Tolerance to B toxicity may differ significantly not only between species, but also within the same species. The identification and characterization of varieties of crops that are tolerant to B toxicity is consequently a key issue as regards improving agricultural yields in soils with excessive B concentrations. This work has been performed with two local maize cultivars  from B-enriched agricultural areas in Tacna. The aim of this study was to analyze the effect of an excessive B supply on the growth, photosynthetic parameters, B contents, and genes related to B transporters in both local maize landraces in order to investigate physiological mechanisms that could improve their tolerance to B toxicity.In A. thaliana plants grown with elevated B concentration, a drop in water flux to the shoot and in the transpiration rate could diminish B transport from roots to shoots and, consequently, reduce B accumulation in leaves. Although Sama had a higher transpiration rate  than Pachía under excessive B conditions ,its lower leaf B content at 10 mM B  suggests the presence of other mechanisms to explain this fact. In this regard, it has been described that B can be mobilized through the phloem bound to sucrose toward sink cells in some species such as canola and wheat. The significantly higher leaf sucrose content of Sama in comparison to that of Pachía might, therefore, increase the B re-translocation from leaves to roots,leading to a lower leaf B accumulation in Sama under B toxicity conditions. Alternatively, under excessive B conditions, two cooperating mechanisms could lead to a decrease in B in plant tissues, namely, a reduced B xylem loading mediated by active B efflux that would result in a decreased B transport from roots to shoots,and  an active B efflux from roots to soil. Lower root and leaf B levels in Sama under B toxicity conditions consequently suggest that less B xylem loading might be occurring and/or that an active B efflux from roots to soil could be more efficiently induced in this landrace as possible mechanisms to explain the lower accumulation of B in the leaves and roots of Sama. In order to investigate this idea, the expression of some B transporters was analyzed. ZmRTE1 has been reported as a functional ortholog of AtBOR1 with a similar key role in B xylem loading. In both landraces, rolling bench the ZmRTE1 gene was strongly down-expressed in roots when 10 mM B was added to the media. 

Similarly, Huanca-Mamani et al.  observed a down-regulation of the ZmRTE1 gene in a B- and salt-tolerant landrace when it was subjected to high B and salt contents. These authors proposed that the decreased gene expression of ZmRTE1 would reduce B uptake into roots and its mobilization to shoots, leading to a lower B content in Lluteno. The remarkable repression of the ZmRTE1 gene in the roots of both Sama and Pachía would help to limit the amount of B transported to shoots under excess B conditions. As this mechanism would not explain the lower leaf B accumulation in Sama,since both landraces had a similar significant down-expression of root ZmRTE1,other B-related transporters were taken into account. As described above, B exclusion from roots is a key mechanism to prevent toxic B accumulation in plants. In fact, AtBOR4 was the first well-characterized B transporter involved in an active B efflux from roots. Phylogenetic analyses have shown that ZmRTE3 belongs to B-transporter class II, containing members involved in B toxicity tolerance, such as AtBOR4, HvBOR2, and TaBOT-B5B. ZmRTE3 has a close evolutionary relationship with AtBOR4 and has a closer phylogenetic relationship with HvBOR2 and TaBOT-B5B, barley and wheat B transporters, respectively. Analogous to ZmRTE3, Zm00001d030297 has a high amino acid identity  with AtBOR4. In addition, Zm00001d030297 has an even higher amino acid identity  with HvBOR2. A direct relationship between the expression levels of the Bot1 gene  and the degree of tolerance to excess B in Sahara 3771  was observed, and this gene was identified as being responsible for B-toxicity tolerance, performing an active B efflux from the roots. It has consequently also been suggested that an increased number of copies of the ZmRTE3 gene could produce B-tolerant maize plants. Interestingly, transcript levels of ZmRTE3 and Zm00001d030297 genes were significantly higher in Sama roots when compared to Pachía in both B treatments. Moreover, an over expression of the Zm00001d030297 gene occurred under B toxicity conditions. The higher transcript contents of these genes in Sama would allow for greater B exclusion from roots to soil that would prevent the accumulation of high B contents in the roots of Sama and their subsequent toxic effects.

In addition, further resistance to B toxicity could be achieved in Sama by means of enhanced B excretion from leaves. It has been suggested that two mechanisms could explain the less intuitive role of B efflux transporters in shoot B tolerance: 1) an excretion of B via hydathodes that would reduce leaf B levels. In Sahara 3771, a strong expression of Bot1  was, therefore, found in mesophyll cells associated with hydathodes. Similarly, in Sama, the higher leaf mRNA levels of ZmRTE3 and Zm00001d030297 in comparison to Pachía, and particularly Zm00001d030297 under excess B conditions, could lead to increased B excretion via hydathodes, as proposed in Sahara, thus explaining the lower leaf B contents in Sama ; 2) a reallocation of excess B from the leaf symplast to the cell wall, performed by efflux transporters, which would allow a less harmful B accumulation. This B redistribution mechanism was described as a key process to alleviate the toxic effects of high B in the B-toxicity tolerant barley cultivar Sahara  and in AtBOR4-overexpressing transgenic Arabidopsis plants. Once B has been excreted from the leaf symplast to the apoplast via ZmRTE3 and Zm00001d030297, this nutrient could be loaded into the phloem and transported to the roots where it would be removed to the soil. According to this hypothesis, the increased sucrose contents in Sama leaves  would likely induce a higher B re-translocation via phloem from leaves to roots, in which higher expressions of B efflux transporters  decrease root B contents in Sama.In recent years, soil has been severely affected by inorganic pollutants, such as metals, including Cu, Zn and Ni. Some metals that were uncommon in the past have also entered the soil environment with increased industrialization such as La, Se and V.. However, there is limited information on the ecological risks of these elements. It is therefore important to understand the ecotoxicity and risks, and establish assessment models for these emerging elements. Quantitative ion character–activity relationship  models have been used to predict the toxicity of metals on the basis of the relationship between the physicochemical properties of the ions and their toxicity. Newman et al.  first proposed the feasibility of applying the QICAR model to toxicity prediction by exploring the effect of metals on the marine bacterium. 

Subsequently, this model has been increasingly used in the aquatic environment. Li et al.  used the QICAR model to study the metal toxicity of the non-marine ostracod Cypris subglobosa in freshwater, correlating the toxicity with physicochemical properties such as atomic number,electronegativity,and the log of the first hydrolysis constant and so on. Similarly, Wolterbeek and Verburg  correlated the physicochemical properties of a wide range of species with the toxic effects of cations and found accurate prediction of ion toxicity using these properties. A recent study found it difficult to establish a relationship between multiple elements and their ecotoxicity, using one or more physicochemical properties. This may be due to the large differences in the properties of the elements, which resulted in completely different dosage-toxicity responses of organisms to individual elements. Thus, QICARs established after hard–soft grouping  have been used in predicting metal toxicity. For instance, Meng et al.  used the QICAR model to investigate the relationships between metal ion characters and the ecological soil screening levels. They found that QICAR models based on classifying metal ions as either soft or hard can accurately predict Eco-SSLs. Similarly, the QICAR model has also been applied to terrestrial systems. Many studies have attempted to apply QICAR to estimate the toxicity of elements to plants, as plants are among the most commonly used organisms in terrestrial risk assessments. However, there are various ligands of plant roots, such as hard ligands containing oxygen or nitrogen  and soft ligands containing sulfur,which have different complexing ability with metals. Therefore, it is difficult to quantify the binding ability of these ligands to metals by using single properties of these elements. In order to better use QICAR in plants, a normalized hard ligands scale  was proposed by Kinraide,which was obtained by averaging and normalizing the binding strengths of metal cations to 13 hard ligands. Kinraide  believed that most metal ions were bound to hard ligands, and that binding affinity to hard ligands could predict the binding ability of metals to biological materials, such as plasma membranes and cell walls. However, the toxicity of some soft ions, such as Ag+, Cu2+ and Tl+, was found to be poorly correlated with HLScale, indicating that the toxicity of these ions may not reflect the binding ability with hard ligands, and may exhibit different toxicity mechanisms from other ions. As a consequence, Kinraide  presented a consensus scale related to the soft ions, termed the softness consensus scale,grow table hydroponic which was obtained by averaging and normalizing ten other scales related to chemical softness.

He reported that σCon was strongly positively correlated with the binding strength of metals to soft ligands, and that its coupling with charge  could closely predict the phytotoxicity of unknown elements. Kopittke et al.  also studied the toxicity effects of 26 metals on cowpea roots, using both HLScale and σCon methods.Although the soft–hard ligand theory has achieved good results in predicting the toxicity of individual elements, there are still some elements such as Ag+ and Tl+ that have poorly predicted effects. In addition, these predictive results need to be further verified using different plants. Recently, a biotic ligand model  theory has been proposed to predict element toxicity. The BLM assumes bioactive receptor sites to be biotic ligands. The BL site may be a physiologically active site, leading to a direct biological response, or it may be a transport site, leading to an indirect biological response. According to BLM theory, toxicity will occur when a certain number of ions bind to BLs. The toxicity of ions depends on the binding affinity  of metal ions to the BL sites. For the last few years, the BLM theory has been used to determine the binding constants of some common elements, such as Cu, Zn and Ni, to specific organisms. In the present study, we compared the methods of soft–hard ion grouping, soft–hard ligand theory  and K based on the BLM principle to explore the application of the QICAR model in predicting the phytotoxicity of multiple metal elements. The results of this study can potentially be used for deriving guideline values for ecological toxicity and can provide fundamentals for the toxicology database.The present study showed that root and shoot length were inhibited to varying degrees when wheat was exposed to 19 metal ions separately. We tested the inhibitory effects of the 19 metal ions on wheat roots and shoots. The dose-response relationships of root elongation with ions were superior as toxicity indicators to those of shoots, except for V5+ and Sb5+. Generally, the roots, as the initial site of contact for metal ions, were damaged more than shoots and leaves of the same plant. In previous studies, acute toxicity tests on plants  usually employed roots as toxicity indicators. However, Yeasmin et al.,investigating the toxicity effects of As, Cr, Se and Mo on the roots and shoots of cucumber,found that high-valence ions  showed closer dose-response relationships with shoots, a finding which was different from the results reported from the current study. This discrepancy between the two studies may be related to differences in the plant species tested, the age of the plants tested and the duration of the testing period.For example, Blamey et al.determined the median effective concentration for cowpea root elongation in Ag+ solutions, ranging from 0.01 μM to 0.021 μM over the first 4–8 h of exposure.