Beyond single-element toxicity: chromium-salinity interactions, food-chain transfer and mitigation in sugarcane agroecosystems

1 Introduction

Sugarcane (Saccharum officinarum L.) is a major C4 crop cultivated predominantly across tropical and sub-tropical regions (Satpathi et al., 2025; Zhao and Li, 2015). It is a perennial species native to South Asia that is now grown in more than one hundred nations, with global output rising from under 448 million tonnes on about 8.9 million hectares in 1961 to over 1.9 billion tonnes on roughly 27 million hectares in recent decades [Food and Agriculture Organization of the United Nations, 2019; USDA (U.S. Department of Agriculture), Foreign Agricultural Service, 2020]. Brazil is the world’s leading producer, contributing close to 39% of global production, while India ranks second at about 15.7% of the total (Food and Agriculture Organization of the United Nations, 2019; Zulu et al., 2019). In São Paulo, the crop is economically dominant and it accounts for roughly 15% of rural land use and a large share of national agrochemical and fertilizer sales (Armas et al., 2005; Ogura et al., 2022). In Thailand, sugarcane and related products have contributed around 21% of agricultural gross domestic product and supported more than 425,000 households, underscoring the crop’s socio-economic weight in producing regions (Bridhikitti et al., 2023).

In general, the formation of cane yield and the accumulation of sucrose are strongly modulated by both biotic and abiotic stresses. Thus, the productivity of these agroecosystems is inherently sensitive to environmental degradation (Mehdi et al., 2024). Besides, Sugarcane belts are increasingly embedded in landscapes where industrial and agrochemical inputs deliver heavy metals and salts simultaneously to soils and waters, affecting its productivity. Mineral fertilizers supply NPK, but also carry potentially harmful elements (Pb, Cd, Cu, Zn, Cr and Ni) (Peris et al., 2008). Fertilizers applied in sugarcane cultivation have also been shown to contain variable concentrations of Cd, Pb, Cr and Ni (Bizarro et al., 2007; da Silva et al., 2016). The diversity of these pathways encompassing tannery effluent, electroplating discharge, vinasse fertigation and saline irrigation and their convergent effect on the soil–plant-consumer continuum are summarized in Figure 1.

Reflecting this input pathway, stream sediments from sugarcane areas in central São Paulo carried higher metal concentrations than sediments in preserved catchments and Cd concentrations rose markedly between 2006 and 2016 (Corbi et al., 2018). In Thailand, application of filter cake produced soils from ethanol industries enriched in Cr, Fe, As, Cd and Mn, whereas vinasse application generated soils with elevated Zn, Cu, and Fe and a clayey, saline character (Bridhikitti et al., 2023). Effluents discharged into streams are routinely reused to irrigate sugarcane in Ghana, transferring Pb, Cd, Cr, Cu and Fe into the ecosystem (Oppong et al., 2018). Long-term effluent irrigated water and monoculture of sugarcane in the Nile alluvial belt of Egypt degraded groundwater quality. Mean Cr, Cd, Ni and Pb concentrations (~1,220, 60, 1,320 and 1,250 μg L⁻¹) frequently exceeded the WHO irrigation limits, while continued use of such water was projected to aggravate salinity in the alluvial soils (Hussien and Yousif, 2024). However, Salinity itself is an important issue of global scale, with an estimated minimum of one-third of the world’s arable land affected by salinization arising from saline irrigation, seawater intrusion and other natural and anthropogenic processes (Guarino et al., 2020).

Among the heavy metals, Chromium (Cr) necessitates particular attention. Cr has become one of the most important inorganic contaminants as a consequence of industrial activity, agricultural practice and waste disposal (Peralta-Videa et al., 2009; Farid et al., 2019). Although chromium is the seventeenth most abundant element in the Earth’s mantle, its toxicity is governed by its oxidation state and it is widely emitted by Cr plating, tanneries, mining, steel, and chemical industries (Shahid et al., 2017; Pushkar et al., 2021). Of its several oxidation states, the trivalent Cr(III) and hexavalent Cr(VI) forms are the most stable and environmentally prevalent (Singh et al., 2013) and these are also the most persistent (Chug et al., 2016). The two species differ profoundly in behaviour and hazard. Cr(VI) is carcinogenic, mutagenic and oxidizing (Wei et al., 2022), with compounds reported to be roughly a thousand times more cytostatic and carcinogenic than those of Cr(III) (Mamais et al., 2016). Cr(VI) is additionally far more soluble and bioavailable than Cr(III) (Xiao et al., 2017). Because chromium has no known biological function in plants (Srivastava et al., 2021) and plants lack dedicated Cr transporters. It is acquired through carriers like Fe channel for Cr(III) and phosphate and sulphate transporters for Cr(VI) (Anjum et al., 2016). Cr enters ecosystems as contaminated water, airborne particulates and sludges that can degrade soil and water quality far from the emission source (Ali et al., 2013; Ma et al., 2024; Zulfiqar et al., 2024).

Conventional toxicology has mainly focused on the effects of individual elements. However, multiple stress factors often occur together and their combined effects cannot be predicted from its individual analysis. Throughout this review, the terms “synergistic” and “cocktail effect” refer to non-additive interaction among stress. These interactions may either enhance or reduce toxicity or uptake, depending on speciation, redox conditions and salinity, rather than implying uniform enhancement of toxicity. For example, soils in semi-arid and mining-affected landscapes often contain elevated concentrations of both heavy metals and salts (Lutts and Lefèvre, 2015; Zaman et al., 2018). Under such conditions, salinity can alter heavy metal bioavailability and plant exposure through interactions with co-occurring contaminants, rather than by influencing the bioavailability of individual metals in isolation (Din et al., 2024; Sánchez-Castro et al., 2023).

Mechanistic studies of combined stress confirm non additive interactions. In a halophyte proxy study using quinoa, Cr(III) accumulated mainly in roots and reached its highest concentration in the presence of 150 mM NaCl. Moderate salinity modulated the transcription of stress protective genes, indicating that salt status can alter the chromium response (Guarino et al., 2020). Convergent evidence from a proxy study using wheat showed that combined elevated NaCl and Cr (30 μM) stress reduced biomass and impaired water relations and stomatal regulation more severely than either Cr or salinity applied alone (Ali et al., 2025). Such multi stress factor interactions can simultaneously influence soil chemistry, metal bioavailability, root uptake, plant physiology and ultimately human health risk (Figure 1). These findings underscore the need to move beyond single element assessment and explicitly consider the interactions among Cr, salinity and co-occurring metals.

The sugarcane specific field evidence was reported in Brazil (Corbi et al., 2018), Egypt (Hussien and Yousif, 2024), Ethiopia (Dejen and Taye, 2024), Ghana (Oppong et al., 2018) and Thailand (Bridhikitti et al., 2023). It is complemented by mechanistic and review treatments of chromium biogeochemistry and phytotoxicity (Zulfiqar et al., 2023), heavy metal food chain risk (Shetty et al., 2025) and the physiological basis of sugarcane stress responses (Mehdi et al., 2024). A central limitation is that controlled studies of the interaction between chromium and salinity have been conducted almost exclusively in non-sugarcane species, so that direct sugarcane-specific data on Cr x salinity synergy at the soil, root and stalk scales remain scarce.

Two features distinguish this synthesis from existing reviews of metal behaviour in salt-affected soils. First, the salinity metal literature is built almost entirely on divalent cations, whose mobility rises through chloride complexation and which compete with Na⁺ and Ca²⁺ at membrane channels; chromate is an oxyanion that forms no chloro-complexes and enters roots through sulphate and phosphate carriers, so findings for Cd, Pb and Ni cannot be carried across to Cr without justification. Second, the pathway from agro-industrial input through soil speciation and root uptake to stalk, juice and consumer exposure has not previously been assembled for sugarcane in a single account. Because the evidence supporting this pathway is uneven, four evidence tiers are distinguished throughout and identified wherever a mechanism is introduced: direct field observation in sugarcane; controlled experiment in sugarcane under a single stressor; evidence from another C4 species; and evidence from non-C4 species, including halophytes and hyperaccumulators used as mechanistic proxies.

The subsequent synthesis follows the movement of chromium through the sugarcane system. It discusses soil geochemistry, which includes Cr(VI)/Cr(III) speciation under pH, Eh and salinity as well as the agro-industrial inputs that modify it. Root zone processes are examined, including rhizosphere control, ion transport and PHT1 mediated chromate entry. The physiological effects are subsequently addressed by ROS damage, C4 carbon metabolism, and sugarcane specific PEPC dependent vulnerabilities. Food chain risk, product quality and bioaccumulation are evaluated. Finally, mitigation strategies including microbial inoculants, bagasse biochar, tailored nano amendments and integrated management were also discussed. Evidence in this review is graded on four tiers according to how closely the experimental system resembles a sugarcane field: direct field observation in sugarcane; controlled experiment in sugarcane under a single stressor; controlled experiment in another C4 species; and controlled experiment in a non-C4 species. Each mechanistic claim below names the species and experimental system it rests on, so the tier is evident where the finding is given.

2 Geochemistry of chromium and co-contaminants in sugarcane agroecosystems

2.1 Multi-metal contamination patterns in sugarcane soils and waters

Chromium rarely arrives in sugarcane soils as a solitary contaminant. It is delivered as part of a suite of trace elements. In a cane belt irrigated with paper-mill effluent in Uttarakhand, India, topsoil carried Cr along with Cu, Pb, Ni, Cd, Zn, Fe and Mg, with Cr reaching 24.10 to 67.58 mg kg⁻¹ and the effluent itself containing 48.41 mg L⁻¹ Cr against a permissible limit of 2 mg L⁻¹ (Pandey et al., 2016). The co-occurrence of these metals reflects a shared anthropogenic origin, since the multivariate structure of the data grouped Pb, Cu, Cd, Ni, Cr and Zn into a single industrial association (Pandey et al., 2016; Zhu et al., 2017). In Thailand, a parallel multi-element burden of As, Fe, Mn, Zn, Cu, Cr, Pb, Cd and Hg was documented, with As frequently exceeding the national agricultural soil standard of 25 mg kg⁻¹ and Cd, As and Zn accumulating broadly while Pb accumulated only in localized patches (Bridhikitti et al., 2023). Beyond the field, the metals propagate into adjacent waters. Long term irrigated sugarcane monoculture in the Nile alluvium elevated groundwater Cr, Cd, Ni and Pb above WHO irrigation thresholds (Hussien and Yousif, 2024).

The spatial template of contamination is strongly governed by proximity to point sources. Tannery clusters generate the most extreme enrichment, with more than 90% of agricultural soils across 65 locations in the Vellore district exceeding 200 mg kg⁻¹ Cr (Rangasamy et al., 2015). Groundwater across the same Vellore tannery belt carried electrical conductivity between 1.95 and 13.78 dS m⁻¹ and was judged unsuitable for irrigation on grounds of both salinity and sodicity hazard, placing chromium and salt loading together in a single Indian cane-growing corridor (Rangasamy et al., 2015). Similarly, lagoon soils at a Hebei tannery averages 6,493 mg kg⁻¹ Cr with a maximum of 12,971 mg kg⁻¹ and it is 211 times greater than the Chinese soil threshold of 61 mg kg⁻¹ (Xie et al., 2023). In China, Cr in industrial regions spanned 0.74 to 37,967 mg kg⁻¹ and was apportioned principally to Cr salt production and leather tanning, with concentrations rising between 2002 and 2009 before entering a sustained decline through to 2021 (Li et al., 2023). Even decades after closure, an old tannery waste lagoon retained total soil Cr as high as 24,340 mg kg⁻¹, demonstrating the persistence of the legacy load (Stepniewska et al., 2001). Thus, sugarcane agroecosystems become metal rich wherever they intersect tannery, electroplating, pulp-and-paper or fertilizer-intensive corridors. The geographic distribution of these contamination patterns, the co-metal suites documented at each site and the regulatory exceedances recorded across global sugarcane producing regions are compiled in Table 1.

Table 1. Occurrence of heavy metals in sugarcane ecosystems across major producing regions.

BCF, bioconcentration factor; CR, carcinogenic risk (acceptable threshold typically 1 × 10⁻⁶ to 1 × 10⁻⁴); PLI, pollution load index; WHO, World Health Organization; μg L⁻¹, micrograms per litre; mg kg⁻¹, milligrams per kilogram dry weight.

𝐼hina and Brazil entries are not sugarcane-specific field studies; they provide national/watershed contamination context for industrial and agricultural belts in which sugarcane is grown.

ᵇStream sediments represent a distinct matrix from sugarcane soil; comparison with soil-based rows should account for matrix differences.

2.2 Cr and soil physicochemical properties

Soil properties regulate chromium retention and mobility. In sugarcane soils irrigated with industrial effluent, chromium accumulation was closely associated with soil pH, Eh and salinity, indicating that soil physicochemical properties control the proportion of chromium present in mobile forms rather than total chromium alone (Kumpiene et al., 2007; Pandey et al., 2016). Table 2 summarizes the principal soil systems and the pH, Eh and salinity conditions governing chromium speciation. The controls described below are established in general soil and sediment systems rather than in sugarcane soils, and their expression in cane agroecosystems is inferred from those systems together with the small number of sugarcane field surveys available.

Table 2. Factors influencing chromium speciation, mobility and partitioning in soil and aqueous systems.

COPR, chromite ore processing residue; Eh, redox potential (mV, millivolts); HPLC-ICP-MS, high-performance liquid chromatography coupled to inductively coupled plasma mass spectrometry; Fe, iron; Mn, manganese; CrO₄²⁻, chromate anion; Cr(OH)₃, chromium hydroxide precipitate; CrFeOOH, iron–chromium oxyhydroxide.

^aDescribes an analytical speciation method applied to contaminated soils, not a distinct field soil system; it is included to document methodological considerations for Cr(VI)/Cr(III) speciation data interpretation.

2.2.1 pH and Eh

The environmental behaviour of chromium is dictated almost entirely by the interconversion of its two stable oxidation states and their controlling master variables are pH and redox potential (Eh). Cr(VI) is stable in oxidizing environments at Eh above 0.6 V and pH above 6.0, whereas Cr(III) is thermodynamically favoured across pH 2–8 and Eh from +0.295 V down to −1.0 V (Chin, 1994; Losi et al., 1994). Because these two species differ in charge and solubility, their distribution governs mobility. The oxidized Cr(VI) form is highly soluble and therefore mobile, while reduced Cr(III) is nearly insoluble and effectively immobile (Kotaś and Stasicka, 2000). Aqueous Cr concentrations are set by the interplay of adsorption, mineral dissolution and precipitation, with Cr(VI) sorbed onto aluminium and iron oxides especially at low pH. On the other hand, Cr(III) retained at higher pH where it precipitates above pH 5.5 as a sparingly soluble amorphous hydroxide unless complexed by organic acids (James and Bartlett, 1983). Redox oscillations are also commonly induced by monsoon flooding or flood irrigation in sugarcane systems, and it reorganizes metal partitioning in the soil. Cr(VI) was preferentially released under oxidizing to weakly reducing conditions (Eh > 100 mV) but was gradually and irreversibly sequestered by reductive precipitation under moderately reducing conditions (Eh between 100 and −100 mV) (Matern and Mansfeldt, 2016). The reductants responsible are the redox active soil constituents like Fe(II) generated from the reductive dissolution of Fe hydroxides, sulphide and organic matter, all drive Cr(VI) to Cr(III) so that Cr cycling is mechanistically coupled to the redox chemistry of Fe, Mn and S (Couture et al., 2015).

Iron and manganese oxides are key regulators of chromium speciation in soils. Iron oxides act as major sorbents for both Cr(III) and Cr(VI), whereas manganese oxides promote the oxidation of relatively immobile Cr(III) to the more mobile Cr(VI). Conversely, under reducing conditions, dissolution of Fe and Mn oxides releases previously sorbed chromium into the soil solution while Fe(II) promotes Cr(VI) reduction to Cr(III). Thus, Fe-Mn oxides function as both sinks and secondary sources of chromium, depending on prevailing soil redox conditions (Couture et al., 2015; Xie et al., 2023).

2.2.2 Salinity

Salinity introduces a second axis of control that operates part independently of redox. Saltwater intrusion raises ionic strength and adds Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻ and SO₄²⁻, ions that compete for sorption sites and can form mobile complexes that enhance metal solubility (Sricharoenvech et al., 2024). Elevated ionic strength also compresses the electrical double layer and favours the formation of soluble chromium complexes, thereby increasing chromium mobility in the soil. Elevated ionic strength acts non-specifically, compressing the electrical double layer and displacing sorbed species through competition for surface sites, whereas chloride acts specifically, forming soluble complexes with divalent cations such as Cd²⁺. Chromate forms no such chloro-complexes, so the chloride pathway that dominates the Cd literature has no chromium analogue, and salinity effects on Cr must be attributed to ionic strength, redox coupling and competition at oxyanion transporters instead. In coastal soils dominated by chromite and Fe-Cr hydroxide coprecipitates, the net effect of seawater plus anoxia was to lower Cr release three to eight times more Cr mobilized under aerobic, freshwater conditions. Because reduction by Fe(II) and sulphide outweighed the salt driven competition for surfaces (Sricharoenvech et al., 2024). Thus, salinity can promote or suppress Cr mobility depending on whether it co-varies with oxidizing or reducing conditions, a duality of obvious relevance to saline, flood-irrigated cane soils.

2.3 Cr and agro industrial inputs

Agricultural amendments and industrial effluents modify chromium behaviour not only by introducing additional metals but also by altering soil physicochemical properties. Sugarcane vinasse is the clearest example. Characterization of distillery vinasse revealed total dissolved solids exceeding 4,000 mg L⁻¹ and electrical conductivity above 6.7 dS m⁻¹, establishing a high risk of soil salinization upon land disposal. While the same effluent carried the priority pollutants Cu, Cr, Ni, Pb and Zn alongside phytotoxic Al and Fe (Christofoletti et al., 2013; Fuess et al., 2017). The salinity hazard from vinasse is dominated by its ionic load rather than by sodium specifically, since low Na concentrations relative to Mg and Ca limit sodification even as overall EC remains high (Fuess et al., 2017).

Field evidence confirms that organic residues reshape soil chemistry differentially. Filter cake application produced sugarcane soils enriched in Cr, Fe, As, Cd, and Mn with a clayey, organic-rich character, whereas vinasse fertigation generated soils higher in Zn, Cu and Fe and distinctly clayey and saline (Bridhikitti et al., 2023). These contrasting outcomes matter because clay content, organic matter and alkalinity directly govern the retention of accompanying metals with each promoting accumulation of specific elements (Bridhikitti et al., 2023). Wastewater irrigation operates similarly, delivering plant nutrients and organic matter but simultaneously loading the soil with Cr, Cu, Pb, Ni, Cd and Zn whose retention and subsequent transfer to the crop depend on the resulting pH and organic-carbon status (Pandey et al., 2016). A critical mechanistic point is that these inputs are double-edged with respect to Cr lability. Added organic matter and reductants can immobilize Cr by driving Cr(VI) reduction and Cr(III) retention, but the elevated EC and the competing major ions introduced by saline effluents can re-mobilize sorbed metals through ionic-strength and complexation effects (Sricharoenvech et al., 2024). The acidity of vinasse, with pH typically 3.5–5, further complicates the balance, since lower pH favours Cr(VI) reduction yet also increases the solubility of several co-contaminant metals (Fuess et al., 2017). Therefore, the environmental behaviour of chromium depends not only on its concentration but also on the combined effects of salinity, organic matter and soil chemistry introduced by agro-industrial inputs.

2.4 Chromium speciation and bioavailability in soils

Total Cr is the whole solid-phase inventory recovered by strong-acid digestion. Potentially mobilizable Cr is the sum of the non-residual sequential-extraction fractions. Bioavailable Cr is the portion actually taken up over a given exposure and is operationally defined by the extractant used rather than measured directly. Cr species denotes the Cr(VI) or Cr(III) oxidation state, resolved by alkaline extraction with HPLC-ICP-MS or by X-ray absorption spectroscopy. DTPA-extractable Cr is a mixed pool that resolves neither fraction nor oxidation state and is treated here as an index rather than a measure. Bioavailability of chromium is determined by the speciation of chromium and its partition among the different soil fractions rather than the total chromium. The total chromium concentration is a measure of chromium contamination level, while the bioavailable fraction determines the mobility and the risks. Soil chromium can be partitioned using sequential extraction to determine bioavailability based on chromium in exchangeable, carbonate bound, reducible (Fe/Mn oxide), oxidizable (organic/sulphide) and residual (lattice) fractions. The exchangeable and carbonate bound chromium fractions are easily available, while the residual fraction is highly immobile since chromium is immobilized within mineral lattices (Mandal et al., 2011). Based on this sequential extraction technique applied in tannery affected soils and sludges near Ranipet, approximately 63.7% of total Cr in soils was found to be potentially mobilizable, while 36.3% was non-bioavailable. The reducible (Fe/Mn oxide) fraction had the largest portion (31.3%) in soils, while oxidizable fraction had the largest portion (56.3%) in sludges (Avudainayagam et al., 2003; Mandal et al., 2011). Based on the sequential extraction carried out on spiked soil, Cr had majority in the organic (61.3%), Fe-Mn (26.9%), and residual (7.6%) fractions (Karimah et al., 2021). Fe-Mn rich soils are able to scavenge and immobilize Cr in potentially bioavailable forms. Thus, oxide content acts as both the sink and reservoir controlling the future release of chromium to biota. It implies that there is a significant percentage of chromium present in mobile fractions, meaning that soil conditions such as acidic pH, cation exchange capacity (CEC), organic carbon and clay contents can increase chromium release even at unvarying total chromium levels (Pandey et al., 2016).

The critical variable is the Cr(III): Cr(VI) ratio in soil solution. Based on a proxy study involving growth of wheat on long term tannery effluent irrigated soils, the soil Cr(III): Cr(VI) ratio determined the tissue chromium partitioning and was a better predictor of plant responses than total chromium concentration (Riaz et al., 2019). Speciation specific analytical models involve the further resolution of the different oxidation states. Ultrasound assisted alkaline (Na₂CO³) extraction followed by Ion Pair Reverse Phase HPLC-ICP-MS identified mostly Cr(VI) in contaminated soils with Cr(III) present in organic soils due to humic acid reduction of extracted Cr(VI) and Fe(II) presence (Leśniewska and Godlewska-Żyłkiewicz, 2019). On the other hand, the conventional single extractant index, such as DTPA extractable metals is only partly informative in the case of chromium since it extracts a mixed fraction without any oxidation state differentiation. There is no information that they are capable of separating the Cr(VI) pool from relatively less toxic Cr(III) pool. Therefore, speciation resolved or sequential extraction methods would be preferable, especially in tannery affected sugarcane soils where redox conditions may change the chromium mobility over time.

2.5 Analytical considerations for assessing chromium

Accurate assessment of chromium bioavailability requires more than measuring total chromium concentration because total content does not represent the fraction available for environmental transport or transformation. Therefore, bioavailability studies should combine measurements of total chromium with speciation and fractionation analyses. However, speciation methods carry their own artefacts. Alkaline extraction is favoured precisely because it limits the reduction of Cr(VI) that occurs under neutral or acidic conditions. Even then the recovered Cr(III) may be partly an extraction artefact generated by humic acids and Fe(II) rather than an in-situ species (Leśniewska et al., 2017; Leśniewska and Godlewska-Żyłkiewicz, 2019). Matrix effects including organic matter, Fe and Mn content and soil moisture were shown to determine the amount of Cr(III) formed during analysis. So, speciation results must be interpreted against the specific soil matrix rather than reported as absolute (Leśniewska and Godlewska-Żyłkiewicz, 2019). Spectroscopic techniques such as XANES and EXAFS directly identify solid-phase chromium species but may underestimate total chromium where much of the metal is locked within residual chromite minerals, thereby affecting mass balance estimates (Sricharoenvech et al., 2024).

Sampling strategy is equally important for reliable chromium assessment. In sugarcane soils irrigated with effluent, chromium and several co-metals showed variable distribution between topsoil and subsoil, implying that sampling at a single depth may not accurately represent chromium inventory or mobility (Pandey et al., 2016). Seasonality and water status are equally critical because redox is dynamic. The same soil released Cr(VI) under drained, oxidizing conditions but retained chromium under saturated, reducing conditions, demonstrating that sampling during dry and monsoon periods can produce contrasting estimates of chromium mobility (Matern and Mansfeldt, 2016). Contamination and ecological risk indices provide another layer of interpretation but should be used with caution. Enrichment factors, geo accumulation index and contamination factors can be used for identification of anthropogenic enrichment in contaminated soils (Moni et al., 2023). However, these indices are based on total chromium concentrations and do not reflect bioavailable chromium or differentiate between Cr(VI) and Cr(III). Similarly, ecological risk indices can identify pollution sources and vulnerable populations but cannot replace chromium speciation when evaluating environmental and health risks (Li et al., 2023).

Despite progress in analytical techniques, some important challenges remain. Reports in sugarcane soils are mostly limited to total chromium concentrations with very little information available on chromium speciation, operational fractions and their response to changing soil and salinity conditions. Future research should combine sequential extraction, chromium speciation and long-term field monitoring to better evaluate chromium mobility and environmental risk in sugarcane agroecosystems.

3 Cr-salinity and rhizosphere interaction

The rhizosphere is where chromium speciation, ion transport and root physiology converge, and it therefore sets the boundary condition for everything downstream. This section examines what is known about that interface under saline conditions, distinguishing throughout between processes demonstrated in sugarcane and those inferred from proxy species, since controlled work on combined chromium and salinity exposure has been conducted almost entirely outside this crop.

3.1 Root ion transport systems under salinity

The root plasma membrane is the primary barrier regulating ion uptake from the soil. Under saline conditions, ion transport is controlled by cation and anion channels responding to steep electrochemical gradients created by Na⁺ and Cl⁻. In cereals such as wheat and maize, Na⁺ enters roots mainly through voltage independent non-selective cation channels (NSCCs), many of which are inhibited by external Ca²⁺. Accordingly, whole-root Na⁺ influx closely follows the Ca²⁺ sensitivity of NSCC activity. Anion channels also facilitate transient Cl⁻ influx and contribute to membrane depolarization under increasing salinity (Tyerman and Skerrett, 1999). Membrane depolarization reduces K⁺ selectivity and alters the root ionome. This change influences the uptake of nutrients, trace-metal cations and oxyanions by modifying the electrochemical environment surrounding root transport systems (Lindberg and Premkumar, 2024). To maintain ion homeostasis, plants rely on a coordinated transport network that includes the Na⁺ transporter AtHKT1;1 (Rus et al., 2004), the plasma membrane Na⁺/H⁺ antiporter SOS1 (Zhu, 2003), the vacuolar Na⁺/H⁺ antiporter AtNHX1 (Apse et al., 1999) and the high-affinity K⁺/Na⁺ transporter TaHKT1 (Schachtman and Schroeder, 1994). Together, these transporters limit cytosolic Na⁺ accumulation while maintaining K⁺ homeostasis.

Sugarcane employs similar mechanisms to tolerate saline conditions. Excess Na⁺ is excluded from the cytoplasm through vacuolar sequestration, while HKT1, SOS1 and NHX mediated transport systems regulate Na⁺ and Cl⁻ transport and maintain ionic balance (Ali et al., 2019; Dhansu et al., 2026). As salinity increases, sugarcane tissues accumulate Na⁺ and Cl⁻, whereas K⁺ and Ca²⁺ concentrations decline. Consequently, the K⁺: Na⁺ and Ca²⁺: Na⁺ ratios decrease, reflecting disruption of cellular ion homeostasis (Rasheed et al., 2016). These changes are particularly relevant for chromium uptake because salinity also influences transport systems responsible for nutrient acquisition. Members of the PHT1 phosphate transporter family are located in the plasma membrane of sugarcane roots and are strongly upregulated under salt stress (Murugan et al., 2022). Since Cr(VI) enters plant roots through phosphate and sulphate transport pathways owing to its structural similarity to these oxyanions, salinity-induced changes in transporter activity may directly influence chromium uptake. This study characterised transporter expression under salinity alone and applied no chromium, so this route is proposed but not demonstrated, and Figure 2 presents it as a hypothesis.

3.2 Salinity mediated uptake of chromium and co-metals

Chromium differs from the metals that dominate the salinity literature in the most basic respect: its behaviour is governed by oxidation state rather than by cationic charge. Cr(VI) persists in solution as the chromate oxyanion and crosses the root plasma membrane through sulphate and phosphate transporters, whose substrates resembles closely enough to be carried by them (Cervantes et al., 2001). Cr(III), by contrast, is sparingly soluble at circumneutral pH and is taken up largely by passive pathways, so the two species reach the root by unrelated routes (Shanker et al., 2005). The immediate consequence is that neither of the two mechanisms ordinarily invoked to explain salinity effects on metal uptake chloride complexation and competition at cation channels has a chromate analogue, because an oxyanion neither forms chloro-complexes nor competes with Na⁺ or Ca²⁺ for entry through cation-selective pathways. Salinity effects on chromium must therefore be sought in ionic strength, redox coupling and competition at oxyanion transporters, and the extensive literature on cationic metals is best used as a contrast rather than as a mechanistic template. That contrast is instructive because the cationic case is both well characterised and internally divided. Chloride forms soluble complexes with divalent cations, and the CdCl⁺ and CdCl₂⁰ species raise Cd mobility in the soil solution above what free-ion activity alone would predict (Norvell et al., 2000). Major cations supplied by saline water Na⁺, K⁺, Ca²⁺ and Mg²⁺ simultaneously displace trace metals from exchange sites and release them into solution (Du Laing et al., 2009). Where these processes act together the amplification can be substantial, with combined Cd and NaCl treatment raising Cd accumulation up to 42-fold in lettuce and 23-fold in strawberry, an outcome attributed to chloride and organic complexation together with Na⁺ saturation of cation-exchange sites suppressing re-adsorption (Ondrasek et al., 2021). The opposite result is equally well documented: in edible amaranth, rising NaCl reduced Cd adsorption by 43–58% and Cd uptake by 32–36%, most markedly in a salt-sensitive cultivar., and channel-blocking experiments located Cd²⁺ entry at Ca²⁺ transport channels where Na⁺ competes for the same pathway, with chloride complexation exerting no measurable effect under those conditions (Mei et al., 2014). Membrane competition rather than soil speciation governed the outcome in that system, so even within the divalent cations the direction of the salinity effect depends on which of the two processes prevails.

For chromate the corresponding controls are the concentration of competing oxyanions in the soil solution and the Cr(VI):Cr(III) ratio, which determines how much chromium is present in a transportable form at all. Direct evidence remains sparse. In rice grown under concurrent NaCl and Cr stress, growth was reduced through osmotic stress, ionic imbalance and oxidative damage, and inoculation with the endophyte Arthrobacter woluwensis lowered Cr(VI) translocation by 38% and bioaccumulation by 42%, while the epiphytic strain tested alongside it achieved reductions of only 11 and 10% (Sobahan et al., 2025). That chromium fluxes responded to a microbial intervention which also restricted Na⁺ uptake, but responded to differing degrees, suggests the two are regulated at least partly independently. The distinction matters most in soils affected by tannery, electroplating and wastewater discharge, where Cd, Pb and Ni co-occur with chromium and three processes operate simultaneously on different elements within the same rhizosphere: chloride complexation of cations, competition at cation channels, and chromate entry through oxyanion carriers. These parallel pathways are illustrated in Figure 2. To our knowledge, no study has directly examined the interaction between salinity and chromium uptake in sugarcane roots. In the absence of a direct sugarcane experiment on combined Cr and salinity, the most informative mechanistic evidence comes from proxy systems including halophytes, glycophytic crops and hyperaccumulators, interpreted against the cane root physiology. The single most relevant proxy is a study in quinoa (Chenopodium quinoa Wild), a halophyte grown on Cr(III) contaminated soil with or without 150 mM NaCl. Results showed that Cr accumulated predominantly in roots reaching ~2.6 mg g⁻¹ DW and peaking under saline conditions while leaf Cr remained very low (≈0.03 mg g⁻¹ DW). The response proved transcriptionally coordinated and reciprocal, with NaCl modulating genes encoding sulphate, Fe and phosphate transporters together with phytochelatin, metallothionein and glutathione synthetase genes and Cr(III) in turn significantly lowering foliar Na⁺ on saline soil (Guarino et al., 2020). The relevance of the oxyanion uptake route to a C4 grass is reinforced by evidence from the fern hyperaccumulator Pteris vittata. In this, chromate and sulphate enhanced one another’s uptake and translocation, confirming the sulphate mimicry pathway by which Cr(VI) enters roots (de Oliveira et al., 2016). Because PHT1 transcripts are upregulated in sugarcane roots under salinity, and because chromate enters roots through phosphate and sulphate carriers in other species, salt-induced PHT1 expression offers a plausible route by which salinity could raise chromate entry in cane (Murugan et al., 2022). The proposition is untested, no measurement of chromate influx in sugarcane roots under contrasting phosphate supply and salinity has been reported (Figure 2).

These proxies can be collectively used to extrapolate for sugarcane and must specify both the metal’s entry pathway and the prevailing salinity chemistry rather than assuming a uniform response. In particular, the salt enhanced root Cr retention seen in quinoa should be regarded as a hypothesis for cane rather than an established outcome, given sugarcane’s glycophytic and distinctly salt sensitive physiology (Dhansu et al., 2026). Besides, controlled experiments evaluating Cr(VI) and Cr(III) uptake under different NaCl, sulphate and phosphate concentrations are therefore needed to clarify chromium transport under saline conditions in sugarcane ecosystem.

3.3 Root architecture and rhizosphere regulation

Besides membrane transport processes, the structural characteristics of roots and rhizosphere processes significantly impact the chromium migration into the plants from the soil. Endodermal Casparian strip is known as an apoplastic barrier that blocks the passive transport of Na⁺ and other ions. It is constituted by the Casparian strip domain protein family and strengthened by lignin and suberin. Endodermal suberization additionally restricts apoplastic bypass when the plant grows in a saline environment. HKT1;5 proteins, found in rice and maize, also contribute to Na⁺ removal from the xylem, thus preventing it from moving up to the shoot (Karahara and Horie, 2021). As water and dissolved metals are transported along the same route, namely, via the transpiration stream, alterations in root barrier permeability and transpiration rate due to saline conditions significantly affect the metal transport to the xylem (Tyerman and Skerrett, 1999). Sugarcane has a unique system of shoot-borne root (SBR), that comprises its permanent root system. SBRs response to stress conditions varies depending on genotypes. In commercial hybrids, root cells undergo irreversible damage upon exposure to stress, while wild species keep root integrity and development of lateral roots via interaction of sugars, calcium, and hormones (Valarmathi et al., 2023). Thus, it can be hypothesized that effectiveness of root barrier to chromium migration may also differ in different sugarcane genotypes. Rhizosphere also provides control over chromium bioavailability. Using rice as a proxy for sugarcane, scientists demonstrated that alternating wetting and drying led to 40% increase in root concentration of chromium compared to continuous flooding. In this case, microbial reduction of Fe and sulphate activity increased bioavailability of chromium and formation of mobile Cr(III) + OM + Fe colloids under changes in pH (Guan et al., 2023).

3.4 Future research and modelling perspectives

Despite these studies, several important questions remain unanswered. The interaction between salinity and chromium uptake has not been directly investigated in sugarcane. Because chromate enters roots through sulphate and phosphate transporters rather than the cation channels used by Cd, Pb and Ni, its response to salinity cannot be inferred from studies of other metals (Cervantes et al., 2001). Controlled hydroponic experiments should therefore evaluate Cr(VI) and Cr(III) uptake under different NaCl, sulphate and phosphate concentrations using ion-channel inhibitors. Since sugarcane upregulates plasma membrane PHT1 phosphate transporters during salt stress, phosphate availability should be included as an experimental factor (Murugan et al., 2022). Future studies should also examine the combined effects of salinity and rhizosphere redox. Factorial experiments incorporating alternate wet and dry cycles and salinity would clarify whether saline, flood irrigated sugarcane soils ultimately promote chromium mobilization or immobilization (Guan et al., 2023). Similarly, chromium uptake studies should distinguish between Cr(VI) and Cr(III), building on combined salinity-chromium experiments already demonstrated in rice, where endophytic inoculation reduced Cr(VI) translocation and bioaccumulation (Sobahan et al., 2025).

From a modelling perspective, total chromium concentration alone is insufficient to predict plant uptake. Future models should integrate chromium speciation, chloride complexation, cation competition, sulphate and phosphate transporter activity, rhizosphere redox dynamics and the structural integrity of the sugarcane shoot borne root system (Karahara and Horie, 2021; Ondrasek et al., 2021; Valarmathi et al., 2023). Such integrated models have not been developed or validated for sugarcane grown in industrially contaminated environments. Field validation in tannery and wastewater affected sugarcane regions therefore remains a key research priority.

4 Cr-salinity effects on physiology and C4 carbon metabolism

Oxidative homeostasis, photosynthetic carbon assimilation, nutrient and hormonal regulation, and source-sink partitioning are examined under chromium and salinity stress, with a clear distinction maintained between responses reported in sugarcane under individual stressors and those observed in other plant species exposed to combined chromium and salinity stress. These responses involve oxidative homeostasis, photosynthetic carbon assimilation, nutrient and hormonal regulation and source-sink carbon partitioning. In sugarcane, the specialized C4 photosynthetic pathway further influences these responses, making C4 carbon metabolism a key determinant of tolerance under combined stress.

4.1 Oxidative stress and antioxidant defence

The primary impact of combined Cr and salinity stress on plant physiology is associated with increased production of reactive oxygen species (ROS) including superoxide radicals and hydrogen peroxide (H₂O₂). If production of ROS exceeds the capacity of cells for antioxidative protection, then oxidative stress occurs which results in lipid peroxidation, disruption of cellular functions and membranes. The development of oxidative stress and injuries to cell membranes is caused by the accumulation of Na⁺, Cl⁻, and H₂O₂ in combination with decreased concentrations of K⁺ and Ca²⁺ due to salinity in sugarcane (Rasheed et al., 2016; Kumar et al., 2023).

The oxidative injury to the cell membrane is commonly estimated using the increase in MDA content and electrolyte leakage. Experiments carried out with the use of C4 plants revealed the significant increase in the concentration of MDA and electrolyte leakage along with the decrease in the chlorophyll content which demonstrates oxidative injury to cell membranes caused by chromium toxicity (Iftikhar et al., 2025). These data demonstrate the importance of cell membranes as the targets of the combined effect of Cr and salinity. Antioxidant defence system of plants includes enzymatic antioxidants (superoxide dismutase—SOD, catalase—CAT, ascorbate peroxidase—APX, peroxidase—POD, glutathione reductase—GR, and glutathione-S-transferase—GST) and non-enzymatic (ascorbate, glutathione and proline). Under mild stresses, increased activity of antioxidant enzymes allows maintaining cellular redox homeostasis. However, heavy chromium stress reduces antioxidant enzyme activities and increases the level of ROS and their oxidation (Kumar et al., 2019). Therefore, the intensity of antioxidant induction determines the level of tolerance.

In rice, exposed concurrently to NaCl and chromium, hydrogen peroxide and malondialdehyde accumulated beyond the levels induced by either stress alone, and rhizobacterial inoculation lowered both alongside chromium translocation (Sobahan et al., 2025). Whether this super-additive oxidative response generalizes to sugarcane has not been established, since no comparable combined-stress experiment has been reported for the crop. The studies showed the increase in H₂O₂ and MDA concentrations under combined effect and the reduction in the level of oxidative injury and chromium toxicity due to the enhancement of antioxidant activity (Sobahan et al., 2025). It means that the efficient removal of ROS and the integrity of cell membranes are the major physiological mechanisms of tolerance to the combined Cr and salinity stress in sugarcane.

4.2 Photosynthesis and carbon metabolism

Salinity impairs both the light reactions and the C4 carbon-concentrating mechanism in sugarcane, and chromium impairs photosynthesis in other species; how the two combine in cane is the open question this subsection addresses. As photosynthesis in C4 plants relies on the cooperation between mesophyll cells and bundle sheath cells, any impairments in the activity of photosystems or key enzymes responsible for carbon fixation negatively affect carbon assimilation and limit the production of biomass and sucrose. Among the listed factors, phosphoenolpyruvate (PEP) carboxylase plays the crucial role catalysing the first step of carbon dioxide fixation; therefore, the activity of this enzyme is important to ensure the efficiency of the C4 photosynthesis pathway under the conditions of stress (Figure 3) (von Caemmerer, 2021). In sugarcane, salinity applied alone alters the abundance of proteins central to C4 carbon metabolism, and the pattern is cultivar-dependent rather than uniform: PEP carboxylase was downregulated in the salt-sensitive cultivar, whereas in the salt-tolerant cultivar the affected proteins were pyruvate phosphate dikinase, the Photosystem II P680 apoprotein, ATP synthase subunit gamma and ferredoxin (Chiconato et al., 2021). Neither cultivar therefore escaped disruption of the C4 apparatus, and whether chromium co-exposure compounds it has not been tested in this crop. Likewise, the studies conducted with the help of a proxy C4 plant like sweet sorghum have shown that the increase in salinity leads to a decline in the maximal quantum efficiency (Fv/Fm) and effective quantum yield of Photosystem II along with the impairment in electron transport. Salt-tolerant genotypes have higher level of protection of photosystems and can maintain carbon assimilation under stress (Yang et al., 2020). It means that photosystem integrity is the main physiological factor that determines stress tolerance. In a non-C4 proxy study using oilseed rape, a Cr-tolerant cultivar maintained higher net photosynthetic rate, stomatal conductance and leaf mineral nutrient concentrations than a sensitive cultivar under identical Cr(VI) exposure, indicating that photosynthetic and nutritional sensitivity to chromium is genotype-dependent (Xu et al., 2025).

In addition to the impact on the process of light harvesting, combined stress affects carbon metabolism and sucrose biosynthesis. As it was shown for sugarcane, genotypic differences in stress tolerance are linked to the capability of preserving stomatal conductance, Rubisco activase activity and photosynthetic capacity under salinity conditions and, therefore, maintaining the source strength regardless of stress (Negi et al., 2025). The efficient carbon assimilation is crucial for sucrose synthesis since the expression and activity of enzymes such as sucrose phosphate synthase (SPS), sucrose synthase (SuSy) and invertases rely on the continuous supply of carbon via photosynthesis (Verma et al., 2019). Thus, the reduction in PEPC activity and photosynthetic efficiency caused by stress leads to the decrease in the flux of carbon into sucrose synthesis and, consequently, lower productivity of canes. Though no physiological experiments related to the combined effects on C4 photosynthesis in sugarcane were conducted, the existing information proves that the oxidative stress, impairments in photosystem integrity and reduced activity of C4 enzymes limit carbon assimilation under stress conditions.

4.3 Nutrient and hormonal regulation

Chromium toxicity does not limit only to oxidative and photosynthetic processes but impacts the mineral nutrition of the plant. Namely, the presence of Cr leads to the impairment of mineral nutrients uptake and distribution (Wakeel and Xu, 2020). Indeed, as Cr(VI) enters the cell with the help of sulphate and phosphate transporters, it competes with these nutrients, thus interfering with their metabolism (Wani et al., 2024). Furthermore, salinity additionally exacerbates the nutritional imbalance in the form of increased uptake of Na⁺ and Cl⁻ ions and decreased absorption of K⁺ and Ca²⁺ (Rasheed et al., 2016; Kumar et al., 2023). Therefore, a combination of disturbed mineral nutrition and ionic balance presents a severe physiological limitation of combined stress.

Physiological reactions to these perturbations are controlled by the hormone signalling and abscisic acid (ABA) is considered as a key hormone controlling stress adaptation. As it was mentioned earlier, salinity activates the ABA production in sugarcane roots, and this hormone is responsible for control of the stomatal conductance, water retention and activation of stress-responsive genes that protect cells from stress (Kumar et al., 2023). Moreover, it becomes evident now that the beneficial endophytes can improve stress tolerance of sugarcane by manipulation of ABA-dependent signalling pathways and thus improve physiological adaptation (Vargas et al., 2014; Kumar et al., 2023). Maintenance of cellular hydration is also achieved with the help of accumulation of compatible osmolytes. Indeed, in sugarcane, proline, glycine betaine, trehalose and soluble carbohydrates adjust osmosis, stabilize proteins and membranes and provide maintenance of cellular metabolism under the conditions of salinity stress (Vargas et al., 2014; Kumar et al., 2023). Tolerant varieties are characterised by the higher content of these osmoprotectants along with the lower content of sodium ions than salt-sensitive cultivars, which proves the importance of osmotic adjustments as one of physiological determinants of stress resistance (Chiconato et al., 2021). However, the synthesis of osmolytes needs much energetic investments of cells and diverts carbon and energy resources from development and sucrose synthesis, which contributes partially to the reduction of crop productivity (Mehdi et al., 2024).

Apart from osmotic adjustments, plants reduce chromium toxicity through intracellular detoxification mechanisms. Metal chelators such as phytochelatin, metallothionein and glutathione-S-transferase complex immobilize chromium to less active forms and enable its sequestration in cellular organelles, thus minimizing interference of Cr with vital physiological processes (Wani et al., 2024). Although these adaptive strategies allow plants to survive in the conditions of combined Cr and salinity stress, the interconnection of these strategies with mineral nutrition, hormones regulation and carbon metabolism remains poorly understood in the case of sugarcane.

4.4 C4-specific physiological vulnerabilities and research gaps

The physiological responses discussed in the preceding sections indicate that combined chromium and salinity stress poses a unique challenge to sugarcane because of its C4 photosynthetic pathway. Unlike C3 species, efficient carbon assimilation in C4 plants depends on the coordinated functioning of mesophyll and bundle sheath cells, together with the activities of phosphoenolpyruvate carboxylase (PEPC), pyruvate orthophosphate dikinase (PPDK) and the decarboxylating enzymes that maintain the CO₂ concentrating mechanism (Figure 3; von Caemmerer, 2021; Calsa and Figueira, 2007). Consequently, disruption of any component of this integrated pathway can substantially reduce photosynthetic efficiency and carbon assimilation.

Current evidence indicates that salinity alone downregulates PEPC and other photosynthetic proteins in sugarcane, with the identity of the affected proteins differing between salt-tolerant and salt-sensitive cultivars rather than the tolerant genotype escaping the effect (Chiconato et al., 2021). Similarly, differences in stress tolerance under saline conditions are noted among genotypic variation in stomatal regulation and Rubisco activase activity (Negi et al., 2025). While these studies demonstrate the sensitivity of the C4 photosynthetic machinery to salinity, comparable physiological evidence for the combined effects of Cr and salinity in sugarcane is still scarce. Most of the information derives from rice (Sobahan et al., 2025), wheat (Ali et al., 2025) and quinoa (Guarino et al., 2020), which limits direct interpretation for sugarcane. All three are C3 species, so no C4 proxy for the combined stress exists, and every mechanistic inference for sugarcane must cross both a species and a photosynthetic-pathway boundary. Future work should focus on integrated physiological analyses of combined Cr and salinity stress, with special emphasis on PEPC activity, photosynthetic electron transport, bundle sheath function, source-sink carbon partitioning and sucrose metabolism. Physiological and biochemical approaches for comparative evaluation of tolerant and sensitive sugarcane genotypes will be necessary to identify traits associated with combined stress resilience. Such studies will contribute to our understanding of C4 specific stress physiology and help in developing sugarcane cultivars with improved productivity under multiple environmental constraints.

5 Chromium bioaccumulation, product quality and food chain risk

The distribution of chromium from roots to the harvested stalk and juice in sugarcane is determined by a combination of internal translocation, plant uptake and soil characteristics. These processes influence product quality, food safety and potential human and ecological health risks, particularly under wastewater irrigation and industrial contamination. The following sections examine chromium partitioning, its implications for sugarcane products and current approaches for risk assessment and monitoring.

5.1 Chromium partitioning and translocation

Chromium accumulation in sugarcane is governed by its partitioning from soil to roots and subsequent translocation to the stalk and juice. In a cane belt of Uttarakhand irrigated with paper mill effluent, soil Cr ranged from 24.10 to 67.58 mg kg⁻¹ in topsoil, edible stem Cr reached 10.54 to 60.22 mg kg⁻¹ (Pandey et al., 2016). On the other hand, wastewater irrigated cane near Mysore showed that the accumulation sequence in the crop was Cd > Zn > Co > Pb > Mn > Ni > Cu > Cr > Fe, placing Cr among the less efficiently translocated elements despite significant soil enrichment and confirming that the more mobile cations such as Cd and Zn out compete Cr for transfer into biomass (Alghobar and Suresha, 2015). In Thailand, filter cake amendment produced soils enriched in Cr, Fe, As, Cd and Mn. However, sugarcane biomass preferentially accumulated Hg rather than Cr, underscoring that high soil Cr does not translate into proportionate stalk Cr (Bridhikitti et al., 2023). Chromium reaches the consumable fraction at concentrations that vary by two orders of magnitude between sites. In the Uttarakhand cane belt, juice Cr averaged 5.06 mg L⁻¹ across thirty sampling locations and ranged from 1.79 to 10.23 mg L⁻¹ (Pandey et al., 2016), whereas juice Cr fell below the analytical detection limit at wastewater-irrigated wetland sites in Ghana despite measurable soil enrichment in other metals (Oppong et al., 2018). Root-to-stalk transfer of chromium is low at most sites but not at all of them, and the disagreement between studies is itself informative. At Mysore, Sanka and Kanchanaburi, Cr ranked among the least efficiently translocated elements, whereas in the paper-mill-irrigated soils of Uttarakhand the mean Cr bioconcentration factor of 0.62 ranked second of eight metals and stem Cr reached 60.22 mg kg⁻¹ (Pandey et al., 2016). The Uttarakhand soils were slightly acidic, which favours both Cr(III) solubility and chromate persistence, so the contrast is better explained by soil chemistry governing the bioavailable pool than by any intrinsic exclusion capacity of the crop.

The extent of Cr uptake and partitioning is strongly influenced by soil properties, including pH, organic matter, cation exchange capacity and redox conditions, all of which regulate Cr speciation and mobility (Pandey et al., 2016). Under highly contaminated environments (industrial effluents or wastewater irrigation), increased bioavailable Cr leads to greater plant accumulation and higher bioconcentration. In contrast, soils with lower metal availability generally show limited Cr transfer despite elevated total soil concentrations. These observations indicate that bioavailable Cr, rather than total soil Cr, is the principal determinant of plant uptake. Chromium partitioning is further influenced by interactions with co-occurring metals and plant transport processes, resulting in comparatively low translocation from roots to stalks even under elevated soil Cr levels. Nevertheless, detectable Cr concentrations in sugarcane juice demonstrate that the edible fraction remains a potential pathway for human exposure. Therefore, understanding Cr partitioning from soil to harvested tissues is essential for assessing product quality, food safety and the risks associated with sugarcane cultivated in contaminated environments (Figure 4). The quantitative bioconcentration factors, transfer factors and juice chromium concentrations from these field studies are compiled comparatively in Table 3.

Table 3. Comparative assessment of chromium accumulation, bioconcentration and translocation in sugarcane field studies.

DW, dry weight; mg kg⁻¹, milligrams per kilogram; μg kg⁻¹, micrograms per kilogram; mg L⁻¹, milligrams per litre.

^aEach source calculated its bioconcentration factor over the plant matrix.

^bAccumulation sequences are the author’s own rankings, based on their soil-to-plant ratios. They are not kinetic uptake sequences. The Sanka sequence is a transfer-factor order.

5.2 Quality and processing implication

Chromium accumulation in sugarcane influences the quality and safety of downstream products, although its effects vary with the degree of contamination and the processing stage. Available evidence indicates that field level Cr contamination does not necessarily reduce juice sugar concentration or sucrose accumulation under moderate contamination. However, the presence of Cr and other heavy metals in harvested cane raises concerns regarding product quality and food safety, particularly where sugarcane is cultivated using industrial effluents or wastewater irrigation (Xie et al., 2014).

The impact of heavy metals becomes more pronounced during sugar processing and bioethanol production because contaminants are redistributed among processing fractions, including juice, molasses, bagasse, press mud, vinasse and wastewater. Although fresh cane juice may contain metal concentrations that do not substantially impair fermentation, concentration of metals in molasses can inhibit yeast growth and fermentation efficiency. Pretreatment of molasses to remove metal impurities has been shown to improve ethanol yield and fermentation performance, highlighting the importance of controlling metal accumulation during industrial processing (Sánchez et al., 2018; Zohri et al., 2022). Therefore, processing plays a critical role in determining the final distribution and potential risk of Cr in sugarcane derived products. Because metals partition differently among processing byproducts, their concentrations cannot be inferred directly from those measured in raw cane or juice. Consequently, individual products should be evaluated separately to ensure product quality, industrial efficiency and environmental safety (Zohri et al., 2022).

Overall, current evidence suggests that sugarcane grown in moderately contaminated soils can remain suitable for sugar and bioethanol production when appropriate processing and quality control measures are implemented. Nevertheless, continuous monitoring of Cr and co-occurring metals throughout the processing chain is essential to minimize contamination of food and industrial products while supporting the safe utilization of sugarcane cultivated in polluted environments (Xie et al., 2014; Zohri et al., 2022).

5.3 Human and ecological risk assessment

Methodological issue must be considered when interpreting the comparisons presented below. Oral reference doses for the two chromium species differ by approximately 500-fold, and the carcinogenic slope factor applies to Cr(VI) alone, yet all the sugarcane risk assessments reviewed here quantified total Cr by acid digestion, and most do not state which toxicity reference value was then applied. Hazard quotients and cancer risks derived on this basis are consequently not comparable between studies and are conservative by an unquantified factor wherever a Cr(VI) value has been applied to a total-Cr measurement. Risk assessment for sugarcane based systems reveals a recurring and important pattern. Assessment based on individual metals may underestimate overall hazards because combined exposure to Cr and co-occurring metals can substantially increase cumulative health risks. Consequently, integrated risk indices provide a more realistic evaluation of contamination than single element assessments (Dejen and Taye, 2024).

Human exposure to Cr occurs through multiple pathways, including direct consumption of sugarcane juice, incidental soil ingestion dermal contact and inhalation, with the relative importance of each pathway varying among production systems (Dejen and Taye, 2024; Wang et al., 2020). In the Ethiopian plantation system, non-dietary oral intake contributed most to total chromium exposure in children, followed by dermal contact, juice drinking and inhalation, while in adults dermal contact ranked first (Dejen and Taye, 2024). In a mining-affected rural area of Guangxi, China, consumption of juice was the dominant route and Cr together with Cd in sugarcane juice posed the most significant non carcinogenic risks to children (Wang et al., 2020).

Children consistently emerge as the most vulnerable group across these assessments, a pattern attributable to higher intake relative to body mass and greater physiological sensitivity (Figure 4). Across the Ethiopian and multi-crop assessments, individual hazard quotients remained within acceptable limits while cumulative hazard indices and carcinogenic risks rose above them under multi-metal exposure, particularly where chromium was present (Dejen and Taye, 2024; Shetty et al., 2025). The Guangxi system departs from this pattern: there, juice consumption alone produced hazard quotients of 1.63 for chromium and 1.38 for cadmium, so the exceedance did not require aggregation across metals to appear (Wang et al., 2020). These findings emphasize that ecological degradation and human health risks should be evaluated collectively rather than on an element-by-element basis. Such approaches provide a more robust basis for evaluating food safety and environmental sustainability in sugarcane production systems affected by chromium contamination (Table 4).

Table 4. Human health risk assessment metrics for sugarcane and comparable cropping systems.

Cd, cadmium; CR, carcinogenic risk (USEPA acceptable range 1 × 10⁻⁶ to 1 × 10⁻⁴); Cr, chromium; Cr(III), trivalent chromium; Cr(VI), hexavalent chromium; HI, hazard index (sum of the individual hazard quotients across all metals assessed; HI < 1 = acceptable combined risk); HM, heavy metal; HQ, hazard quotient (HQ < 1 = acceptable non-carcinogenic risk); THQ, target hazard quotient. An en dash (−) indicates a metric the source did not calculate.

5.4 Comparative risk across cropping systems

Comparison with other wastewater irrigated cropping systems indicates that sugarcane generally presents a lower risk of heavy metal transfer to edible tissues than many food crops. This difference largely reflects its capacity to retain a substantial proportion of absorbed metals within the root system, thereby limiting translocation to the harvested stalk (Dejen and Taye, 2024; Alghobar and Suresha, 2015). In contrast, leafy vegetables readily accumulate heavy metals in edible tissues, resulting in greater dietary exposure and higher human health risks under wastewater irrigation (Othman et al., 2021).

Despite this relative advantage, sugarcane cannot be considered risk free. Detectable concentrations of chromium and other heavy metals in cane juice demonstrate that contaminants can still enter the food chain, particularly where irrigation relies on industrial effluents or contaminated wastewater (Wang et al., 2020). Furthermore, direct consumption of fresh sugarcane juice represents an important exposure pathway that may offset the benefits of restricted root to stalk translocation. Evidence from wastewater irrigated agricultural systems also suggests that appropriate management practices can substantially reduce consumer risks, even under long term wastewater use (Woldetsadik et al., 2017). Nevertheless, the degree of risk depends on site specific factors, including contamination intensity, wastewater quality, soil properties and crop characteristics, emphasizing the need for crop specific assessments rather than generalized assumptions. However, continued monitoring of juice quality and heavy metal accumulation remains essential to ensure food safety and sustainable cultivation in contaminated environments (Othman et al., 2021; Wang et al., 2020).

5.5 Monitoring and future risk assessment

Effective monitoring is essential for minimizing chromium (Cr) contamination and ensuring the safe cultivation of sugarcane in industrial and wastewater irrigated agroecosystems. Long-term irrigation with contaminated water promotes the accumulation of Cr and other heavy metals in soils and groundwater, often accompanied by increasing salinity, emphasizing the need for integrated monitoring of both metal contamination and soil quality (Hussien and Yousif, 2024). Consequently, monitoring programs should encompass the entire soil plant continuum, including irrigation water, soil, roots, stalks, and juice, to accurately assess contaminant transfer and potential exposure pathways (Pandey et al., 2016). Risk assessment should extend beyond total Cr concentrations by incorporating Cr speciation, particularly the differentiation between Cr(III) and the more toxic Cr(VI), together with multi metal pollution indices and cumulative health risk assessments (Dejen and Taye, 2024). Because sugarcane is frequently exposed to contaminant mixtures rather than individual metals, integrated assessment approaches provide a more realistic evaluation of environmental and human health risks than single element analyses.

Routine monitoring of sugarcane juice and processed products should also be prioritized, as these represent the primary routes of human exposure in many production systems (Wang et al., 2020). Furthermore, age specific risk assessment should be incorporated into monitoring frameworks (Sharafi and Salehi, 2025). Integrating contaminant monitoring with pollution indices, Cr speciation and population specific risk assessment will provide a comprehensive framework for safeguarding product quality, public health and the long-term sustainability of sugarcane production in contaminated environments.

6 Mitigation strategies for Cr-salinity stress

No single intervention addresses metal toxicity, salinity and their interaction simultaneously, and at field scale combined strategies have consistently outperformed isolated treatments in both efficiency and durability (Sánchez-Castro et al., 2023). The approaches reviewed here therefore span soil amendments, engineered materials, microbial inoculants and agronomic water management, considered individually and in combination. Because removal efficiency measured in a stirred aqueous system is not a prediction of field performance, each intervention is identified below by the crop in which it was tested and by its experimental scale aqueous batch, soil incubation, pot trial or field trial.

6.1 Sugarcane derived biochar and processing residues

Biochar made from sugarcane processing residues such as bagasse has become a potential sustainable amendment for soil under co-stress with chromium and salts. Due to high surface area, functional groups and alkaline nature, sugarcane biochar effectively immobilizes heavy metals, alters physicochemical and microbial properties of soil, thus decreasing bioavailability of heavy metals and improving plant growth (Bashir et al., 2018). Experimental research shows that bagasse based biochar (SCB) significantly decreases bioavailable form of Cr and coexisting metals, while minimizing their uptake by plants. The effect is associated with higher soil pH, cation exchange capacity and efficient adsorption of metal ions on the surface of biochar (Nie et al., 2018). In case of soil contamination by combined metals and salts, biochar improves soil organic carbon content, increases microbial and enzymatic activities and decreases the availability of Cd and Pb, meaning that biochar can overcome both metal toxicity and salt stress but not any individual one (Azadi and Raiesi, 2021).

The efficiency of SCB also depends on properties of soil and biochar preparation process. Biochar prepared at higher temperatures demonstrate higher metal immobilizing capacity and contribution to soil carbon sequestration, while those prepared at lower temperatures promote microbial activity of soil (Azadi and Raiesi, 2021; Hafez et al., 2024). There might be differences in biochar efficiency in case of salinity and saline sodicity of soils. Two distinct effects were reported in salt-affected soils. Biochar applied at 2 and 4% raised exchangeable Cd in saline and saline-sodic soils, an outcome attributed to the material’s capacity to sorb salts and so relieve the ionic competition that had been holding Cd in solution; separately, and in all four soil types tested, the organic-matter-bound fraction became the dominant Cd pool after amendment (Zahedifar, 2020). This evidence emphasizes the need to use certain biochar features depending on soil and salinity conditions. The efficiency of SCB can be additionally improved through combination with other materials. Sugarcane bagasse applied alone at 10 t ha⁻¹ lowered the exchangeable sodium percentage by 30.8% and raised microbial biomass by 151.1% relative to the untreated control. Combining it with zinc oxide nanoparticles lowered shoot Cd by 74.1%, against 43.3% for the nanoparticles alone, and raised grain yield by 75.2%, against 42% for bagasse alone, in a field trial on wheat (Hafez et al., 2024). Besides bagasse, there are other residues of sugarcane processing which can be used as raw materials for biochar, such as straw, press mud, bagasse ash, vinasse and molasses (Ungureanu et al., 2022).

6.2 Engineered biochar and nano amendments

Engineered biochar and nano-amendments have emerged as promising technologies for remediating Cr contaminated soils. Because they not only adsorb metals but also transform toxic Cr(VI) into the less mobile and less toxic Cr(III). Among these materials, nano zero-valent iron (nZVI) modified biochar has attracted considerable attention owing to its combined adsorption, reduction and immobilization capabilities, making it particularly suitable for contaminated environments (Dermawan et al., 2025). Sugarcane bagasse biochar modified with nZVI exhibits substantially greater Cr(VI) adsorption than unmodified biochar which reduces Cr(VI) to Cr(III), followed by adsorption and precipitation on the biochar surface (Bai et al., 2021; Dermawan et al., 2025). Similar improvements have been reported in soil-based studies demonstrated the efficacy of biochar supported nZVI in soil in proxy systems. nZVI@BC at 8 g kg⁻¹ achieved 100% Cr(VI) and 91.94% total Cr immobilization within 15 days, converting exchangeable Cr almost entirely to Fe-Mn oxide and organic matter fractions while reducing the leachability of iron (Su et al., 2016). The biochar support also enhances nanoparticle dispersion, supplies an electron-shuttle function and improves soil properties, thereby increasing remediation efficiency.

The effectiveness of engineered biochar extends beyond chromium. Multi-functional composites containing biochar, bentonite and nZVI have simultaneously immobilized Cr(VI), Pb and Cd through complementary mechanisms including adsorption, reduction, co-precipitation and surface complexation (Jin et al., 2023). However, the performance of nano amendments depends on application rate and material composition. Excessive nZVI concentrations may induce phytotoxicity, suppress plant growth and increase iron accumulation in roots. In general, biochar supported formulations generally reduce these adverse effects and provide greater environmental stability than unsupported nanoparticles (Sun et al., 2020). In a non-C4 proxy study using oilseed rape, foliar silicon nanoparticles applied under 100 μM Cr(VI) lowered root and leaf chromium by 33 and 55%, respectively, and partly restored shoot biomass, although the system was hydroponic and no salinity was applied (Batool et al., 2025). However, field scale validation is therefore required before these technologies can be widely adopted for sustainable management of contaminated sugarcane production systems.

6.3 Biological interventions and agronomic management

Biological interventions offer environmentally sustainable approaches for mitigating combined Cr-salinity stress by enhancing metal detoxification, improving nutrient acquisition and restoring plant physiological functions (Hoque et al., 2022). Plant growth promoting rhizobacteria (PGPR), endophytes and halotolerant microorganisms have demonstrated considerable potential to reduce Cr toxicity while simultaneously improving plant growth under saline conditions through multiple complementary mechanisms, including Cr(VI) reduction, phytohormone production, nutrient solubilization and antioxidant enhancement (Tirry et al., 2021).

Among these biological approaches, Cr resistant PGPR enhances plant tolerance by reducing oxidative stress, improving chlorophyll content, and promoting root and shoot growth while limiting metal toxicity. In addition, halophilic bacteria capable of reducing Cr(VI) under high salinity provide an effective strategy for remediation in saline environments, where conventional microorganisms often exhibit poor survival and activity (Mishra et al., 2012). These findings demonstrate that microbial detoxification can remain effective even under the saline conditions commonly encountered in sugarcane growing regions.

Evidence from combined stress studies further indicates the potential of microbial inoculation to restore ion homeostasis and reduce metal accumulation. Endophytic rhizobacteria have been shown to simultaneously decrease Na⁺ and Cr uptake, reduce Cr translocation and bioaccumulation and strengthen antioxidant defence under concurrent Cr and salinity stress (Sobahan et al., 2025). Microorganisms can also be combined with engineered biochar to compound the benefit. Iron-enriched rice husk biochar applied at 3% with the fungus Trichoderma viride reduced DTPA-extractable chromium by 96.8% after 40 days, while the most effective bacterial pairing, zinc-enriched biochar with Pseudomonas fluorescens, reached 92.81% over the same interval (Batool et al., 2022). Agronomic management practices also influence Cr mobility and availability. Water management with the regulation of soil redox conditions, alters metal speciation and promotes the formation of stable Fe/Mn oxide-bound and sulphide bound metal fractions, thereby reducing Cr bioavailability (Matern and Mansfeldt, 2016; Wan et al., 2024). Although biological and agronomic interventions show considerable promise individually, their combined application with derived biochar and other amendments remains largely unexplored. The major remediation strategies and their reported outcomes are summarized in Table 5. A structural feature of this evidence base warrants stating plainly: across the twelve interventions surveyed, none used sugarcane as the test plant. The connection to the crop lies in the feedstock of the biochars rather than in the species on which they were assessed.

Table 5. Performance of biochar, nano and biological based interventions for chromium remediation.

SCB, sugarcane bagasse biochar; nZVI, nano zero-valent iron; PGPR, plant growth-promoting rhizobacteria; DTPA, diethylenetriaminepentaacetic acid (extractable metal fraction); CEC, cation exchange capacity (cmol kg⁻¹); SOC, soil organic carbon; ESP, exchangeable sodium percentage; EC, electrical conductivity; DW, dry weight; MDA, malondialdehyde; qmax, Langmuir maximum adsorption capacity (mg g⁻¹); d, days; IAA, indole-3-acetic acid. Negative values (−) denote reductions; positive values (+) denote increases relative to untreated control.

^aAt 2–4% application rates in saline and saline-sodic soils, biochar raised exchangeable Cd rather than lowering it, highlighting the need for careful dose optimisation in salt-affected systems.

6.4 Integrated management framework

The recurring lesson across these strategies is that no single intervention resolves the Cr-salinity-co-metal cocktail, because each tactic addresses a different facet of a coupled problem. Organic amendments such as biochar, vermicompost, green manure and farmyard manure simultaneously alter the plant’s ionic balance, enhance photosynthetic machinery, boost antioxidant systems and reduce oxidative damage, which positions them as multifunctional rather than single-target tools under combined salinity and metal stress (Irin and Hasanuzzaman, 2024). A coherent management framework therefore integrates complementary measures and review of agricultural soil remediation organize these into low metal cultivar selection and breeding, physiological blocking, water management and soil amendment, followed by phytoremediation where extraction is the goal (Wan et al., 2024). The evidence that combinations outperform single measures is strongest at field scale, where the best efficiencies have been reported for combined strategies such as physical containment paired with assisted phytoremediation, in contrast to the more modest and less durable outcomes of isolated treatments (Sánchez-Castro et al., 2023).

The components reviewed here can be assembled into an evidence-informed framework for sugarcane, offered in Figure 5 as a testable configuration rather than a validated protocol. None of its four elements has been evaluated in sugarcane, and of the twelve interventions surveyed only one was tested at field scale, in wheat (Hafez et al., 2024). It includes cane bagasse biochar to immobilize cationic co-metals and rebuild soil organic carbon and microbial function, nZVI-modified biochar to reduce and fix Cr(VI), halotolerant and Cr-reducing inoculum to detoxify chromium under saline conditions and redox-oriented water management to shift metal partitioning toward stable fractions (Figure 5). The translation of laboratory validated combinations into field validated management for industrial sugarcane corridors consequently represents a priority for future field scale validation and the design of such trials should explicitly cross amendment type with salinity and co-metals so that the interactions will provide the evidence required to develop resilient, field ready management strategies (Chen et al., 2025; Wan et al., 2024).

7 Synthesis and discussion

7.1 Integrated pathway

The evidence compiled here reveals a pathway that is well established in its initial stages but remains incomplete in the later stages. Agro-industrial inputs demonstrably alter soil electrical conductivity, redox potential and pH; those variables demonstrably govern Cr(VI)/Cr(III) partitioning; and that partitioning demonstrably determines the size of the mobilizable pool. From the mobilizable pool onward the chain becomes inferential. No study has measured chromate uptake by sugarcane roots under controlled salinity, none has measured C4 carbon assimilation in cane under combined chromium and salinity exposure, and the mitigation options proposed for the crop have been validated in other species. Field observations resume at the far end of the chain, in stalk and juice concentrations and in the risk metrics derived from them, but they cannot be connected mechanistically to the soil conditions that produced them. This discontinuity, more than any single quantitative finding, is what the present synthesis establishes.

7.2 Research limitations

Although considerable progress has been made in understanding Cr contamination in sugarcane systems, important knowledge gaps remain.

• Field observations of chromium contamination and bioaccumulation in sugarcane exist, but they are few and unevenly distributed across a small number of sites in seven countries (Alghobar and Suresha, 2015; Corbi et al., 2018; Oppong et al., 2018; Wang et al., 2020; Bridhikitti et al., 2023; Dejen and Taye, 2024; Hussien and Yousif, 2024). They are almost entirely cross-sectional, report total rather than speciated chromium, and seldom record salinity alongside metal concentrations, so they establish that contamination occurs without constraining the processes that govern it (Pandey et al., 2016).
• Mechanistic evidence on combined chromium and salinity stress is very limited, deriving from rice (Sobahan et al., 2025), wheat (Ali et al., 2025) and quinoa (Guarino et al., 2020) rather than from sugarcane itself. Most remediation studies rely on laboratory or pot experiments with limited validation under field conditions in sugarcane production systems (Sánchez-Castro et al., 2023).
• Consequently, many current recommendations rely on extrapolation across crop species and experimental scales, underscoring the need for sugarcane specific field validation (Wan et al., 2024).

7.3 Future directions

Future research should focus on resolving the major knowledge gaps identified throughout this review and developing field validated management strategies for combined Cr-salinity stress in sugarcane. Major future research priorities identified from the current literature include,

• Conduct factorial experiments evaluating different Cr species [Cr(VI) and Cr(III)] across salinity gradients to quantify their interactive effects on sugarcane growth, physiology, and productivity.
• Investigate Cr uptake mechanisms in sugarcane roots, particularly the role of phosphate and sulphate transporters in mediating Cr acquisition under saline conditions (Murugan et al., 2022).
• Characterize rhizosphere processes controlling Cr speciation, redox transformations and root exudate interactions under combined Cr-salinity stress.
• Examine the effects of combined Cr and salinity on C4 photosynthesis, PEP carboxylase activity, bundle sheath function, sucrose metabolism and genotype-specific stress tolerance in sugarcane (Negi et al., 2025; Dhansu et al., 2026).
• Determine Cr speciation in harvested stalks and juice to improve assessments of food safety and human health risk (Dejen and Taye, 2024).
• Validate sugarcane derived biochar, nano amendments, microbial inoculants and their integrated application under long-term field conditions and ratoon cropping systems (Jin et al., 2023; Sánchez-Castro et al., 2023; Mishra et al., 2012; Irin and Hasanuzzaman, 2024).
• Develop integrated monitoring frameworks that simultaneously evaluate salinity indicators (EC and SAR), Cr speciation, co-metal contamination and human health risks in industrial sugarcane production systems (Hussien and Yousif, 2024; Dejen and Taye, 2024).

8 Conclusion

Chromium contamination in sugarcane production systems rarely occurs in isolation but is commonly associated with salinity and other co-occurring heavy metals arising from industrial effluents, wastewater irrigation and long-term agricultural practices. This review finds the chromium-salinity interaction to be well established at the soil scale, where ionic strength, redox coupling and competitive desorption jointly govern speciation and mobility. At the plant scale the position differs. Salinity and chromium each disturb nutrient balance, redox homeostasis and photosynthetic carbon metabolism in sugarcane when applied alone, but their combined action has not been examined in this crop, and the expectation that they interact non-additively rests on evidence from rice, wheat and quinoa rather than from cane. Root-to-stalk transfer of chromium is low at most sites examined but not at all of them, and measurable chromium reaches cane juice; in a mining-affected system, juice consumption alone was sufficient to carry the chromium hazard quotient beyond its acceptable threshold, so food safety remains a legitimate consideration in product quality evaluation.

Mitigation options exist but have not yet been demonstrated for this crop. Sugarcane-derived biochar, nano zero-valent iron composites, halotolerant chromium-reducing inocula and redox-oriented water management each lower chromium bioavailability and improve soil physical and microbial condition in the systems in which they have been evaluated, and combinations consistently outperform single measures. None has been tested in sugarcane, however, and field-scale validation exists only in other crops, so the integrated framework proposed here is a testable configuration rather than a recommendation for deployment. The use of bagasse, filter cake and other processing residues as feedstock for these amendments nonetheless aligns remediation with the circular bioeconomy, returning a waste stream of the industry to the soils that sustain it.

Three requirements follow for industrial cane corridors. Field trials must evaluate chromium and salinity together in sugarcane across ratoon cycles, resolving chromium speciation, rhizosphere redox and the response of the C4 photosynthetic apparatus rather than inferring them from other species. Monitoring must extend across the irrigation water, soil, root, stalk and juice continuum, pairing salinity indicators reported as both electrical conductivity and sodium adsorption ratio with speciated chromium and co-metal determinations, since total chromium resolves neither mobility nor toxicity. And risk assessment must state which chromium species its toxicity values assume, because the trivalent and hexavalent forms differ so widely in potency that an unstated assumption about speciation can dominate the resulting estimate. Until these are in place, management of chromium in salt-affected sugarcane systems will continue to rest on extrapolation from other crops and other stresses.

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Footnotes

Edited by: Emmanuel Oliver Fenibo, Hensard University, Nigeria

Reviewed by: Ahsan Ayyaz, Zhejiang University of Science and Technology, China

Pratyush Kumar Das, Centurion University of Technology and Management, India