Metals are ubiquitous and normally co-occur as mixtures in soil, but there remains much to do regarding the development of appropriate models which incorporate mixture interactions and bioavailability to estimate their phytotoxicity and phytoaccumulation. Here, we developed a probability-based electrostatic toxicity model (ETM) and a Langmuir-type electrostatic uptake model (EUM) to predict and normalize toxicity and uptake of zinc-copper mixtures in Hordeum vulgare L. in different soils. For model development, the electrical potential (ψ0) and metal ion activities ({M2+}0) at the cell-membrane surface was computed based on plant physiological properties and soil solution chemistry. Single metal toxicity correlated more closely to their corresponding {M2+}0 than to ion activities in soil solution or total soil metal concentrations. The ETM explained up to 89% of the variance in mixture toxicity across different soils. Incorporation of ψ0 into the EUM improved the model's ability for predicting metal uptake. Besides, cell-surface H+ appeared to significantly inhibit copper uptake via competition or other mechanisms, beyond its effect upon ψ0. Our results for the first time demonstrate that electrostatic theory can be used to predict and reconcile mixture toxicity and uptake data in different soils, indicating the potential of electrostatic-based models in risk assessment of multimetal-contaminated soils.

The role of root hairs in Cd acquisition from soil was investigated in three pot experiments using a root hairless mutant (bald root barley, brb) and its wild-type (WT) cultivar of barley (Hordeum vulgare). brb had significantly lower concentrations and lower total amounts of Cd in shoots than WT. The Cd uptake efficiency based on total root length was 8–45% lower in brb than in WT. The difference between brb and WT increased with increasing extractable Cd in soil under the experimental conditions used. Additions of phosphate to soil decreased Cd extractability. Both soil and foliar additions of phosphate decreased root length, and root hair formation in WT. These effects resulted in decreased Cd uptake with increasing P supply. Cd uptake in WT correlated significantly with root length, root hair length and density, and soil extractable Cd. Root hairs contribute significantly to Cd uptake by barley.

A Biotic Ligand Model was developed predicting the effect of cobalt on root growth of barley (Hordeum vulgare) in nutrient solutions. The extent to which Ca2+, Mg2+, Na+, K+ ions and pH independently affect cobalt toxicity to barley was studied. With increasing activities of Mg2+, and to a lesser extent also K+, the 4-d EC50Co2+ increased linearly, while Ca2+, Na+ and H+ activities did not affect Co2+ toxicity. Stability constants for the binding of Co2+, Mg2+ and K+ to the biotic ligand were obtained: log KCoBL = 5.14, log KMgBL = 3.86 and log KKBL = 2.50. Limited validation of the model with one standard artificial soil and one standard field soil showed that the 4-d EC50Co2+ could only be predicted within a factor of four from the observed values, indicating further refinement of the BLM is needed. Biotic Ligand Models are not only a useful tool to assess metal toxicity in aquatic systems but can also be used for terrestrial plants.

There has been a growing concern with the environmental influences of nanomaterials due to recent developments in nanotechnology. This study investigates the impact and fate of hematite nanoparticles (α-Fe₂O₃ NPs) (∼14 nm in size) on a crop species, barley (Hordeum vulgare L.). For this purpose, hematite NPs (50, 100, 200, and 400 mg/L) were hydroponically applied to barley at germination and seedling stages (three weeks). Inductively coupled plasma mass spectrophotometry (ICP-MS) along with vibrating sample magnetometer (VSM) techniques were used to track the NPs in plant tissues. The effects of NPs on the root cells were observed by scanning electron microscopy (SEM) and confocal microscopy. Results revealed that α-Fe₂O₃ NPs significantly reduced the germination rate (from 80% in control to 30% in 400 mg/L), as well as chlorophyll (36–39%) and carotenoid (37%) contents. Moreover, the treatment led to a significant decline in the quantum yield of photosystem II (Fv/Fm). Leaf VSM analysis indicated a change in magnetic signal for NPs-treated samples compared with untreated ones, which is mostly attributed to the iron (Fe) ions incorporated within the leaf tissue. Besides, Fe content in the roots and leaf had gradually increased by the increasing doses of NPs, which was confirming NPs’ translocation to the aerial parts. Microscopic observations revealed that α-Fe₂O₃ NPs altered root cell morphology and led to the injury of cell membranes. This study, in the light of our findings, shows that α-Fe₂O₃ NPs (∼14 nm in size) are taken up by the roots of the barley plants, and migrate to the plant leaves. Besides, NPs are phytotoxic for barley as they inhibit germination and pigment biosynthesis. This inhibition is probably due to the injury of the cell membranes in the roots. Therefore, the use of hematite NPs in agriculture and thereby their environmental diffusion must be addressed carefully.

Environment pollution often occurs as an obvious combined effect involving two (or more) elements, and this effect changes with the concentrations of the different elements. The effects on barley root elongation were studied in hydroponic systems to investigate the toxicity of Cu–Ni combined at low doses and at a fixed concentration ratio. For low doses of Cu–Ni, the addition of Ni (<0.5 μM) to Cu significantly decreased Cu toxicity for barley, but the addition of Cu (<0.25 μM) had no significant effect on Ni toxicity. At a fixed concentration ratio, according to the single effective concentration (EC) (barley root elongation inhibitory concentration) values of Cu and Ni, five sets of Cu–Ni fixed ratios were used: ECn(Cu)+ECm(Ni) (n + m = 100) (ECn and ECₘ indicate toxicity unit value for n% and m% inhibition of barley root length, respectively). The calculated toxicity unit value for 50% inhibition of root length ranged from 0.44 to 0.98 (i.e., <1), indicating a synergistic effect. To consider the interactions between the metal ions, the extended concentration addition model (e-CA) was established by integrating the Cu–Ni interaction into the concentration addition model (CA), and the data of two groups (the low doses of Cu–Ni and at a fixed concentration ratio) were respectively fitted. The e-CA accurately predicted the root length of barley under the Cu–Ni combined action. The correlation coefficient (r) and the root-mean-square error (RMSE) between predicted and observed values were 0.97 and 6.6 (low-dose group) and 0.96 and 8.12 (fixed-ratio group), respectively, and e-CA significantly improved the prediction accuracy compared to the traditional CA model without consideration of the Cu–Ni competition (r = 0.89, RMSE = 14.16). The results provided a theoretical basis for evaluation and remediation of soil contaminated with heavy metal composites.

Cadmium (Cd) in the food chain poses a serious hazard to human health. Therefore, a greenhouse hydroponic experiment was conducted to examine the potential of exogenously strigolactone GR24 in lessening Cd toxicity and to investigate its physiological mechanisms in the two barley genotypes, W6nk2 (Cd-sensitive) and Zhenong8 (Cd-tolerant). Exogenous application of 1 μM GR24 (strigol analogue) reduced the suppression of growth caused by 10 μM Cd, lowered plant Cd contents, increased the contents of other nutrient elements, protected chlorophyll, sustained photosynthesis, and markedly reduced Cd-induced H₂O₂ and malondialdehyde accumulation in barley. Furthermore, exogenous GR24 markedly increased NO contents and nitric oxide synthase activity in the Cd-sensitive genotype, W6nk2, effectively alleviating the Cd-induced repression of the activities of superoxide dismutase and peroxidase, increasing reduced glutathione (GSH) and ascorbic acid (AsA) pools and activities of AsA-GSH cycle including ascorbate peroxidase, glutathione peroxidase, glutathione reductase, dehydroascorbate reductase and monodehydroascorbate reductase. The findings of the present study indicate that GR24 could be a candidate for Cd detoxification by decreasing Cd contents, balancing nutrient elements, and protecting barley plants from toxic oxidation via indirectly eliminating reactive oxygen species (ROS), consequently contributing to reducing the potential risk of Cd pollution.

The main objective of this study was to assess the uptake and translocation of MnFe₂O₄ magnetic nanoparticles (MNPs) in hydroponically grown barley (Hordeum vulgare L.). Hydrothermally synthesized and well characterized MNPs (average crystallite size of 14.5 ± 0.5 nm) with varied doses (62.5, 125, 250, 500, and 1000 mg L⁻¹) were subjected to the plants at germination and early growing stages (three weeks). The tissues analyzed by vibrating-sample magnetometer (VSM) and transmission electron microscopy (TEM) revealed the uptake and translocation of MNPs, as well as their internalization in the leaf cells. Also, elemental analysis proved that manganese (Mn) and iron (Fe) contents were ∼7–9 times and ∼4–7 times higher in the leaves of MNPs-treated plants than the ones for non-treated control, respectively. 250 mg L⁻¹ of MNPs significantly (at least p < 0.05) promoted the fresh weight (FW, %10.25). However, higher concentrations (500 and 1000 mg L⁻¹) remarkably reduced the increase to %8 and %5, respectively, possibly due to the restricted water uptake. Also, catalase activity was increased from 91 (μM H₂O₂ min⁻¹ mg⁻¹) to 138 in leaves, and decreased to 66 in roots upon 1000 mg L⁻¹ of MNPs application. Chlorophyll and carotenoid contents were not significantly changed, except chlorophyll a (%6 increase at 1000 mg L⁻¹, p < 0.05). Overall, MnFe₂O₄ NPs were up-taken from the roots and migrated to the leaves which promoted the growth parameters of barley. Hence, MNPs can be suggested for barley breeding programs and can be proposed as effective delivery system for agrochemicals. However, the possible negative effect of MNPs due to its potential horizontal transfer from plants to animals via the food chain must be also considered.