INTRODUCTION

The issues of the accumulation of chemical elements (CEs) by plants are very topical at present and are widely considered both in Russia and abroad. A detailed study of CE accumulation processes is necessary for an understanding of the chemical cycle and is also important in practical terms: the quality and safety of food and medicinal plants (MPs), the search for bioindicators of ore deposits, assessment of the level of anthropogenic pollution of territories, expansion of the range of plants for phytoremediation, etc. (Kovall’skii, 1974; Kovalevskii, 1991; Bargagli, 1998; Pollard et al., 2002; Kabata-Pendias, 2010; Gravel et al., 2012; Lovkova et al., 2014; Ufimtseva, 2015; etc.). Such a wide range of issues related to the CE concentration in plants led to the emergence of various terminological and classification contradictions that need to be identified and resolved.

TERMINOLOGY AND PROBLEMS OF CLASSIFICATION

In English-language works, the term “accumulation” is commonly used, while the words “accumulation” and “concentration” are used as synonyms in Russian-language articles, often at the same time in one publication. The terms “excluders,” “accumulators,” and “indicators” are widely used in the naming of plant with different features of CE accumulation abroad (Baker, 1981). In Russian works, the terminology is much more diverse (Ufimtseva, 2015): group and selective (Vinogradov, 1957), adapted and nonadapted (Kovall’skii, 1974), barrier and barrier-free (Kovalevskii, 1991), concentrators and exclusions (Seregin et al., 2015), etc. The authors often focus on different classification features, which does not allow their adequate comparison.

Ent et al. (2013) proposed a very logical conceptual scheme to vary the CE concentration in plant leaves depending on the content of their mobile forms of compounds in the soil (Fig. 1).

Fig. 1.
figure 1

Relationship between the concentration of metals/metalloids in the leaves/shoots of plants and the available concentration of metals/metalloids in the soil (according to: Ent et al., 2013). The dotted line indicates the hyperaccumulation threshold for metals/metalloids.

Full size image

It is known that the state of the geochemical environment is dynamic: the forms and concentrations of CEs in the soil are constantly changing both in time and space, so the same plant species can find itself in a wide variety of geochemical conditions in which it will be forced to either adapt to these conditions or perish (Kovalevskii, 1991; Dobrovol’skii, 2003). The “normal” plants highlighted in Fig. 1 can tolerate only low concentrations of mobile forms of CE compounds in the soil. In the work of Koval’skii (1974), such plants are called nonadapted: when the concentrations of available forms of CE compounds in the soil change, they develop various physiological disorders that lead to endemic diseases; development is inhibited, generative functions are disrupted, and the population dies out. An insignificant number of individuals (1–3% of the population) can survive, and nonadapted physiological forms can turn into adapted ones as a result of a long, gradual process.

Among the CEs, heavy metals (HMs) currently attract the greatest attention of researchers. Various definitions are given in the literature for the terms “heavy metals,” “toxic elements,” “trace elements,” etc. Thus, for example, according to various classifications, HMs include elements with a relative atomic mass of more than 40–50 and a density over 5–8 g/cm3; noble metals are excluded from this group, but metalloids, etc., are included. From an ecological point of view, the chemical and physical properties of elements included in this category and their biological activity, toxicity, and other properties are taken into account.

The study of the influence of HMs on ecosystems as a whole and individual taxa is closely related not only to natural phenomena and processes but also to a wide range of anthropogenic sources that cause environmental pollution.

It was established that the toxic effect of HMs on a living organism is based on three main mechanisms: enzymatic toxic action, membranotropic action, and oxidative stress (Skugoreva et al., 2016). Similarly, there are three ways to protect plants from this unfavorable factor: prevention or restriction of penetration, chelation and localization, and the work of the antioxidant system.

A variety of approaches to the identification of mechanisms of plant adaptation to elevated CE concentrations (including HMs) is provided by a combination of these processes at different stages of advancement and various classification criteria. For example, one system is based on different levels of response: in the tissue (due to different rates of CE transport from roots to shoots), intracellular (CE binding in the cell wall and vacuoles), intercellular (activation of mechanisms for cellular excretion), and molecular (the synthesis of enzymes resistant to toxic CE effects), etc. (Fenik et al., 1995; Seregin, 2001; Seregin and Ivanov, 2001). More interesting is the approach that combines not only the levels of CE promotion but also the methods of their detoxification (Manara, 2012).

The mechanisms of plant adaptation to HMs have been considered in detail in the works of Titov et al. (2007, 2014), which take into account the following strategies: “avoidance” (i.e., limiting HM entry into tissues) or “resistance” (“tolerance,” i.e., the detoxification mechanisms in a plant organism) (Regvar and Vogel-Mikuš, 2008) (Fig. 2). The implementation of these pathways and their combination probably led ultimately to the emergence of adapted plant species, as shown in Fig. 1 by the other three groups. They turned out to be well adapted to high concentrations of mobile CE forms; however, they use different strategies for this, so their ability to accumulate elements is expressed to varying degrees.

Fig. 2.
figure 2

Main mechanisms of plant adaptation to high concentrations of heavy metals. Bold font indicates terminology of Regvar and Vogel-Mikuš (2008); ordinary font indicates terminology of Titov et al. (2014); italics indicate taxon- and element-specific methods. ROS, reactive oxygen species; LPO, lipid peroxidation; FR, free radicals.

Full size image

The adapted group primarily includes plants that grow for a long time in a given area, which acquire resistance to adverse environmental conditions as a result of natural selection. These mainly include wild flora and plants that have been cultivated in this region for a long time.

“Excluders” or “deconcentrators” (“excluders”) can grow in a wide range of concentrations of mobile forms of CE compounds in the soil as long as the level of their accumulation is correctly controlled by physiological processes. Violations in these processes cause an unregulated absorption of elements, which leads to plant death. Ufimtseva (2015) notes that this group includes plants with clearly functioning biogeochemical barriers, primarily in the root system.

“Bioindicators” absorb CEs over a wider range, and the concentration of elements in leaves is determined by their concentration in soils until it becomes phytotoxic, i.e. it prevents the further life of plants and leads to their death.

“Hyperaccumulators” are capable of withstanding and accumulating much higher CE concentrations. The dotted line in Table 1 shows the threshold values for the content of elements (µg/g: Cd, Se, Tl, 100; Cu, Co, Cr, 300; Ni, As, Pb, 1000; Zn, 3000; Mn, 10 000). If these values are not reached, the plants are referred to as “concentrators” or “accumulators” (Ent et al., 2013).

Russian monographs and textbooks dealing with plant biogeochemistry (Vinogradov, 1957; Koval’skii, 1974; Kovalevskii, 1991; Perel’man and Kasimov, 1999; Dobrovol’skii, 2003; etc.) use a different terminology. The closest in meaning to the above definitions is the selection of habitual (typical) and unusual (atypical) concentrators among CE concentrating plants. Both of them can be group (simultaneous accumulation of several CE) and selective (accumulation of a single CE).

The listed options for the grading of the adaptive abilities of plants to increased concentrations of CEs in the environment are most often successfully combined. A classification scheme that takes into account the most common biogeochemical criteria was drawn up for higher plants based on the CE accumulation strategy (Fig. 3).

Fig. 3.
figure 3

Scheme of classification of higher plants based on the strategy of accumulation of chemical elements. Bold indicates grouping and terminology by Ent et al. (2013); the ordinary font indicates the terminology of Russian-speaking authors (Vinogradov, 1957; Koval’skii, 1974; Kovalevskii, 1991; Perel’man and Kasimov, 1999; Dobrovol’skii, 2003; Seregin et al., 2015; Ufimtseva, 2015).

Full size image

Unusual concentrators include most plants of the local flora with a wide range of CE tolerance. Plants can grow at elevated concentrations of elements in the soil and at a normal content. Under normal conditions, plants do not extract large amounts of the element from unenriched soil. When settling in areas more enriched in CE, unaccustomed concentrator plants accumulate significant amounts of CE, approaching the usual concentrators in terms of the degree of accumulation. Thus, the terms “bioindicator” and “unusual (atypical) concentrator” are very close.

Habitual (typical) concentrators, as a rule, always extract significant amounts of CE from the environment, even at an average, normal content in the soil, because they have already developed a special metabolism due to the genetically fixed need of the corresponding species for specific elements. Among them, the group of “hyperconcentrators” stands out: plants with an extremely pronounced adaptation to very high CE concentrations in the environment.

PLANT CONCENTRATORS (ACCUMULATORS)

The terms “concentrator” and, especially, “hyperconcentrator” are widely and freely interpreted and used by different researchers. For example, Vinogradov (1957) distinguished two types of concentrator organisms: (1) those living in areas with a high content of any CE and having its high content and (2) those accumulating any CE in any environment. Kovalevskii (1991) proposed to compare the relative content of CEs in plant species (RTC) growing under comparable conditions and to classify as concentrators those species for which the RTC exceeded 2.5. Various biogeochemical coefficients were also used. Perel’man and Kasimov (1999) pointed out that CEs accumulate in a plant if the coefficient of biological absorption Ax (aka BBP, Kb), the ratio of the CE concentration in the plant ash to its total content in the soil, is greater than 1. M.Ya. Lovkova et al. (2014) classified as concentrators those plants for which the coefficient of biological accumulation (CBN, also known as the coefficient of biogeochemical mobility Bx), the ratio of the CE concentration in the dry plant matter to the content of its mobile forms in the soil, is higher than 1. If it was not possible to calculate this coefficient, they considered concentrators to be plants with CE contents that were several times higher than the clarke values and hyperconcentrators to be plants that were higher by an order of magnitude or more.

Similar approaches were also used in foreign literature. Markert (1992) proposed to compare the CE content with the elemental composition of the “generalized standard plant” (Reference Plant, RP); Dunn (2007) later somewhat clarified the recommended values. Bargagli’s monograph (Bargagli, 1998) gives the concentration factor (CF). Numerous articles use synonymous terms: “enrichment factor” (Branquinho et al., 2007), “bioaccumulation factor” (Kovacik et al., 2012), “transfer factor” (Overesch et al., 2007), etc. In general terms, the coefficient is calculated as the ratio of the CE concentration in the aerial parts of plants to its concentration in the soil. However, the authors use the CE content in both dry and wet biomass and not only their total amount in the soil but also various mobile forms, which greatly complicates the comparative analysis of literature data. Reimann et al. (2001) point out that this indicator should be used with caution, since the use of the total CE content in the soil or mobile forms of CE compounds extracted by different extractants will lead to completely different results. It is also noted that, due to the high variation in the CE composition of soils and plants, the calculation of various coefficients based on the CE content cannot be accurate and allows only general conclusions to be drawn (Zeiner et al., 2015).

HYPERCONCENTRATOR PLANTS (HYPERACCUMULATORS)

A review by Ent et al. (2013) indicates that the term “hyperconcentrator” has been used millions of times since the mid-1970s by thousands of people with varying degrees of accuracy, relevance, and understanding, in ways that did not always correspond to the views of the creators of the terminology and current authors. In this regard, it is necessary to clarify the circumstances under which the term “hyperconcentrator” is appropriate and to define the conditions that must be met when using this term.

The term “hyperaccumulators” was used to describe nickel accumulation by Jaffré et al. (1976). It was applied later to plants that accumulated more than 1000 µg/g Ni in dry leaves (Baker, 1981). This threshold is 100–1000 times greater than what is typically found in plants on non-ultramafic soils and 10–100 times higher than most other plants on nickel-rich ultramafic soils. In addition, this value separates two modes of the bimodal frequency curve: more or less lognormally distributed concentrations up to 1000 µg/g and a remote cluster at exceptionally high concentrations (Brooks et al., 1979). Due to the relative rarity of hyperaccumulators of CEs other than nickel, even a high level of CE accumulation often does not make it possible to clarify whether hyperaccumulators form a qualitatively different group (bimodal structure) or are the tail of a positively distorted continuous (lognormal) distribution (Broadley et al., 2001).

A more precise definition was later given (Reeves, 1992): “A nickel hyperaccumulator is a plant in which a Ni concentration of at least 1000 µg/g has been recorded in the dry matter of any above-ground tissue in at least one specimen growing in its natural habitat.” Thus, the use of this term is inappropriate for the analysis of underground organs or the entire plant. First, this is due to the difficulty of the collection and preparation of samples that are not contaminated with soil particles. Second, the immobilization of metals in the root system is a very common phenomenon, while the active accumulation of CEs in aboveground plant tissues makes the diagnosis of hyperaccumulators more accurate (Baker et al., 1994). Ent et al. further clarified (2013):

— plant leaves must be used to establish the status of a hyperaccumulator;

— accumulation should refer only to the active accumulation of elements within the leaf tissue of the plant through the roots;

— the passive accumulation of elements via atmospheric deposition on plant leaves should not be considered.

In additional, attention should be focused on the end of the definition given above, “…grows in a natural habitat”: hyperconcentrators must accumulate high concentrations of metals while remaining healthy enough to maintain a self-sustaining population. Baker and Whiting (2002) pointed out that new species of hyperaccumulator plants are often erroneously reported from experimental studies in artificial conditions, when high doses of HM salts are added to the soil or nutrient solution. However, experiments in such cases almost never go so far as to demonstrate the formation of viable seeds. Moreover, a “forced” or “induced” uptake of metals often leads to plant death in the end. Although this is of interest for some phytoremediation strategies, it may have nothing to do with the ongoing life cycle of natural metallophyte populations.

Among hyperaccumulators, “obligate” (“mandatory,” “strict”) and “facultative” (“optional”) types are distinguished (Pollard et al., 2002). Obligate hyperaccumulators are endemic to some types of metal-bearing soils and always accumulate high concentrations of the element. Facultative hyperaccumulators are representatives of populations in which only some plant specimens are hyperaccumulators. In a review by Pollard et al. (2014) on hyperaccumulation issues, it was noted that, despite attempts to understand the general patterns of intraspecific variations, there are still species with paradoxical behavior. Such species have wide geographic ranges and adapt to various ecological conditions, including both “normal” and “serpentine” soils; however, they manifest themselves as hyperaccumulators only on a small number of sites on each of them. It is assumed that facultative hyperaccumulation can be associated both with genetic differences between individual populations of the species and with the difference in the content and availability of CEs in soils.

CRITERIA FOR HYPERACCUMULATION

At present, hyperaccumulators include plants with an aerial part that contains CEs in the following amounts (μg/g of dry matter): more than 100 for Cd, Se, and Tl; 300 for Co, Cr and Cu; 1000 for As, Ni and Pb; 3000 for Zn and 10 000 for Mn. The concentration should be two to three orders of magnitude higher than that in the aerial part of most species on normal soils and at least one order of magnitude higher than the usual range found in plants from metal-bearing soils (Ent et al., 2013). However, the nominal threshold values should not be considered “ultimate truth”; for example, plants exhibiting extreme physiological behavior and stably accumulating 900 µg/g Ni can be considered hyperaccumulators of this metal. At present, criteria have been developed for 11 elements, but some of them are already in question. The hyperaccumulation criterion for Cu is considered too high (Faucon et al., 2007). Krämer (2010) recommended lowering the criteria for Cu and Co, and Ent et al. (2013) supported this proposal.

In addition to nominal thresholds, there are other criteria that define hyperaccumulation. In particular, hyperaccumulators have a very high concentration coefficient (see above the ratio of the CE content in the aerial parts of plants to its content in the soil), since their physiological characteristics provide active metal binding and an increase in their concentration. Hobbs and Streit (1986) suggested that this coefficient be considered an indicator of hyperaccumulation. However, it was later shown that the use of the concentration coefficient alone was not sufficient to determine hyperaccumulation based on field-collected material (Pollard et al., 2002). This coefficient manifests itself as the interaction of the genotype with the environment and is controlled both by genetically determined plant physiology and local edaphic conditions. In this regard, there is no unambiguous relationship between the content of the element in the leaves and soils (Pollard et al., 2002). In addition, it was found that the plant intensively absorbs the available amount of the forms of this element available to it at a low concentration of the element in the soil; only in the case of a high CE content in the soil does the regulatory mechanisms begin to work (Bargagli, 1998; Kabata-Pendias, 2010). A complicating factor is that no chemical extractant universally and accurately extracts the plant-available fraction of the metal contained in the soil (Meers et al., 2007; Menzies et al., 2007).

Another criterion proposed for the determination of hyperaccumulation is the translocation factor—in hyperaccumulator plants, the content of metals in the roots is lower than that in the aboveground part (Macnair, 2003). It should be noted that various synonymous terms are found in the literature for assessments of the ratio of the concentrations of elements in the underground and aboveground parts of plants: the movement coefficient (MC) (Kovalevskii, 1991), the root-barrier coefficient (RBC) (Praktikum po agrokhimii…, 2001; Afanaseva and Ayushina, 2018), the acropetal coefficient (AK) (Sibgatullina et al., 2014), the transition coefficient (TC) (Zhuykova and Zinnatova, 2014), the translocation factor (TF) (Branquinho et al., 2007), the transfer factor (TF) (Lajayer et al., 2017), etc. At the same time, both the ratio of CEs in the underground part to the aboveground part is calculated, and vice versa, which leads to additional contradictions. It was suggested that the reduced amount of many CEs in the aboveground parts of plants may be associated both with the low need for them and with the existence of mechanisms that regulate their accumulation and distribution between roots and aboveground organs (Seregin and Kozhevnikova, 2008). However, a low content of available CE forms in the soil can lead to a more intense uptake of elements by plants, and the root-barrier effect appears only at high concentrations (Bargagli, 1998; Kabata-Pendias, 2010). The translocation coefficient is also not sufficient to prove hyperaccumulation for various reasons: (1) the difficulty of correct extraction of the root mass of plants, especially woody ones; (2) the difficulty of complete removal of soil particles from the roots ; (3) the conversion of the CE concentrations in various plant organs to dry matter may be incorrect, since it depends on the proportion of structural material in them; (4) the high CE content in the aerial part may depend on an external source of intake, etc. (Ent et al., 2013).

High values of the concentration and translocation coefficients are necessary, but not sufficient, criteria for hyperaccumulation, since they are also characteristic of concentrator species (Wei et al., 2008); therefore, the nominal threshold criteria, despite a number of shortcomings and a narrow range of elements, are considered a more mandatory condition for the identification of hyperaccumulators.

In matters of CE accumulation and detoxification, the tissues in which the element is localized are also of great importance (Isaure et al., 2006; Seregin and Kozhevnikova, 2008; Titov et al., 2014) For hyperconcentrators, the entry of the element into the mesophyll is typical, while clusters are observed only in epidermal structures in other cases (Kupper et al., 2000; Choi et al., 2001; Ma et al., 2005).

SURFACE POLLUTION OF PLANT MATERIAL

The determination of the source of CE entry into plants is a serious methodological problem. The foliar route of CE entry into plants can play a significant role under the conditions of anthropogenic atmospheric pollution, including fine soil particles (Amato et al., 2009; Kopylova, 2013) enriched in CE (Uzu et al., 2010). The content of some elements in these particles can be an order of magnitude higher than that in the soil as a whole (Siromlya et al., 2015). In such cases, high concentrations of a number of CE in the aerial parts of plants are the result of their passive accumulation through atmospheric deposition. At the same time, CEs do not actually accumulate in plants but are contained in dust particles on their surface (Zagurskaya and Siromlya, 2018).

The dust-holding capacity of plants depends not only on the type, size, and number of particles but also on many biological features of the taxon. An important role is played by both macrostructural features (plant height, size, shape and arrangement of leaves, etc.) and microstructural features (surface roughness, various types of pubescence, folding, specific excretory organs, the presence of a wax layer, the size and number of stomata and etc.) (Latyshevskaya and Strekalova, 2006; Sæbø et al., 2012; Leonard et al., 2016; etc.). For example, Kurinskaya et al. (2012) use only the leaf architectonics and the area of the assimilation apparatus to explain the highest Pb content in wrinkled leaves in comparison with other studied species of Lolium perenne L. Korelskaya and Popova (2012) indicated that the content of Zn, Cu, and Pb in dust particles that settled and sorbed on the surface of plants increased the content of metals in leaves and grass by more than 30%.

Ent et al. (2013) questioned the results of a number of previously published works on the identification of new hyperconcentrator plant species precisely because of the possible contamination of the plant surface by atmospheric aerosol particles containing significant amounts of HMs. At the same time, it was indicated that they need to be confirmed with samples treated to remove surface contamination. For example, most copper hyperaccumulating plant species that were described in the Democratic Republic of the Congo (Brooks et al., 1982) did not appear to be such in a later study that used intensive washing of the plant leaf material (Faucon et al., 2007). The scientists found that 12 species that had previously been studied and reported as hyperconcentrators of copper did indeed have high concentrations of the element, but the values rarely exceeded the nominal thresholds. They concluded that a large change in the concentrations of the element in plants within the same plot, a significant linear correlation between its content in soils and plants (characteristic of bioindicators), and a relatively low concentration of copper in many samples are not typical for hyperconcentrators, and they also suggested that most of the previously analyzed samples were contaminated with dust. For example, the addition of 0.2 mg of malachite in the form of dust to 100 mg of plant leaves containing 10 μg/g copper is sufficient to increase the apparent concentration of Cu by more than 1.150 μg/g (Reeves and Baker, 2000). On the same basis, Ent et al. (2013) also questioned the examples of chromium hyperaccumulation described in previously published works (Zhang et al., 2007; Redondo-Gomez et al., 2011) and explained such phenomena by passive accumulation via deposition from the air on the plant leaves.

In the conditions of the southeast of western Siberia, the influence of surface pollution on the CE composition of plants was manifested in a study of Leonurus quinquelobatus Gilib. (Zagurskaya and Siromlya, 2018), as well as Artemisia sieversiana Willd. and Urtica cannabina L. (Siromlya, 2019). It was found that a significant part of the samples did not meet the requirements of the State Pharmacopoeia of the Russian Federation (2018) for the total ash content and ash residue insoluble in 10% HCl. The latter indicator makes it possible to estimate the proportion of impurities of predominantly mineral origin, i.e., it actually reflects the dust content of plants. Comparison of the CE concentration in plant samples that did (“pure”) and did not correspond (“dusty”) to the above requirements found a statistically significant difference. It was also found that the amount of CE in “pure” samples generally corresponded to the upper part of the range of their content in plants of the other studied species. This phenomenon is explained by the specific anatomical and morphological features of the above species: these plants are densely pubescent with long soft hairs (Konspekt flory…, 2005). Minkina et al. (2017) also showed that, under conditions of technogenic pollution, the greatest amounts of CE accumulate in the aerial part of pubescent plant species: Ambrosia artemisiifolia L. and Artemisia austriaca Jack. Belonogova (2009) notes that, among the numerous studied types of medicinal plant materials, an increased content of total ash insoluble in 10% HCl and HMs is characteristic of nettle leaves (Urtica dioica L.) and coltsfoot (Tussilago farfara L.). The fact that pubescent plant species concentrate CE to a greater extent than less pubescent ones was also noted in other publications (Latyshevskaya and Strekalova, 2006; Leonard et al., 2016).

It is also interesting to note that the content of many of the studied CEs in the underground part of A. sieversiana and U. cannabina turned out to be lower than that in the aboveground part. A root-barrier coefficient less than 1 in this case indicated not the CE concentration in the aerial parts of plants but their increased dust content and foliar intake of a significant proportion of these elements, which was also noted in the literature (Kovalevskii, 1991; Ent et al., 2013; Kopylova, 2013).

In addition to the high ash content and the residue insoluble in 10% HCl, the dustiness of plants can be assessed by the Si content (Zagurskaya and Siromlya, 2018) and the chromium/nickel ratio (Syso, 1998). Reimann et al. (2001) point out that high values of Al and Fe can be taken as the first indication of the possible influence of mineral dust on the observed concentration of elements in plants. Given the problems of the detection of a number of elements in some species (e.g., Cr, Li, Sc and Th), Al, Fe, Si, V, Y, and Zr are the best indicators of dust content in plant material. Bargagli (1998) recommended that the concentrations of soil indicator elements (Al, Ti, Pu, Sc, Zr, Fe, Si) should be determined in plant samples together with the elements of interest in order to assess plant contamination with soil particles. It is then necessary to calculate the enrichment factor (EF): the concentration of the studied element in the plant is related to the concentration of Al or Ti in the same plant, and this ratio, in turn, is divided by the corresponding ratio of these elements in the soil or the Earth’s crust. As a general rule, the closer the EF is to one, the more likely is the soil origin of the element in question, and values much higher than one are indicative of atmospheric pollution.

CONTRADICTIONS DURING THE IDENTIFICATION OF CONCENTRATE SPECIES

Analysis of the literature data shows that the use of different accumulation criteria by different researchers leads to contradictory results.

For example, Masarovicova et al. (2010) report an inconsistent classification of Matricaria chamomilla L. (syn. M. recutita L.). It is considered by different authors to be a hyperaccumulator, an optional metallophile, and an exclusive species due to the use of various parameters and their diversity. Somewhat later, Kovacik (2013) drew attention to the fact that it is meaningless to draw conclusions about the accumulation potential of plants grown in hydroponics. He also noted that it is impossible to focus only on the values of the concentration and translocation coefficients without taking into account the decrease in plant biomass, etc., and also agreed with the recommendations (Ent et al., 2013) on the need to prove the Cd accumulation by plants M. chamomilla growing on naturally polluted soil.

Lovkova and Buzuk (2013) presented the results of a mass survey (about 200 species) of medicinal plants of the Russian flora, of which more than 80% are classified as concentrator species. At the same time, the basis for such conclusions is a comparison of the CE content in the MPs with the average Clarke values. An earlier publication by Lovkova and Buzuk (2011) indicated that the clarke values for copper (5 mg/kg) were taken from the monograph by Kabata-Pendias and Pendias (1984). There are no clarke values in this monograph, but the ranges of the Cu content in mature leaf tissues are given: 2–5 mg/kg, deficiency; 5–30 mg/kg, normal concentration. Focusing on these values, it can be noted that almost all of the studied types of MPs fit into the range of normal concentrations: the copper content in them is 15.5–27.0 mg/kg. However, the authors refer them to concentrators. The yellow capsule is referred to the superconcentrators: the yellow water lilly Nuphar lutea L. (33.3 mg/kg), the swollen Lobelia inflata L. (41.1 mg/kg), the common anise Pimpinella anisum L. (43.3 mg/kg), and the cudweed Gnaphalium uliginosum L. (105.1 mg/kg) (moreover, the rhizomes were studied in the water lily, while the fruits were studied in the anise). However, their copper content is much lower than the threshold value for hyperconcentrators of 300 mg/kg proposed by Ent et al. (2013).

The data on the CE concentration of other MPs (Lovkova and Buzuk, 2013) are also in significant conflict with large-scale studies (Ent et al., 2013) and raise many questions. For example, the value of chromium clarke is 0.15 mg/kg, while the monograph by Kabata-Pendias and Pendias (1984) indicated the normal range of its content is 0.1–0.5 mg/kg, and the clarke values are 1.8 mg/kg (Dobrovolsky, 2003) and 1.5 mg/kg (Markert, 1992; Dunn, 2007). The use of such a low clarke value, 0.15 mg/kg, is probably why more than half of the studied types of MPs are classified as concentrators. At the same time, Lovkova and Buzuk (2013) indicated that there actually include no moderate accumulators and that there are only concentrators and superconcentrators of this element (the latter include plants accumulating 6–17 mg/kg of chromium). Ent et al. (2013) cited 300 mg/kg as a threshold value and, as noted earlier, generally questioned chromium hyperaccumulation, considering it to be an effect of dust. Our results (Zagurskaya and Siromlya, 2018; Siromlya, 2019) also showed that there were statistically significant correlations between the total chromium content and the amount of ash insoluble in 10% HCl for certain plant species characterized by a high level of dust content due to anatomical and morphological features. This is quite consistent with the data of Perel’man and Kasimov (1999), who classified chromium as an element with a low level of accumulation.

IS HYPERICUM PERFORATUM A CADMIUM HYPERCONCENTRATOR?

The results obtained by us in the study of Hypericum perforatum L. (St. John’s wort), growing in the southeast of Western Siberia. Many researchers consider H. perforatum to Cd hyperconcentrators (Schneider et al., 2002; Jurca et al., 2011; Lovkova et al., 2014; Badea, 2015). According to the definition (Ent et al., 2013), the plants in this case should accumulate more than 100 mg/kg Cd. However, studies (Siromlya, 2019; Zagurskaya and Siromlya, 2020) showed that elevated Cd concentrations of up to 1.7 mg/kg are found only in individual samples of H. perforatum (both cultivated and wild). In general, samples with a relatively low content of this element, 0.4–0.5 mg/kg, predominate in the region. Similar high intraspecific differences in Cd accumulation were noted in other species, e.g., Brassica napus L. (Grispen et al., 2006).

A large amount of literature data showed that the range of concentrations (mg/kg) of this element is small in the aerial part H. perforatum: 0.1–1.7 in Russia; less than 0.1 in Asia; 0.05–0.26 in America (Zagurskaya and Siromlya, 2020).

Upon closer examination of publications in which H. perforatum is called a Cd hyperaccumulator, there are doubts about the validity of this statement. M.Ya. Lovkova et al. stated in a monogram (2014) that H. perforatum is one of eight types of MPs concentrating Cd, but the Cd content is not given; only the coefficient of biological accumulation, 7.2, is indicated. In an article by Schneider et al. (2002), a similar coefficient changes from 13 to 888. In Badea’s work (Badea, 2015) H. perforatum are classified as hyperaccumulators because the Cd content in it was 0.13 mg/kg and turned out to be higher than in other types of studied plants (0.04–0.12 mg/kg) in earlier publications (Gasser et al., 2009; Đurović et al., 2013). However, Đurović et al. (2013) noted only a high Cd concentration (0.73–1.12 mg/kg), which is three to four times higher than the values proposed by WHO (0.3 mg/kg of dry matter). Gesser et al. (2009) gave data showing that the amount of Cd in 90% of the studied samples does not exceed 0.95 mg/kg, and they proposed the introduction of exceptions for its maximum permissible content in the grass H. perforatum: 1.0 mg/kg compared with 0.5 mg/kg in the European Pharmacopoeia 2008. Thus, none of the reviewed publications allows the assignment of H. perforatum to Cd hyperaccumulators. The authors of another work (Pavlova and Karadjova, 2013) come to the same conclusion, pointing out that the Cd content in H. perforatum samples are no higher than those in plants of other species growing under the same conditions.

Thus, H. perforatum cannot be attributed to Cd hyperaccumulators; the content of this element in the aerial parts of plants does not exceed 100 mg/kg. The relatively high levels of this CE in some cases may be due to the fact that the samples were taken in the vicinity of industrial enterprises (Glavač et al., 2017) or on a mountain slope (Đurović et al., 2013), as a result of which an increased amount of some CEs may be associated with the presence of ore (Gravel et al., 2012). In other works (Radanovic et al., 2002; Jurca et al., 2011; Owen et al., 2016; Zagurskaya and Siromlya, 2020), elevated Cd concentrations (i.e., exceeding the maximum allowable concentrations for medicinal plant materials and plant-based dietary supplements) are noted only for single samples.

CONCLUSIONS

For correct scientific communication, it is necessary to unify and standardize the conceptual apparatus and to specify the terms related to the accumulation of CEs by plants. The summary of a large amount of literature data identified two main groups of plants: those that adapted only to low/normal concentrations of mobile forms of CE compounds in soils and those that adapted to a wider range of concentrations, including significantly increased ones. The use of different accumulation criteria by different researchers often leads to conflicting results. The following approaches are used to determine hub species: (1) comparison of the CE content in plants with their content in other species, clarkes, a “generalized standard plant,” etc., and (2) various biogeochemical coefficients. However, they do not contradict each other and become much more informative when used together.

It should be emphasized that, the reliable detection of CE accumulation requires the use of leaves of plants growing in the natural habitat that are healthy enough to maintain a self-sustaining population. In this case, only the active accumulation of the element inside the plant leaf tissue through the roots should be taken into account; passive accumulation through atmospheric precipitation should not be considered in this case. The dust content of plant material can be estimated from the total ash content, the amount of ash residue insoluble in 10% HCl, the chromium/nickel ratio, the enrichment factor, and the content of Si, Al, Fe, etc.

Biogeochemical coefficients are also used to identify hyperconcentrator species; however, the initial condition is that the nominal threshold values for CE content are exceeded. Unfortunately, these values are currently set for only 11 elements, and these boundaries are often subject to revision. In all other cases, it makes sense to speak of hyperaccumulation only when the concentration of the element in the plant is two to three orders of magnitude higher than that in the aboveground part of most species on normal soils and at least one order of magnitude higher than that in the usual range found in plants from metal-bearing soils.

Thus, the use of a versatile and multifaceted approach to the assessment of the accumulation potential of plants will help to eliminate most of the currently existing contradictions associated with the identification of strategies for plant adaptation to high CE concentrations.