226 Annales Universitatis Paedagogicae Cracoviensis Studia Naturae, 6: 226–250, 2021, ISSN 2543-8832 DOI: 10.24917/25438832.6.13 Peiman Zandi1,2*, Joanna Puła3, Xing Xia2, Elke Bloem4, Aminu Darma2, Yaosheng Wang2, Ingrid Turisová5, Qian Li6, Luu Ngoc Sinh7, Na Li2 1International Faculty of Applied Technology, Yibin University, Yibin 644000, P.R. China; z_rice_b@yahoo.com 2Institute of Environment and Sustainable Development in Agriculture, Chinese Academy of Agricultural Sciences, Beijing 100081, P.R. China 3Department of Agroecology and Crop Production, Faculty of Agriculture and Economics, University of Agriculture, Mickiewicza 21 Ave, 31-120 Krakow, Poland 4Institute for Crop and Soil Science Julius Kühn-Institut, Federal Research Centre for Cultivated Plants, Bundesallee 69, 38116 Braunschweig, Germany 5Department of Biology and Ecology, Faculty of Natural Sciences, Matej Bel University in Banská Bystrica, Tajovského 40, Banská Bystrica 974 01, Slovakia 6Institute of Vegetables and Flowers, Chinese Academy of Agricultural Sciences, Beijing 100081, P.R. China 7Faculty of Sciences and Technology, Hanoi Metropolitan University. 98 Duong Quang Ham, Cau Giay, Hanoi, Vietnam More insight into the concept of iron plaque formation and its characteristics in rice (Oryza sativa L.) Introduction Rice (Oryza sativa L.) is the second most widely cultivated cereal crop, and around half of the world’s population depends on rice consumption (Bazrkar Khatibani et al., 2019). However, the enhanced concentration of toxic metal(loids) in paddy soil due to anthropogenic activities has stimulated their increased accumulation in rice (Norton et al., 2014; Clemens, Ma, 2016), posing a signi�cant negative impact on human health via the food chain pollution. As a typical semi-wetland plant, rice is characterised by releasing oxygen (O2) from the root surface in the rhizosphere when growing in a saturated or anaerobic environ- ment; this induces the oxidation of ferrous (Fe2+) to ferric (Fe3+) and subsequently their precipitates as Fe oxides on the outermost cell layers of the root (Sundby et al., 1998; Hu et al., 2014). Iron oxide deposits are known as irregular porous coatings on the roots of hydrophytes (Tripathi et al., 2014). According to the literature reviewed by Khan et al. (2016), several studies have analysed the iron plaque (IP) deposits by employing various techniques, including X-ray di�raction and energy-dispersive X-ray microanalysis. �ese deposits consisted mainly of a mixture of lepidocrocite [g-FeO(OH)], goethite [α-FeO(OH)], and ferric phosphate (FePO4), in which manganese (Mn) and Fe usually 227 co-occur (co-precipitate/adsorb). Under laboratory conditions, Bacha and Hossner (1977) reported a ratio of Fe (43): Mn (1) in the IP of rice roots. Either amorphous or crystalline in structure, IP is composed dominantly of 63% ferrihydrite [(Fe3+)2O3 × 0.5 H2O], 32% goethite, and 5% of siderite (FeCO3) (Hansel et al., 2001). �e latter is only formed on microsites where CO2 concentration from respiration is high due to insu�cient eÈux (Hansel et al., 2001). Sey�erth et al. (2010) showed that IPs are not evenly distributed on rice roots. Predominantly mature roots show a high level of IP, while thin immature roots show minimum IP formation (Sey�erth et al., 2010, 2011). In contrast, the lowest and highest incorporation rate of rice root sections into IP coating from (oxyhydr) oxides (ferrihydrite 81–100%) sources were attributed earlier to the root base and root tips, respectively (Liu et al., 2006). Fe hydroxides’ mineralogy in IP of rice root showed that ferrihydrite was the main constituent of IP (Sey�erth et al., 2011). Frommer et al. (2011) showed that the accumulation of Mn and Fe highly depends on the root thickness. Based on their observation, Fe accumulation near �ne immature roots (< 100 µM) was more discernible than at tick mature roots (⁓500 µM) compared with Mn. A  recent study on primary roots by Zandi et al. (2020) showed that root surface plaque deposition was predominantly built of Fe, regardless of root sections. �e unique accumulation pattern of IPs was later attributed to Fe’s kinetics and thermodynamics, Mn redox transformations, and the extent of root aeration (Khan et al., 2016). �e disparities in observations may emanate from genotypic di�erences in the roots’ oxidising capacities and the biogeochemical conditions used in these studies. Root IP may also contain a variety of other metals and metalloids such as alumini- um (Al), arsenic (As), cadmium (Cd), chromium (Cr), mercury (Hg), nickel (Ni) and lead (Pb) (Tripathi et al., 2014). �is feature can indicate the involvement of IP in the adsorption of both anions and cations (Liu et al., 2004a; Yang et al., 2020) and thus introduce it as an essential barrier for heavy metal uptake and accumulation (Hossain et al., 2009a, b; Hu et al., 2014; Sun et al., 2016; Xia et al., 2020; Zandi et al., 2020). It should be pointed out that sequestration mechanisms of heavy metals on IP remain mostly unknown. Growing evidence suggested that dissolved organic carbon (DOC) and sulphur (S) could participate in the precipitation of Fe hydroxides on the root surface (Liu, Huang, 2003; Sun et al., 2016; Yang et al., 2016). �ey probably modify Fe hydroxide’s surface properties and molecular structures in IP, thus in�uencing the binding mechanisms of heavy metals to IP. �e speciation of S has also been shown to a�ect and improve the barrier feature of IP against Cr uptake and accumulation in rice roots (Zandi et al., 2020). Furthermore, a�er being reduced in upper plant parts, S by-products can chelate Cr and Cd, and thereby actively reduce their mobility and toxicity in roots and shoots (Zhang et al., 2013; Cao et al., 2018; Yamazaki et al., 2018; Zandi et al., 2021). Among di�erent biotic or abiotic factors involved in IP formation and development, root-released O2 is speculated to show a strong involvement (Khan M ore insight into the concept of iron plaque form ation and its characteristics in rice (O ryza sativa L.) P ei m an Z an di , J oa nn a P uł a, X in g X ia , E lk e B lo em , A m in u D ar m a, Y ao sh en g W an g, In gr id T ur is ov á, Q ia n Li , L uu N go c S in h, N a Li 228 et al., 2016; Huang et al., 2020). �e physical barrier of IP has been ascribed to the pro- portion of roots coated by iron oxides (Xia et al., 2020), the composition type of iron oxides (Amaral et al., 2017) and the ability to develop new roots (Zandi et al., 2021). �is review primarily aimed at providing more insight into the intrinsic features of iron oxide plaques, the in�uencing factors on their root surface build-up, and their key role in growth parameters and heavy metal sequestration and immobilisation in rice as a semi-wetland plant. Characterisation of iron plaque Wetlands sediments are composed of iron (Fe) elements. Ferrous ion (Fe2+) is a unique species in anoxic sediment pore water capable of donating electrons (Tripathi et al., 2014). Ferric ion (Fe3+) is an electron acceptor Fe species available dominantly in oxic pore water. In addition to the Fe plaque, Mn plaque may also be formed on hydrophyte roots through oxidisation of Mn2+ by root-released O2 (Hu et al., 2007). �e formation process of IP (Fig. 1) primarily refers to the following chemical reactions (Amaral et al., 2017): Fe3++ e− (reducing factors in the bulk/rhizosphere soil) Fe2+ (1) Fe2+ + O2 (released/di�used from root) Fe3+ (Iron plaque) (2) Mn2++ O2 (released/di�used from root) Mn4+ (Manganese plaque) (3) �e concentration gradient is an essential factor in triggering the depletion of Fe2+ from bulk sediment and its further movement into the hydrophyte rhizosphere. In the rhizosphere, root-released O2 oxidises Fe 2+ to Fe3+ that subsequently precipitates as FeOOH (or iron oxyhydroxide) and accumulates on the root surface of hydrophyte and soil particles as IP (Fig. 1) (Hu et al., 2007; Yang et al., 2014). �e interior root penetration of iron deposition (plaque distribution) varies among species. �ese di�erences in penetration may result from the dissimilar potential of various species in oxidising rhizosphere because of their variant characteristics of root radial oxygen loss (ROL), root respiration and/or variance in the extent of soil oxygen demands (St-Cyr, Crowder, 1989). Inner parts of rice roots viz. epidermis, hypodermis and exodermis coated with iron spots/stains, were detected so far (Pereira et al., 2014). Internal iron hydroxide precipitation is possible at low ROL. In addition to the typical root plaque formation, shoot bases and rhizomes of hydrophytes also can partially be covered by IP (Povidisa et al., 2009). 229 �e conception of iron plaque formation: in�uencing factors �e formation of IP occurs during the oxidation of ferrous (Fe2+), formed during pe- riods of anaerobic soil conditions, to ferric (Fe3+) iron (or iron oxyhydroxides) and the precipitation of resultant ferric oxide on the outer surface of roots (Khan et al., 2016). Many physicochemical characteristics of soils and/or sediments including organic matter, soil texture, soil redox potential (Eh) (Syu et al., 2013), soil pH (Zhang et al., 2019a), soil water management (Zhang et al., 2019b), Fe/ Mn availability (Shi et al., 2004), phosphorus (P) (Hu et al., 2005), selenium (Se) (Huang et al., 2020), As (Lee et al., 2013) and S supply (Hu et al., 2007), root-released O2 (radial oxygen loss-ROL) (Huang et al., 2020), root exudates (Becker, Asch, 2005; Wu et al., 2014), root enzymatic activates (Lee Fig. 1. Iron plaque formation in Oryza sativa L.; A − roots (cv. Xiangzaoxian 31) and rhizosphere; B1–3 – roots of di�erent rice cultivars (B1 – cv. Hashemi, B2 – cv. Lemont, B3 – cv. Xiangzaoxian 31); C1–2 – roots of rice seedlings cv. Xiangzaoxian 31 (upper row) and cv. Hashemi (lower row) before and a�er iron plaque induction, respectively, under laboratory conditions; D1–3 − roots of rice seedlings cv. Kazemi (D1), cv. Hashemi (D2) and cv. Xiangzaoxian 31 (D3) a�er harvest (Photo. P. Zandi and X. Xia) M ore insight into the concept of iron plaque form ation and its characteristics in rice (O ryza sativa L.) P ei m an Z an di , J oa nn a P uł a, X in g X ia , E lk e B lo em , A m in u D ar m a, Y ao sh en g W an g, In gr id T ur is ov á, Q ia n Li , L uu N go c S in h, N a Li 230 et al., 2007), microbial activities (Neubauer et al., 2007; references therein; Huang et al., 2012a), plant genotypes (Syu et al., 2014) and plant age a�ect IP formation (Fig. 2). �e formation of IP may arise (or result) from neutrophilic Fe2+ oxidising bacteria (Neubauer et al., 2007, 2008). �e discovery of both lithotrophic Fe-oxidising bacteria (FeOB) and Fe-reducing bacteria (FeRB) (King, Garey, 1999) on the root surface of many wetland plants indicate that plaque-associated microbes may directly in�uence IP formation. �e FeOB could contribute substantially to the formation of IP (Neubauer et al., 2007, 2008). Such bacteria have been shown to account for over 40–60% of Fe (III) mineral precipitation on roots of wetland hydrophytes under microoxic (⁓5–30 μM O2) conditions (Neubauer et al., 2002; Maisch et al., 2019). �ere is a general agreement that the reductive dissolution of IP results from rice root decay mediated by FeRB (Wang et al., 2009; Huang et al., 2012a). Root surface microbial Fe (III) reduction o�en results in rapid decomposition and subsequent release of crystalline iron minerals and their associated toxic metal(loids) to the immediate rhizosphere (Weiss et al., 2004, 2005). �is event usually leads to some notable envi- ronmental problems. While biological oxidation of Fe2+ may occur naturally, its biotic oxidation from ROL is likely to be of greater signi�cance (Tripathi et al., 2014; Huang et al., 2020). �e local conditions dramatically impact the extent and importance of biological oxidation at the root interface. Based on the heterogeneous distribution of soil oxygen due to microbial behaviour, root distribution, and soil morphology, paddy soils’ Eh and/or conditions vary (Yamaguchi et al., 2014). Maisch et al. (2019) indicated that ROL central role in ferric iron mineral formation is not only limited to the root surface but also to the whole soil matrix, where it expands potential microoxic living niches for microaerophilic Fe(II) oxidisers (microFeOx) throughout the entire rhizosphere. Soil moisture regime has been suggested to a�ect the amount of iron plaque formed on rice roots (Liu et al., 2010). �is amount was found much lower under lower soil moisture contents than under submerged conditions (Chen et al., 2008). Continuous �ooding conditions in paddy �elds signi�cantly increase the abundance of iron-reducing bacteria (e.g., Latescibacteria, Desulfuromonadales, and Geobacteraceae) under anaer- obic rhizospheric conditions, leading to less IP formation around rice roots (Zhang et al., 2019b). However, because the availability of Cd, zinc (Zn), copper (Cu), and other metals in the rhizosphere has decreased, the absence or reduced formation of root plaques cannot promote their presence in rice grains (Xu et al., 2013; Eduardo et al., 2014; Zhang et al., 2019b). In a continuously �ooded culture, changes in the soil pH (increase) and Eh (decrease) tend to increase the formation of hydrous oxides of Fe, Mn, aluminium (Al) and other metals in �ooded soils with a high a�nity for metal adsorption on them (more immobilisation), leaving less soil-available metal concen- tration for uptake in plant roots (Eduardo et al., 2014; Li et al., 2015). 231 Fig. 2. Comparison of factors a�ecting iron plaque formation in rhizosphere where rice roots environment is located; +, – and +/– are respectively indicative of positive, negative or conditional impacts on iron plaque formation; More explanations are in the text (Courtesy of word diagram: P. Zandi) Elemental S (S0) could induce the formation of Fe and Mn plaques on the root surface and mineral particles in the rhizosphere (Hue et al., 2007; Yang et al., 2014). On the other hand, sulphate (SO4 2-) minerals signi�cantly impacted IP’s formation and barrier function compared to S0 minerals (Hu et al., 2007). Di�erent concentrations of Fe plaque were formed on rice roots depending on the amount of S content in soil M ore insight into the concept of iron plaque form ation and its characteristics in rice (O ryza sativa L.) P ei m an Z an di , J oa nn a P uł a, X in g X ia , E lk e B lo em , A m in u D ar m a, Y ao sh en g W an g, In gr id T ur is ov á, Q ia n Li , L uu N go c S in h, N a Li 232 solution (Fan et al., 2010; Sun et al., 2016). A su�cient S supply has been suggested to play a central role in iron oxide formation and suppression of potentially toxic metals uptake and bioavailability in rice (Hu et al., 2007; Khan et al., 2016; Sun et al., 2016; Yang et al., 2016; Zandi et al., 2021). Excessive addition of S supply reduced the pro- portion of oxidised iron minerals through extra rhizosphere Fe2+ depletion via binding with monosul�de (S2-) to form pyrite (FeS/FeS2) minerals. �e resultant precipitates of FeS/FeS2 in bulk soil have been reported to diminish the mass �ow of Fe 2+ (and/or Mn2+) ions from bulk soil to the rhizosphere zone, leading to less IP formation on root surfaces (Hu et al., 2007; Fun et al., 2010; Sun et al., 2016). Moreover, the abundance of S2– ions in rhizosphere soil could also deplete more oxygen owing to S2- oxidisation. Figure (3) brie�y illustrates S cycling and its relationship with IP formation in sub- merged conditions. In several studies, authors have demonstrated that the absence of P in the rhizos- phere (P de�ciency) enhances the oxidising capacity of roots and consequently leads to more IP formation (Chen et al., 2008; Hussain et al., 2009b; Fu et al., 2010, 2014). Se-mediated enhancement of IP formation was attributed to its function in elevating the ROL of rice roots (Huang et al., 2020). ROL is assumed to be the most important driving force in the formation of plaques (Stcyr, Crowder, 1989; Xu, Yu, 2013; Yang et al., 2014). Wetland plant species, including rice cultivars, are di�ered based on their ROL oxidising capability (Deng et al., 2009; Yao et al., 2011). Rice roots porosities, aerenchyma structure and exodermis layer are three main controlling factors for mo- lecular oxygen transfer and radial oxygen loss (Yamada et al., 2005; Wu et al., 2011; Yamauchi et al., 2013). Among these, aerenchyma tissues enable the transport of oxygen to the roots under hypoxia conditions, and high root porosity tends to ROL from root to rhizoplane (Kirk et al., 2014; Huang et al., 2020). �ere is a general agreement that rice seedlings with their weak aerenchyma structure in their primary roots are more susceptible to paddy �elds’ anaerobic conditions than older rice plants. �ere were signi�cant disparities in ROL, IP formation and As/Cd accumulation in roots of rice plants among plant growth stages studied, the lowest occurred at the grain-�lling period (Wang et al., 2013). Some studies reported a signi�cant di�erence among various rice cultivars in IP formation, regardless of whether they were grown in hydroponic, glass beads or soil culture (Liu et al., 2006; Wu et al., 2012; Lee et al., 2013; Syu et al., 2014). In a  comparison study by Zhang et al. (2021), it was demonstrated that na- no-Fe3O4-modi�ed biochar (BC–Fe) was more e�cient in IP formation and Cd im- mobilisation in rice roots than ferrous sulfate (FeSO4), or chelated iron (EDTA-Fe). �ey have also shown that biochar loaded with nanoparticles of Fe could signi�cantly raise the proportion of crystallised IP on rice roots by up to 31.8%–35.9%. It has been reported that As feeding can accelerate the formation of plaque on hydrophilic plants’ 233 roots (Lee et al., 2013). �e underlying mechanism in accelerated IP formation under As feeding conditions has been earlier attributed to As stress-induced formation of superoxide anion (H2O2, O2 −) in di�erent plant parts, which later tend to form more plaque by di�using free radicals and O2 − out of the root (Mishra et al., 2011). Plaque formation has been generally reported to have negative associations with organic matter, inorganic carbonates (CO3) and Eh, and a positive association with ex- tractable Fe in the substrate (Batty et al., 2000). Iron immobilisation in the presence of inorganic carbonates (e.g., FeCO3) is a typical phenomenon in wetlands. High organic matter content in soil matrix has been found to bind iron and make it less available for cycling (Perret et al., 2000; Weiss et al., 2003). �e relationship between the root plaques and the concentrations of organic matter in sediments has been linked to the degree of organic matter decomposition and the creation of anoxia (reducing) condi- tions around the root surface. Syu et al. (2013) suggested that the Eh of the soil and the extent of Fe2+ released into soil solution are determined by two factors, the content of Fig. 3. Conceptual model illustrating sulphur cycling and iron plaque formation under the in�uence of submerged conditions. (Courtesy of conceptual model: P. Zandi) M ore insight into the concept of iron plaque form ation and its characteristics in rice (O ryza sativa L.) P ei m an Z an di , J oa nn a P uł a, X in g X ia , E lk e B lo em , A m in u D ar m a, Y ao sh en g W an g, In gr id T ur is ov á, Q ia n Li , L uu N go c S in h, N a Li 234 iron oxides and organic matter in �ooding soils. �ey further concluded that soil iron budget alone is not su�cient to form IP on roots of hydrophytes in �ooding conditions but when coincides with high contents of organic matter elevates the extent of Fe2+ ions release into soil solutions which further are oxidised to form Fe3+ on root surfaces (Syu et al., 2013, 2014). Amongst the leading factors a�ecting the extent of IP formation are organic matter and clay composition of soil texture. Soils with low clay contents are better candidates for IP deposition on roots than soils with high clay contents. �is report rests on the speculation that the low clay soils boundless iron, making it more available in solution than the higher clay soils. It is generally accepted that organic matter contents in clay soils with iron reservoir characteristics (Bacha, Hossner, 1977) are more than in sandy soils (Dou et al., 2016). St-Cyr, Crowder (1989) stated that soil organic matter usually counterbalances the e�ect of low/high clay contents on IP formation. In other words, clay minerals compete with organic matter for iron adsorption. Organic matters can instead adsorb iron desorbed from clay colloids to bound carbonates in paddy soils rich in organic matter. �e binding of iron and carbonates has been proposed as a pre- requisite step in the formation of plaques. Overall, it may be concluded that the e�ect of clay content on IP formation depends signi�cantly on the amount of soil organic matter and the positive correlation between clay and carbonate-bound iron, which can be achieved on the condition that organic matter proportion in the soil is constant. A su�cient amount of Fe2+ (or Mn2+) present in the soil sediment is required for plaque formation (Yang et al., 2011; Jia et al., 2018; Singha et al., 2019). �e degree of iron deposition on roots has been related to iron concentration in solution (Taylor et al., 1984) and to the dominant form of iron. Taylor et al. (1984) demonstrated that chelated iron, whether it is in oxidised (Fe-EDDHA-ferric-ethylenediamine di (o-hydroxy-phe- ny1 acetic acid), Fe-EDT (a-ferric-ethylenediaminetetraacetic acid) or reduced (Fe- BPDS-ferrous-4,7-di[4-pheny1sulfonate] 1,10-phenanthroline) states, had little chance to produce root plaques compared with Fe2+ in solution. �eir �nding supported the contention that supplying the soil culture medium with Fe3+, Fe3+ chelates, or Fe2+ che- lates results in less extensive plaque formation. However, at lower solution culture pH values (<4), ferric iron could produce more plaques compared with ferrous iron. At these lower pH values, signi�cant concentrations of Fe3+ ions were suggested to maintain in a soluble/ colloidal state and plants had utilised their reduction at the solution-root interface for plaque generation. Of di�erent soil iron fractions, the fraction at which iron is bound to carbonates has been central for the formation of root iron plaques. In contrast to hydroponic studies, showing a positive association between IP formation and Fe2+ in solution (Bacha, Hossner, 1977; Taylor et al., 1984), the exchangeable iron fraction in soil samples was not necessarily related to IP accumulation. �e underlying reason for this might be that iron oxyhydroxide plaques are not directly produced from 235 Fe2+ a�er oxidation but rather from iron carbonate siderite (FeCO3) that resulted from exchangeable iron transformation (Fig. 4). Siderites are subsequently oxidised by the oxidation potential of rhizosphere and rhizospheric CO2 concentration to goethite (α-FeOOH) or lepidocrocite (Ƴ-FeOOH). �e extent of Fe and/or Mn plaque formed on roots increases in proportion to the concentration of organic matter responsible for creating an anoxic environment in which these elements (a�er being reduced) are oxidised to form Fe and Mn oxides on root surfaces (Syu et al., 2013, 2014). Mn and Fe plaques are positively correlated to each other (Ye et al., 2003). In an earlier study by Crowder and Coltman (1993), it was shown that the amount of IP formed on the root surface was fully consistent with the amount of pH and Mn2+ in the rhizosphere. Similar to the results of Taylor et al. (1984), the formation level of Fe oxide plaque on rice roots remained higher upon increasing the pH value, as indicated by Zhang et al. (2019a). Soil pH impact on IP deposition on root surfaces in the rhizosphere can be consid- ered from two aspects: direct control of iron concentration in solution and dissolution of precipitated root iron and the indirect in�uence of root oxidising capacity. In other words, the pH of the soil can control the net oxidising capacity of the roots and hence cause the precipitation of iron in the rhizosphere. Earlier reports showing that the IP formation elevated linearly as the soil pH ranged between 3 and 4.6 in Typha latifolia L. (Taylor et al., 1984) or 3 and 5.3 in Oryza sativa L. Probably, the given pH ranges diminished the solubility rate of Fe3+ on the roots, leaving more root iron deposition. As with iron plaque reduction (lower IP accumulation) at pH above 5, it is speculated to have been occurred by soluble iron depletion due to Fe2+ oxidation in the culture solution (Taylor et al., 1984; St-Cyr, Crowder, 1989). �ese researchers found a positive correlation between lower IP accumulation and lower carbonate-bound iron fractions under rhizosphere pH of more than 5, suggesting the iron-bound to carbonate fraction to be the precursor to IP formation on the roots. �e relative quantities and reactivities of the reductants and oxidants in the rhizos- phere portray the extent to which the net oxidative and reductive nature of the rhizos- phere is determined. As the soil pH decreases, phenolic compounds as Fe3+ reductants are released into the immediate rhizosphere as a result of suberin and lignin decomposi- Fig. 4. Conceptual word diagram illustrating the relationship between iron-bound-to-carbonates (Siderite) and iron plaque formation in the rhizosphere (Courtesy of conceptual word diagram: P. Zandi) M ore insight into the concept of iron plaque form ation and its characteristics in rice (O ryza sativa L.) P ei m an Z an di , J oa nn a P uł a, X in g X ia , E lk e B lo em , A m in u D ar m a, Y ao sh en g W an g, In gr id T ur is ov á, Q ia n Li , L uu N go c S in h, N a Li 236 tion/ hydrolysis in root outer cell layers (endodermis and exodermis). In addition, at low pH, rice roots lose much of their oxidising power that will tend to lower the oxidation of Fe2+ in the solution, leading to negligible IP deposition. On the other side, at higher soil pH values, the oxidative capacity of the rhizosphere increases the possibility of IP accumulation to a degree (of pH) at which there is no iron available in the solution. In addition to the concentration and form of Fe, the soil Eh was also characterised as a driving component for plaque formation. �e impact of Eh on plaque formation depends primarily on its in�uence on iron concentration in soil solution (Taylor et al., 1984). An increase in the soil Eh may not restrict the plaque formation process until the soil solution iron is not decreased. On the other side, very negative Eh has been found to initiate an increase in soil oxygen demand more than the oxidising power of roots, resulting in the mitigation or absence of plaques. �e Eh value decreases with depth in the submerged soil pro�le (Schmidt et al., 2011). Plaques are mostly found on roots near the surface of submerged soil where the Eh value is neither too high nor too low to prevent plaque formation. Christensen et al. (1998) proved that IP is formed in sediment with relatively low Eh because high concentrations of reduced Mn and Fe di�use towards the surface of roots where they eventually were oxidised due to root oxygen release. �e oxidising capacity of plant roots, as a biotic factor, is an essential factor in con- trolling IP formation (Xu, Yu, 2013; Cheng et al., 2014; Huang et al., 2020). In addition to this, root plaque formation may also be in�uenced, as outlined above, by root exudates, root enzymatic activities and hydrophyte genotypes (Tripathi et al., 2014). Based on the studies conducted so far, there has been a general belief over the possible implication of root-released O2 on Fe oxidation in the rhizosphere (Khan et al., 2016; Huang et al., 2020). Although root-released O2 has been introduced as the primary cause of iron oxide plaque formation in wetland plants, in some exceptions (e.g., Menyanthes trifoliata L. and Molinia caerulea (L.) Moench), root enzymatic activity could play a decisive role in oxi- dising ferrous iron in the rhizosphere. Root exudates (organic acids, phytosiderophores, etc.) have a high capacity in the reductive dissolution of iron oxides (Lee et al., 2007) and mobilisation of iron in the rhizosphere (Wu et al., 2014). �e �ow rate of oxygen through the aerenchyma system is substantially associated with plant characteristics and environmental conditions (temperature and humidity) (Colmer, 2003; Yamauchi et al., 2013; Amaral et al., 2017). Rice cultivars with higher ROL rates showed a higher potential in root plaque formation (Cheng et al., 2014). However, in some cases, there has been strong evidence of the more appreciable e�ect of root enzymatic activity on oxidising Fe(II) for ROL scavenging (Becker, Asch, 2005; Wu et al., 2014). �e occurrence of Fe oxide plaque, which is believed to be liable for the seques- tration and immobilisation of various metal(loids) (Hansel et al., 2001; Bailey-Serres et al., 2012), is common in wetland and aquatic plant species such as Typha latifolia, Phragmites australis (Cav.) Trin. ex Steud, Aster tripolium L. (= Tripolium pannonicum 237 subsp. tripolium (L.) Greuter), and Spartina alterni�ora Loisel.; root and rhizome sur- face of the Cymodocea serrulata (R. Brown) Ascherson & Magnus; mangrove seedlings viz., Avicennia schaueriana Stapf & Leechman ex Moldenke, Laguncularia racemosa (L.) C.F. Gaertn., Rhizophora mangle L. and Oryza sativa (Tripathi et al., 2014). �ese species are usually grown under partially or all-time �ooded conditions. �is suggests that they possess a versatile development of internal aeration (air lacunae) systems that facilitate oxygen di�usion from the atmosphere to the plant roots for root respiration by basic dispersion through the air spaces within the cortical tissue (Bedford et al., 2001). Because some parts of roots are ‘leaky’ (to some extent suberised), oxygen di�uses outside of roots’ parts where it encounters Fe2+ and precipitates (mostly abiotically) as Fe3+ (�omas et al., 2005). �erefore, the plant’s ability to oxidise the rhizosphere is one of the critical factors underlying root iron oxide formation (Chen et al., 2007). Some wetlands’ seasonal impacts on IP accumulation were exclusively attributed to their closeness to the free-�owing water source containing carbonates. In addition to this, other factors which include ecotypic abilities in biomass and photosynthesis production for providing oxygen for ROL and iron oxidation, as well as site-speci�c characteristics in pH and Eh, were proposed as principals for site di�erences in root Fe deposition (St-Cyr, Crowder 1989). E�ect of rice origin on iron plaque formation Recent studies have pointed to the signi�cant di�erences among various rice cultivars and genotypes in IP formation (Wu et al., 2012; Lee et al., 2013; Cheng et al., 2014; Syu et al., 2014; Zandi et al., 2020). �e amount of Fe content in root plaques of lowland rice cultivars was reported to be higher than upland rice cultivars (Pereira et al., 2014). Moreover, rice cultivars’ di�erence with respect to IPs was attributed to exodermis selectivity, Fe nutrition solutions and variations in ROL (Khan et al., 2016). Lee et al. (2013) reported a signi�cant variation in the amount of root plaque among various rice cultivars grown in hydroponic culture. Similarly, Syu et al. (2014) demonstrated that the degree of root plaque formation in 14 indica rice genotypes was remarkably lower than that of 14 japonica rice genotypes. �e di�erence was ascribed to the lower oxidation capability of indica rice roots compared with japonica rice roots. According to the result by Weiss et al. (2005), plaque accumulation in di�erent plant species is characterised by their ability to release oxygen into the rhizosphere. Such potential in a speci�c plant species can also be related to the age of plants (Chen et al., 2008). �e duration between tillering and the ear emergence stage in rice coincided with a dramatic increase in ROL and Fe plaque (Wang et al., 2013). In another experiment on ten rice genotypes a�er iron treatment, it was found that the di�erence between genotypes in root and plaque Fe content was in close association with iron in�ux regulatory mechanisms in these genotypes (Asch et al., 2007). Additionally, Asch et al. (2007) demonstrated that sensitive M ore insight into the concept of iron plaque form ation and its characteristics in rice (O ryza sativa L.) P ei m an Z an di , J oa nn a P uł a, X in g X ia , E lk e B lo em , A m in u D ar m a, Y ao sh en g W an g, In gr id T ur is ov á, Q ia n Li , L uu N go c S in h, N a Li 238 rice genotypes had more iron content in the root and root iron plaque than resistance rice genotypes. �e amount and order of IP formation among rice cultivars studied so far were found di�erent for hydroponic and soil cultures; thereby indicating the multilateral controlling capability of root plaques (Lee et al., 2013; Syu et al., 2013). In a glasshouse study conducted to assess rice genotypic tolerance against As-rich irrigation water, Liu et al. (2006) found the concentration of IP formation o�en varied among genotypes. �ese authors observed a similar pattern of IP formation (tip> middle> base) in the roots of di�erent rice genotypes. In soil and hydroponic study under various aeration conditions, Wu et al. (2012) similarly found an additional mass of root plaque at di�erent root zones among various genotypes studied. Despite higher discharge of oxygen from root tips (Nishiuchi et al., 2012), the occurrence of root plaque was more apparent in older root parts of rice plants (Williams et al., 2014). �is could be explained by rapid development of immature root parts (growing root tips) (Sharma, Kaur, 2020). Iron plaque: bu�er or barrier for metal translocation? Great endeavours towards scrutinising IP function in metal sequestration and translo- cation have yielded con�icting results (Tripathi et al., 2014; Khan et al., 2016; Amaral et al., 2017). Earlier experiments suggest that IP’s barrier feature suppresses the uptake of elements (Christensen et al., 1998; Batty et al., 2000), while others estimate IP as a bu�- er/reservoir allowing the uptake of elements (Ye et al., 2001). Apparently, the entry of potentially phytotoxic metal(loids) into rice roots can be either decreased or increased based on the degree of IP formation (Tripathi et al., 2014). �e heavy coating of IP may impede the uptake and concentration of Zn in sea aster and rice, Cd (Liu et al., 2007; Xu et al., 2018) and Cr (Xia et al., 2020; Zandi et al., 2020) in rice and As in rice (Liu et al., 2006). Zhou et al. (2007) concluded a direct association between the quantities of Se accumulated in plaque and the amount of IP encircled root surface. �us, their studies demonstrated that Se concentration in shoots and roots of rice plants decreased upon increasing IP formation. According to Batty et al. (2000), under pH conditions of 6.0, IP presence lowers the content of Cu in the shoots of a robust perennial grass, common reed (Phragmites australis). Moreover, Spartina densi�ora Brong showed a higher potential to maintain essential metals around the root surface and control metal absorption by increasing root plaques’ formation (Cambrollé et al., 2008). �ere have been reports of IP mediated reduction in Cd concentration in response to Fe fertiliser application to Cd-contaminated paddy soil (Liu et al., 2008). �e extended formation of IP might be used as a trapping strategy to immobilise Cd and Cr from the rhizosphere through adsorption; however, it may not a�ect their uptake and translocation (Khan et al., 2016). It was found that the presence of IP constitutes a barrier to entry (acquisition) and circulation (uptake and accumulation) of both Cr(III) and Cr(VI) species in rice (Yu et al., 2017). �is result was later contradicted 239 by Zandi et al. (2020) who reported a greater inhibitory function of IPs against Cr(VI) compared to Cr(III). Sorption experiments have shown higher net As enrichment in IP for rice seedlings treated with monomethylarsenate (MMA) compared to monometh- ylthioarsenite (MMTA), and no enrichment for dimethylmonothioarsenate (DMMTA) treatment (Kerl et al., 2019). �eir results also showed that methylated thioarsenates (MTA) have very little binding/sorption capacity to amorphous ferrihydrite, indicating a low barrier capacity of IP for MTA. �e role of IP as a nutrient reservoir/bu�er is recommended, particularly when de- creasing the supply of nutrients (Khan et al., 2016). Its direct impact on exerting changes corresponding to the translocation of nutrients or toxic metal(loids) is o�en plant-species and element-speci�c (Tripathi et al., 2014). Accordingly, it has been demonstrated that IP exhibits greater a�nity for selenite Se (IV) than for Ni or Cu in Typha latifolia; this indicates deferential a�nity and sequestration capacity of IP for di�erent ions. Ye et al. (2001) reported that root plaque could act as a Cu reservoir. �is result was violated by a recent study by Peng et al. (2018) who concluded that root surface iron oxides are strong barriers against Cu oxide uptake and translocation in rice plants. Otte et al. (1989) stated that IP increased Zn and As uptake by Aster tripolium. Besides, P and Zn concen- trations in rice plants were lower in the absence of IP than in those with IP, suggesting that IP acted as a Zn and P reservoir. Ye et al. (1997) reported that the root surface of Typha latifolia without IP had lower concentrations of Cu than those with IP and that the presence of IP enhances Ni translocation. As in hydroponic cultures, the degree of IP formation on root surfaces was positively correlated with net P enrichment in IP both in the �eld (Dwivedi et al., 2010) and pot (Liang et al., 2006) cultures. Unfortunately, there is no report in the literature on the positive/negative impact of IPs on nutrient transporters. �e disparity in uptake patterns (ascending/descending) of metal(loids) in the presence of IP was strongly attributed to the amount of IP formed, the mineral composition of IPs and the type and availability of metallic ions (including anions and cations) in the growth media (Tripathi et al., 2014; Khan et al. 2016). Role of iron plaque in sequestration and uptake of heavy metals Root IPs have been predominately found associated with metal(loid)s sequestration (or deposition), uptake and translocation, including Pb, Cu, Zn, Cd, As, Cr, Al, Sb (Chen et al., 2006; Jiang et al., 2009; Huang et al., 2012b; Xu, Yu, 2013; Xu et al., 2015; Xia et al., 2020). �is is because a noticeable amount of metals bind to the Fe plaque by complex formation due to Fe hydroxide’s high a�nity for di�erent metals (Machado et al., 2005; Khan et al., 2016). In most studies, root IP reduced metal translocation to root and shoot (Xu et al., 2015). �e IP that forms on the roots of aquatic plants is an indispensable sorbent of both cations and anions metal(loid)s (Chen et al., 2006; Cheng et al., 2014) and is liable for the attenuation of metal(loid) uptake into wetland plants M ore insight into the concept of iron plaque form ation and its characteristics in rice (O ryza sativa L.) P ei m an Z an di , J oa nn a P uł a, X in g X ia , E lk e B lo em , A m in u D ar m a, Y ao sh en g W an g, In gr id T ur is ov á, Q ia n Li , L uu N go c S in h, N a Li 240 (Jiang et al., 2014). Reduction in IP formation due to continuous �ooding conditions did not necessarily contribute to an increase in Cd and other metals accumulation in rice grains (Zhang et al., 2019b). �is was ascribed to the �ood-mediated reduction in root-to-shoot translocation and availability of metals in the rhizosphere (Xu et al., 2013; Zhang et al., 2019b). What is probably the most determinative factor in controlling plaque formation is the presence of soluble soil iron and plant species capable of creating an oxidised rhizosphere (Chen et al., 2006). �ese Fe plaques are precipitated at the oxic-anoxic interface of the aquatic rhizosphere where oxygen from radial oxygen loss and Fe(II) from the reductive dissolution of Fe (oxyhydr)oxides intercept (Liu et al., 2006). �is process may be biotic or abiotic (Sey�erth et al., 2011; Xu, Yu, 2013; Khan et al., 2016 – Fig. 2), and the type of Fe phase that forms and its potential to sorb metal(loid)s is dependent upon the solution chemistry and rate of oxidation (Yamauchi et al., 2013; Amaral et al., 2017). Taylor, Crowder (1963) reported about the possible involvement of IP in the im- mobilisation of Ni and Cu in the rhizosphere of Typha latifolia. Zhang et al. (1998) suggested that Zn uptake might be in�uenced both by the presence and amount of IP precipitates on the root surface. Liu et al. (2004a) reported that about ⁓90% of total As was concentrated in IP on the rice root surface. �e formation of IP under P de�ciency conditions had a signi�cant e�ect on As sequestration in IP and reduction in root to shoot translocation of As (Liu et al., 2004b; Hu et al., 2005). Cu accumulation in roots and shoots of rice plants was negatively associated with the extent of IP development (Peng et al., 2018). In other words, the physical presence of IP could dramatically reduce the amount of Cu in the abovementioned rice tissues. In a study of three rice cultivars using Cr (III) treatments, Hu et al. (2014) concluded that the degree of IP formation and Cr concentration in IP increased in the treatment with no P application. In addition to the inevitable function of root plaques in suppressing the transfer of toxic metal(loids) in wetland-living species, the importance of those additional factors a�ecting the uptake process of elements, namely, plant species, ionic species and their concentra- tions, pH of the rhizosphere, particular Fe phases and the development of new lateral roots, should not be overlooked (Khan et al., 2016). For instance, Zandi et al. (2021) showed how newly developed lateral roots negate the bene�t of IP by providing barrier-free absorption conditions. Given the fact that the whole root axis is not covered by IP, an absorption gap may be available around root tips for penetration of nutrients (Batty, Younger, 2003; Yama- guchi et al., 2014). Not to mention that the redox (Eh) state and amount of Fe2+ produced, as well as the speci�c reactive surface area, may also be involved in the uptake of metal/ loids (Tripathi et al., 2014). It should be pointed out that high-pH induced formation of IP and expression of metal transporter genes were responsible for absorption and uptake of toxic metal elements, such as Cd, Mn and Zn (Zhang et al., 2019a). 241 Role of iron plaque in plant growth �ere are con�icting reports regarding the variable in�uence of IP on the growth of di�erent plant parts during IP formation. Investigations of Typha latifolia growth re- vealed that the exposure of plaque-bearing roots to Cu, Ni, Zn, Pb, and Cd addition did not enhance growth in comparison to plaque-free roots that were exposed to the same metals. Experiments with the aquatic plant Lobelia dortmanna L. showed that the IPs located on the root surface did not a�ect the growth of root diameter (Møller, Sand-Jensen, 2008). �e addition of Cu and Ni tended to decrease root length, especially when coated by IP. Despite this, the presence of IP on the roots of seedlings exposed to slightly toxic conditions of Cu and Ni improved the shoot growth of rice seedlings. In plants under excess Cu and Zn toxicity, the IP was demonstrated to positively a�ect the dry weight of roots and shoots and the length of leaves and roots (Tripathi et al., 2014). Additionally, plants without IP had more leaves displaying chlorosis than those bearing IP when treated with excess Cu. �us, it may symbolise the role of IP as a barrier to both toxic and essential metals. Conclusion Anthropogenic activities are the main sources of recent incremental trends in contami- nants loading in wetlands. It is a general belief that IP development on the root surface of hydrophytes is an initial output of ferrous iron oxidation under oxic conditions of the rhizosphere in wetlands. Amorphous and crystalline iron oxyhydroxides are fundamental factors of IP constitution and thus account for sequestration of those of nutrients and contaminants having a high binding a�nity to iron oxides. �e so-called sequestration may in�uence the uptake of nutrients and contaminants. �e distribution and evenness of such sequestration are dependent on several components including bio-physicochemical properties of the rhizosphere, di�usion capability of hydrophytes roots and the nutrient and contaminants availability in soil solution. Adopting careful management of nutrients and contaminants charging can curb their excess loading in wetlands. Above all, much research is still lacking regarding the �uctuate and contro- versial role of IPs (as ine�ectual, barrier or facilitator) concerning the plant uptake of emerging contaminants that need to be addressed. Con�ict of interest �e authors declare no con�ict of interest related to this article. 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Plant and Soil, 290, 17–28. https://doi.org/10.1007/s11104-006-9072-9 Poglądy na koncepcję powstawania blaszki żelazowej i jej cechy charakterystyczne u ryżu siewnego (Oryza sativa L.) Streszczenie Trwały i bioakumulacyjny charakter toksycznych metali(oidów) (TM) jest głównym problemem związanym z ich obecnością w środowisku. Skażenie TM w glebie i osadach zwiększa potencjalne ryzyko utraty zdrowia człowieka, przez narażenie na skażenie łańcucha pokarmowego. Odkładanie płytki tlenku żelaza na korzeniach hydro�towych (np. ryżu) jest wynikiem różnych czynników biotycznych i abiotycznych. Promieniowa utrata tlenu (ROL) odgrywa kluczową rolę w utlenianiu żelaza w ryzosferze, a następnie wytrącaniu nisko- lub wysoko krystalicznych i/lub amor�cznych minerałów żelaza na powierzchni korzeni. Biorąc pod uwagę, że każdy gatunek rośliny ma unikalną zdolność tworzenia utlenionej ryzosfery w warunkach beztlenowych gleby, obecność żelaza w ryzosferze ma ogromne znaczenie. Grupy funkcyjne (-OH) i specy�czne powierzchnie re- M ore insight into the concept of iron plaque form ation and its characteristics in rice (O ryza sativa L.) P ei m an Z an di , J oa nn a P uł a, X in g X ia , E lk e B lo em , A m in u D ar m a, Y ao sh en g W an g, In gr id T ur is ov á, Q ia n Li , L uu N go c S in h, N a Li 250 agujące w blaszkach żelaza mają wysokie powinowactwo do adsorpcji różnych metali śladowych (toksycznych/ nietoksycznych), wpływając na ich wchłanianie i akumulację w roślinach. W akumulacji różnych pierwiast- ków ważną rolę odgrywają płytki żelaza (IP). Gatunki roślin o niskim IP na swoich korzeniach mogą lepiej akumulować metale ciężkie, niezależnie od tego, czy IP jest barierą, czy buforem. Rośliny jadalne o wysokim IP są lepszymi �to-remediatorami potencjalnie �totoksycznych metali(oidów) i mogą być bezpieczniejsze do spożywania przez ludzi. Niniejszy przegląd podsumowuje obecną wiedze dotyczącą czynników związanych z tworzeniem i funkcjami płytki żelaza w zarządzaniu transportem metali w systemie korzeniowym. Key words: iron oxide plaque, paddy �elds, radial oxygen loss, toxic metals immobilisation Received: [2021.07.20] Accepted: [2021.10.16]