Phytoremediation efficiency depends on a complex sequence of factors influencing the number of interactions, one of the most important being the species/variety of used plant. In different studies significant diversity in the phytoextraction/ phytodegradation effectiveness was confirmed, not only within the species but also among varieties of the same plant species (Mleczek et al. 2010). The most important differences of plant traits are as follows: structure, size, immunity and individual traits such as environmental and climatic requirements (water, nutrients or temperature).
Natural bioremediation with selected bacterial strains and fungi is an interesting solution in decontamination of areas polluted with organic compounds (Juwarkar et al. 2010). Like other methods, bioremediation is also limited (by interaction of microbes with existing microorganisms, presence of toxic substances inhibiting microbial development or low bioavailability of xenobiot-ics). Hence, co-operation of both methods has a significant role in increase of phytoremediation efficiency. Along with many strategies focused on plants' accommodation to unfriendly pollutants, a symbiosis with mycorrhizal fungi seems to be very helpful (Vamerali et al. 2010). Fungi in the rhizosphere are a significant factor in phytoaccu-mulation/phytodegradation efficiency increase, to stimulate plant growth as well as to increase the resistance of plants to concentrations of pollutants found in the environment. The synergism is especially significant in the case of hyperaccumu-lators. According to the literature data mentioned above, more than 400 plant species are documented as hyperaccumulators and they belong to the following families: Asteraceae, Brassicaceae, Caryophyllaceae, Cyperaceae, Cununiaceae, Fabaceae, Flacourtiaceae, Lamiaceae, Poaceae, Violaceae and Euphorbiaceae.
The presence of microbial clusters (different genes in rhizoremediation) may decrease levels of plant stress hormone. Especially significant is a combination of plant and plant growth promoting rhizobacteria (PGPR). Inoculation of selected plant species (including hyperaccumulators) with endophytic bacteria, e.g. Achromobacter xylosoxi-dans, Bacillus pumilus, Corynebacterium flave-scens, protects plants against the phytotoxic effects of heavy metals and/or different xenobiotics (Glick 2010). In this case a significant role of plants able to exude pollutant-degrading enzymes into the rhizosphere can be found in plants, fungi, endo-phytic bacteria and root-colonizing bacteria (e.g. root-specific laccase (LAC1), peroxidases, haloal-kane dehydrogenase (DhaA), P450 monooxyge-nases, phosphatases, nitrilases). These enzymes are able to transform pollutants without their uptake (Dowling and Doty 2009; Gerhardt et al. 2009).
The efficiency of selected heavy metals' phy-toaccumulation depends on the mutual relations of macroelements important in plant growth (nutrition) and development in the polluted matrix. When macroelements are present in excess or deficiency, oxidative stress begins but also different ratios of them are important in the phytoremediation process. As an example, when compared to the physiological Ca/Mg ratio (4:1), an increase of calcium ion concentration in relation to magnesium ions in studied soil (Mleczek et al. 2011) resulted in decreased cadmium and lead phytoextraction efficiency by Salix viminalis 'Cinamomea'. Additionally, Salix growth was restrained under 1:10 Ca/Mg ratio while it was stimulated under 20:1 ratio, which is opposite to cadmium and lead sorption.
Additionally, a change of Ca/Mg ratio influences the amount and kind of low molecular weight organic acids (LMWOAs) exuded into the rhizosphere. A model experiment where the efficiency of formation of selected LMWOAs depending on cadmium, copper, lead and zinc concentration was tested and indicated selective exudation of acids depending on the concentration and the kind of heavy metal. In physiological 4:1 Ca/Mg ratio the following acids formed complexes with particular heavy metal ions: citric, lactic, maleic and succinic acids with Zn2+, and malonic acid with Pb2+ and Zn2+. A change of Ca/ Mg ratio to 1:10 caused that citric (Cd2+, Zn2+ complexation), maleic and succinic (Cd.+, Cu2+, Pb2+, Zn2+) acids were observed in the rhizosphere (Magdziak et al. 2011).
The rhizosphere, as the space in the immediate vicinity of roots, is permanently influenced by their exudates. Moreover, it differs - in relation to other soil fractions - in the composition and large amounts of bacterial cells (the phenomenon of bacteriolysis) and fungi (mycorrhiza), with diversified decay of plant roots, soil structure, composition of organic matter, pH, humidity and microorganism activity. The properties often change in a particular site, demonstrating frequently dynamic changes in time (Macek et al. 2007) . All the above-listed factors influence the solubility and uptake of pollutants, both indirectly, through the change of their microbiological activity and the root growth dynamics, and also directly through changes of soil reactions, chelation, precipitation of deposits and oxidation-reduction reactions.
In the broad spectrum of organic compounds present in the rhizosphere, particular attention is focused now on LMWOAs. Organic acids such as: malic, oxalic, acetic or citric are recognized as the most significant ones in many different processes in the rhizosphere. Depending on their degree of dissociation (efficiency), and the amount of carboxylic groups in the molecule, acids can appear in the form of differently charged anions, which in consequence results in the possibility of metal cations' complexation and relocation from the soil. This is the reason that acids are reported as components of the soil environment which in the rhizosphere take part in many processes, e.g. in dissolving and uptake of nutrients (e.g. P and Fe) by plants and microorganisms, decrease of stress associated with anaerobic conditions, dissolving soil minerals leading to pedogenesis, and detoxification of heavy metals by plants (e.g. Al).
LMWOAs exuded by plant roots play a significant role in bacterial microflora composition in relation to nutrients and amounts of available forms of elements in the soil (Magdziak et al. 2011). Moreover, acids influence decomposition of organic matter, and structural formation with particular physical-chemical soil properties. Heavy metals are present in polluted soil in a form insoluble in water, because afore-mentioned water-soluble LMWOAs, as rhizosphere components excluded by the plant root system, change the rhizosphere features, which results in heavy metals' complexation to insoluble forms in soil. LMWOAs usually appear as anions. This allows for instantaneous reaction with metal ions, in the water phase, soil solution and in constant phases, which makes it an important element in the phy-toremediation process. It is worth underlining that interaction of organic acids with metals and other elements closely depends on the kind of soil. For example, for a nutrient such as phosphorus, dissolving and mobilization of ions of this element by selected LMWOAs (oxalic and citric acids) is closely related to the soil type. Similar relations exist for other exuded LMWOAs playing a significant role in macroelement mobility increase (e.g. Cu, Cd, Zn) and mechanisms of heavy metal immobilization (Al, Cd, Ni).
The problems presented above and associated with the type of soil and its physical-chemical properties indicate ambiguous information about LMWOAs' function in the rhizosphere. Some data inform about mobilization and elution from the soil of heavy metals after soluble complex formation with the acids, but this information is fragmentary and insufficient to answer the following questions: (1) do organic acids released by roots influence the mobilization and uptake of heavy metals by plants from the rhizosphere or (2) is their amount dependent on the concentration and chemical character of the metal or (3) does the amount of acids indicate activation of the plant defence mechanism against stress? Such information can also elucidate the role of plant genetic factors in increase of heavy metals' availability and uptake from soil followed by improved effectiveness of heavy metal accumulation. For that reason more detailed studies are needed to fully answer the above questions.
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