in Cd uptake and root-shoot transport in pot culture. Longer root length, smaller root diameter, and more numerous root tips were more conducive to Cd absorption, while thicker roots were able to retain more Cd, thus reducing root-shoot transport and improving Cd tolerance of shoots. Cd stress can induce the deposition of apoplastic barriers in wheat roots, and the deposition of the apoplastic barrier increases under greater stress. The formation of apoplastic barriers can reduce Cd absorption and transfer to the shoot, and the presence of passage cells can weaken this effect. The cell wall thickening induced by Cd stress enhanced Cd adsorption capacity in wheat roots, but there was no significant correlation between Cd content and polysaccharide content in the cell wall. The up-regulated expression of TaHMA3 and TaVP1, which encode proteins related to Cd compartmentalization, was associated with increased Cd tolerance in wheat and decreased Cd translocation to aboveground parts. The morphology and anatomy of roots appear to play critical roles in Cd tolerance, uptake, and translocation in wheat. The present study provides useful information for the selection of wheat cultivars with low Cd accumulation.

Foliage application of chitosan alleviates the adverse effects of cadmium stress in wheat seedlings (Triticum aestivum L.)

Excessive cadmium (Cd) causes toxic effects on crops. The effects of chitosan (CTS) with different molecular weight (MW) (5 kDa, 3 kDa, and 1 kDa) on the growth and biochemical parameters, as well as Cd concentrations in Cd-treated wheat plants were examined in a pot experiment. The results demonstrated that foliar spraying with CTS significantly improve the wheat growth, reduce malondialdehyde content and reactive oxygen species accumulation in leaves and decrease Cd concentrations in roots and shoots of wheat seedling under Cd stress. The alleviation of Cd toxicity by CTS is probably related with the activity of antioxidant enzymes, osmotic adjustment matter and root morphology. The application of CTS enhanced the activities of superoxide dismutase, peroxidase, and catalase in Cd-stressed wheat seedling leaves by 6.6%-13.1%, 17.2%-33.0%, and 19.6%-25.5%, respectively. Besides, exogenously applied CTS also increased the soluble protein and soluble sugar contents by 17.6%-33.8% and 30.1%-36.1% in the leaves of wheat under Cd stress. Furthermore, CTS with a molecular weight of 1 kDa was the most effective in mitigating Cd toxicity in wheat seedlings, which indicates that the activity of CTS is dependent on its molecular weight. It can be concluded that the use of foliar spraying, especially with 1 kDa CTS, could have potential in reducing the damage of Cd stress.

Sexual differences in root growth and antioxidant characteristics in Salix viminalis exposed to cadmium stress

Salix viminalis, a dioecious shrub willow, has been widely used in phytoremediation, yet sexually differences in tolerance to cadmium of which remained unclear. This study focused on different responses to cadmium stress between roots of male and female S. viminalis. Results show that male plants of S. viminalis have stronger cadmium tolerance than female plants, which indicates male S. viminalis should be more considered to be applied for phytoremediation and ecological restoration of cadmium-accumulated soil considering cadmium tolerance characteristics. The findings can provide valuable evidence and insights for researches focused on phytoremediation with dioecious woody plants and sexual dimorphism under abiotic stress.

Effects and interactions of cadmium and zinc on root morphology and metal translocation in two populations of Hylotelephium spectabile (Boreau) H. Ohba, a potential Cd-accumulating species

The interactions between Cd and Zn in their effects on plants are inconsistent and difficult to predict. A hydroponic experiment was conducted to investigate the effects of Cd and Zn and their interactions on root morphology and metal translocation in two populations of Hylotelephium spectabile (Boreau) H. Ohba (Crassulaceae, HB1 and HB2). Both populations showed relative tolerance to high levels of Cd and Zn, except that the leaf biomass of HB1 significantly decreased by 44.6% with 5-mg/L Cd plus 10-mg/L Zn. Root growth was inhibited in both populations by addition of 20-mg/L Zn under Cd stress, while 10-mg/L Zn showed little impact on the root growth inhibition of HB2. Roots with diameter 0.1-0.4 mm contributed most of the total root length (RL) and root surface area (RSA) of H. spectabile. In both populations, these root parameters showed greater suppression with the combined stress of Cd plus Zn than under Cd or Zn single stress, except by adding 10-mg/L Zn under Cd stress. Moreover, HB2 maintained relatively higher RL and RSA than HB1 under the different treatments, which implied that HB2 might possess a more effective mechanism than HB1 for coping in response to Cd and Zn stress. The addition of Zn not only affected the absorption of Cd but also significantly affected the distribution of Cd in different tissues of H. spectabile. A low level of Zn led to increased Cd in the stem of HB2, but an increase in Cd in the leaf and root of HB1. Addition of 10-mg/L Zn led to a significant increase by 188% and 170% in Cd accumulation in aboveground part of HB2 under 2- and 5-mg/L Cd stress, whereas the addition of Zn had little effect on Cd accumulation in HB1. Thus, strong positive interactions of Cd and Zn occurred in HB2, which showed great potential for application in phytoremediation of soil contaminated with both Cd and Zn, warranting further investigation under field condition.

Root morphological response of six peanut cultivars to chromium (VI) toxicity

We investigated the hypothesis that root morphology plays a crucial role in the variation in chromium (Cr) accumulation among peanut (Arachis hypogaea L.) cultivars, using the relationship between Cr accumulation and morphological characteristics of six peanut cultivars determined under 0, 10, 25, 75, and 100 μmol L^-1 Cr(VI) via hydroponic experiment. Significant variations were observed in Cr accumulation and root morphological parameters among peanut cultivars at the five Cr levels. The Cr concentrations in plants exhibited 1.72-, 4.67-, 1.81-, and 2.91-fold variations within cultivars for 10, 25, 75, and 100 μmol L^-1 Cr treatments, respectively. Positive correlations were found for total Cr in plants with total root length (RL), root surface area (SA), and root volume (RV). Negative correlations were also observed between the percentages of Cr in shoots and specific root length. These results suggest that root system morphology may partly explain the variation in Cr accumulation among cultivars. Cultivars with greater RL, SA, and RV showed higher capability for Cr accumulation.

Comparison of Cd subcellular distribution and Cd detoxification between low/high Cd-accumulative rice cultivars and sea rice

Salt-tolerant rice cultivar (sea rice) is a research hotspot worldwide due to its high yield in high salinity soil. However, knowledge regarding the cadmium (Cd) effects on the growth of sea rice is limited. To determine the short-term and long-term impact of Cd stress, relatively low/high Cd-accumulative rice cultivars and sea rice were grown to compare their growth responses to Cd stress over time. The results showed that sea rice presented the highest Cd concentrations in the root, stem, and leaves under 32-days of Cd stress. Cd stress shortened and thickened the rice root, and decreased the proportion of root diameters in the 0-0.2 mm range. Cd stress remarkably increased the Cd and Fe concentration in dithionite-citrate-bicarbonate (DCB) extracts, and the DCB-Cd and DCB-Fe concentrations were the highest in sea rice. The subcellular distribution of Cd in the rice roots indicated that Cd accumulated the most in the soluble fraction and cell wall. The contents of pectin and hemicellulose 2 in the root cell wall of the low-Cd accumulative rice variety CL755 were higher than those in MXZ and sea rice. Collectively, this work provides a general understanding of the Cd effects on sea rice growth and indicates that sea rice has a relatively high Cd accumulation compared with the other two rice cultivars. However, the specifically-related mechanism remains to be further studied.