The promising response of chromium-stressed (Cr(VI)–S) plants to hydrogen sulphide (H₂S) has been observed, but the participation of nitric oxide (NO) synthesis in H₂S-induced Cr(VI)–S tolerance in plants remains to be elucidated. It was aimed to assess the participation of NO in H₂S-mediated Cr(VI)–S tolerance by modulating subcellular distribution of Cr and the ascorbate-glutathione (AsA-GSH) cycle in the pepper seedlings. Two weeks following germination, plants were exposed to control (no Cr) or Cr(VI)–S (50 μM K₂Cr₂O₇) for further two weeks. The Cr(VI)–S-plants grown in nutrient solution were supplied with 200 μM sodium hydrosulphide (NaHS, donor of H₂S), or NaHS plus 100 μM sodium nitroprusside (SNP, a donor of NO). Chromium stress suppressed plant growth and leaf water status, while elevated proline content, oxidative stress, and the activities of AsA-GSH related enzymes, as well as endogenous H₂S and NO contents. The supplementation of NaHS increased Cr accumulation at root cell walls and vacuoles of leaves as soluble fraction to reduce its toxicity. Furthermore it limited oxidative stress, improved plant growth, modulated leaf water status, and the AsA-GSH cycle-associated enzymes’ activities, as well as it further improved H₂S and NO contents. The positive effect of NaHS was found to be augmented on those parameters in the CrS-plants by the SNP supplementation. However, 0.1 mM cPTIO, the scavenger of NO, inverted the prominent effect of NaHS by decreasing NO content. The supplementation of SNP along with NaHS + cPTIO reinstalled the positive effect of NaHS by restoring NO content, which suggested that NO might have a potential role in H₂S-induced tolerance to Cr(VI)–S in pepper plants by stepping up the AsA-GSH cycle.

Antimony (Sb) and arsenic (As) are chemical analogs, but their behaviors in plants are different. To investigate the Sb uptake, translocation and speciation in As-hyperaccumulator P. cretica, a hydroponic experiment was conducted. In this study, P. cretica was exposed to 0.2-strength Hoagland nutrient solution, which contained 0.5 or 5 mg/L antimonite (SbIII) or antimonate (SbV). After 14 d exposure, P. cretica took up 1.4–2.8 times more SbIII than SbV. Since P. cretica was unable to translocate Sb, its roots accumulated >97% Sb with the highest at 7965 mg/kg. In both SbIII and SbV treatments, SbIII was the predominant species in P. cretica, with 90–100% and 46–100% SbIII in the roots. As the first barrier against Sb to enter plant cells, more Sb was accumulated in cell wall than cytosol or organelles. The results suggest that P. cretica may detoxify Sb by reducing SbV to SbIII and immobilizing it in root cell walls. Besides, the presence of SbIII significantly reduced the concentrations of dissolved organic C including organic acids in P. cretica root exudates. Further, increasing Sb levels promoted P accumulation in the plant, especially in the fronds, which may help P. cretica growth. The information from this study shed light on metabolic transformation of Sb in As-hyperaccumulators P. cretica, which helps to better understand Sb uptake and detoxification by plants.

Manganese (Mn) transporter OsNRAMP5 was widely reported to regulate cadmium (Cd) uptake in rice. However, the relationship between OsNRAMP5 expression level and Cd accumulation, impacts of external ion activities on OsNRAMP5 expression level and Cd accumulation are still unclear. Investigations of the relationship between OsNRAMP5 expression level and Cd accumulation in three indica rice genotypes were conducted under various external Mn²⁺ activities ranging from Mn deficiency to toxicity in EGTA-buffered nutrient solution. Results in this work indicated that OsNRAMP5 expression level in roots significantly up-regulated at Mn phytotoxicity compared to that at Mn deficiency, which may stimulate by the increasing uptake of Mn. Our work also demonstrated that root Cd concentration of all the tested rice decreased notably when external Mn²⁺ activity reached the level of toxicity. This may explain by the increasing competition between the excess Mn²⁺ and Cd²⁺ as well as the disorder of element absorption caused by root damage at Mn toxicity. Our work also revealed that the relationship between OsNRAMP5 expression level in roots and Cd accumulation in roots was insignificant for all the tested genotypes. Besides, OsNRAMP5 expression level in roots seemed more related to root Mn accumulation. The fact that function of OsNRAMP5 mainly focuses on Mn uptake, together with the fact that many transporter genes involved in Cd uptake might result in the insignificant correlation between OsNRAMP5 expression level and Cd accumulation in roots. At last, multi-level regulating and processing of the process from gene expression to protein translation might account for the inconsistent relationship between root OsNRAMP5 expression level and Cd accumulation in roots.

Application of Zinc (Zn) is considered an effective measure to reduce Cadmium (Cd) uptake and toxicity in Cd-contaminated soils for many plant species. However, interaction between Zn and Cd in rice plant is complex and uncertain. In this study, four indica rice cultivars were selected to evaluate the effect of Zn exposure in an EGTA-buffered nutrient solution under varying Zn activities and a field level of Cd activity to characterize the interaction between Zn and Cd in rice. Severe depression in shoots’ biomass, tiller number, and SPAD (Soil and Plant Analyzer Development) value were found at both Zn deficiency and Zn phytotoxicity levels among four tested rice cultivars. There existed a strong antagonism interaction between Zn and Cd in both shoot and root from Zn deficiency to Zn phytotoxicity. The reduction of Cd accumulation in roots and shoots could be explained by the competition between Zn and Cd as well as the dilution effect of increasing biomass. The conflicting effect of Zn supply on Cd uptake may be attributed to the increasing transfer ratio of Cd from root to shoot with the increasing Zn²⁺ activities and the strong depression of Fe and Mn in shoots with the increasing Zn²⁺ activities as well as the variation of genotypes. Balance between Zn and Cd should be considered in field application.

Silicon (Si) may decrease the uptake and accumulation of arsenic (As) in rice. However, the effects of Si/As ratios in growth medium on arsenic uptake, arsenite efflux to the external medium and translocation of arsenite in rice are currently unclear. Rice seedlings (Oryza sativa L.) were exposed to nutrient solutions with 10 μM arsenite [As(III)] or 10 μM arsenate [As(V)] to explore the influence of different silicic acid concentrations (0, 10, 100, 1000 μM) on arsenic uptake and translocation of arsenite with or without 91 μM phosphate for 24 h. Arsenic speciation was determined in nutrient solutions, roots, and shoots. In the arsenite treatments, different Si/As ratios (1:1, 10:1, 100:1) did not affect As(III) uptake by rice roots, however they did inhibit translocation of As(III) from roots to shoots significantly (P < 0.001) in the absence of P. In the arsenate treatments, a Si/As ratio of 100:1 significantly decreased As(V) uptake and As(III) efflux compared with the control (Si/As at 0:1), accounting for decreases of 27.4% and 15.1% for –P treatment and 47.8% and 61.1% for + P treatment, respectively. As(III) is the predominant species of arsenic in rice roots and shoots. A Si/As ratio of 100:1 reduced As(III) translocation from roots to shoots markedly without phosphate. When phosphate was supplied, As(III) translocation from roots to shoots was significantly inhibited by Si/As ratios of 10:1 and 100:1. The results indicated that in the presence of P, different silicic acid concentrations did not impact arsenite uptake and transport in rice when arsenite was supplied. However, a Si/As ratio of 100:1 inhibited As(V) uptake, as well as As(III) efflux and translocation from roots to shoots when arsenate was supplied.

Antimony (Sb) is a contaminant of increased prevalence in the environment, but there is little knowledge about the mechanisms of its uptake and translocation within plants. Here, we applied for the synchrotron based X-ray absorption near-edge structure (XANES) spectroscopy to analyze the speciation of Sb in roots and shoots of rye grass (Lolium perenne L. Calibra). Seedlings were grown in nutrient solutions to which either antimonite (Sb(III)), antimonate (Sb(V)) or trimethyl-Sb(V) (TMSb) were added. While exposure to Sb(III) led to around 100 times higher Sb accumulation in the roots than the other two treatments, there was no difference in total Sb in the shoots. Antimony taken up in the Sb(III) treatment was mainly found as Sb-thiol complexes (roots: >76% and shoots: 60%), suggesting detoxification reactions with compounds such as glutathione and phytochelatins. No reduction of accumulated Sb(V) was found in the roots, but half of the translocated Sb was reduced to Sb(III) in the Sb(V) treatment. Antimony accumulated in the TMSb treatment remained in the methylated form in the roots. By synchrotron based XANES spectroscopy, we were able to distinguish the major Sb compounds in plant tissue under different Sb treatments. The results help to understand the translocation and transformation of different Sb species in plants after uptake and provide information for risk assessment of plant growth in Sb contaminated soils.

Many pharmaceutical and personal care products (PPCPs) and endocrine-disrupting chemicals (EDCs) are present in reclaimed water, leading to concerns of human health risks from the consumption of food crops irrigated with reclaimed water. This study evaluated the potential for plant uptake and accumulation of four commonly occurring PPCP/EDCs, i.e., bisphenol A (BPA), diclofenac sodium (DCL), naproxen (NPX), and 4-nonylphenol (NP), by lettuce (Lactuca sativa) and collards (Brassica oleracea) in hydroponic culture, using 14C-labeled compounds. In both plant species, plant accumulation followed the order of BPA > NP > DCL > NPX and accumulation in roots was much greater than in leaves and stems. Concentrations of 14C-PPCP/EDCs in plant tissues ranged from 0.22 ± 0.03 to 927 ± 213 ng/g, but nearly all 14C-residue was non-extractable. PPCP/EDCs, particularly BPA and NP, were also extensively transformed in the nutrient solution. Dietary uptake of these PPCP/EDCs by humans was predicted to be negligible.

Low pH and aluminum (Al)-toxicity often coexist in acidic soils. Citrus sinensis seedlings were treated with nutrient solution at a pH of 2.5, 3.0, 3.5 or 4.0 and an Al concentration of 0 or 1 mM for 18 weeks. Thereafter, malate, citrate, isocitrate, acid-metabolizing enzymes, and nonstructural carbohydrates in roots and leaves, and release of malate and citrate from roots were measured. Al concentration in roots and leaves increased under Al-toxicity, but it declined with elevating nutrient solution pH. Al-toxicity increased the levels of glucose, fructose, sucrose and total soluble sugars in leaves and roots at each given pH except for a similar sucrose level at pH 2.5–3.0, but it reduced or did not alter the levels of starch and total nonstructural carbohydrates (TNC) in leaves and roots with the exception that Al improved TNC level in roots at pH 4.0. Levels of nonstructural carbohydrates in roots and leaves rose with reducing pH with a few exceptions with or without Al-toxicity. A potential model for the possible role of root organic acid (OA) metabolism (anions) in C. sinensis Al-tolerance was proposed. With Al-toxicity, the elevated pH upregulated the OA metabolism, and increased the flow of carbon to OA metabolism, and the accumulation of malate and citrate in roots and subsequent release of them, thus reducing root and leaf Al and hence eliminating Al-toxicity. Without Al-toxicity, low pH stimulated the exudation of malate and citrate, an adaptive response of Citrus to low pH. The interactive effects of pH and pH on OA metabolism were different between roots and leaves.

Chromium (Cr) is a toxic element among which hexavalent chromium [Cr(VI)] is more toxic than trivalent chromium [Cr(III)]. Chromium can be reduced or oxidized in soil because soil is a complex medium and various soil components affect redox reaction of Cr in soil. Therefore, Cr speciation in hydroponics and soil was compared and Cr uptake and speciation by lettuce grown in the media were evaluated. Higher phytotoxicity was found in Cr(III) spiked soil than in Cr(VI) spiked soil, while Cr toxicity was higher in Cr(VI) treated hydroponics than Cr(III) treated hydroponics. Chromium was mainly accumulated in lettuce roots as Cr(III), and more Cr was translocated from roots to shoots grown in Cr(VI) treated hydroponics than Cr(III) treated hydroponics. Accumulation of Cr in roots grown in Cr(III) treated nutrient solution reduced Fe, K, Ca, Mg, and P uptake in lettuce. Chromium valence state was Cr(III) in lettuce leaves and roots grown in both Cr(III) and Cr(VI) treated hydroponics and soil. Chromium speciation in hydroponically grown lettuce roots was Cr(III) coordinated with 6 oxygens in the first shell and 2 or 4 carbons in the second shell as analyzed by X-ray absorption spectroscopy (XAS), which was similar to chromium acetate. The valence state of Cr in Cr(III) and Cr(VI) treated nutrient solution was not changed, while Cr(VI) was reduced to Cr(III) in Cr(VI) spiked soil by soil organic matter. Spiking of Cr(III) induced reduction of Mn in soil, which resulted in an increase of bioavailable Mn concentration in the Cr(III) spiked soil. Therefore, the increased phytotoxic effect for lettuce in Cr(III) spiked soil can be attributed to the reduction of Mn and subsequent release of Mn(II). For Cr(III) contaminated soil, Mn speciation should be considered, and bioavailable Mn concentration should be monitored although Cr existed as Cr(III) in soil.

The increasing discharge of pharmaceuticals and personal care products (PPCPs) into the environment has generated serious public concern. The recent awareness of the environmental impact of this emerging class of pollutants and their potential adverse effects on human health have been documented in many reports. However, information regarding uptake and intracellular distribution of PPCPs in hydrophytes under hydroponic conditions, and potential human exposure is very limited. A laboratory experiment was conducted using ¹⁴C-labeled triclosan (TCS) to investigate uptake and distribution of TCS in six aquatic plants (water spinach, purple perilla, cress, penny grass, cane shoot, and rice), and the subcellular distribution of ¹⁴C-TCS was determined in these plants. The results showed that the uptake and removal rate of TCS from nutrient solution by hydrophytes followed the order of cress (96%) > water spinach (94%) > penny grass (87%) > cane shoot (84%) > purple perilla (78%) > rice (63%) at the end of incubation period (192 h). The range of ¹⁴C-TCS content in the roots was 94.3%–99.0% of the added ¹⁴C-TCS, and the concentrations in roots were 2–3 orders of magnitude greater than those in shoots. Furthermore, the subcellular fraction-concentration factor (3.6 × 10²–2.6 × 10³ mL g⁻¹), concentration (0.58–4.47 μg g⁻¹), and percentage (30%–61%) of ¹⁴C-TCS in organelles were found predominantly greater than those in cell walls and/or cytoplasm. These results indicate that for these plants, the roots are the primary storage for TCS, and within plant cells organelles are the major domains for TCS accumulation. These findings provide a better understanding of translocation and accumulation of TCS in aquatic plants at the cellular level, which is valuable for environmental and human health assessments of TCS.