Chromium-Induced Reactive Oxygen Species Accumulation by Altering the Enzymatic Antioxidant System and Associated Cytotoxic, Genotoxic, Ultrastructural, and Photosynthetic Changes in Plants

Abstract

Chromium (Cr) is one of the top seven toxic heavy metals, being ranked 21st among the abundantly found metals in the earth’s crust. A huge amount of Cr releases from various industries and Cr mines, which is accumulating in the agricultural land, is significantly reducing the crop development, growth, and yield. Chromium mediates phytotoxicity either by direct interaction with different plant parts and metabolic pathways or it generates internal stress by inducing the accumulation of reactive oxygen species (ROS). Thus, the role of Cr-induced ROS in the phytotoxicity is very important. In the current study, we reviewed the most recent publications regarding Cr-induced ROS, Cr-induced alteration in the enzymatic antioxidant system, Cr-induced lipid peroxidation and cell membrane damage, Cr-induced DNA damage and genotoxicity, Cr-induced ultrastructural changes in cell and subcellular level, and Cr-induced alterations in photosynthesis and photosynthetic apparatus. Taken together, we conclude that Cr-induced ROS and the suppression of the enzymatic antioxidant system actually mediate Cr-induced cytotoxic, genotoxic, ultrastructural, and photosynthetic changes in plants.

1. Introduction

Chromium (Cr), heavy metal with a range of oxidation numbers [Cr(II) to Cr(VI)], which is placed in the group (VI-B) of transition elements in the modern periodic table [1]. Chromium, which is the hard silver color metal with 7.19 g/cm³ density, 51.10 g/M molecular weight, and 24 atomic number, has been ranked 21st among the most abundantly found metals on the earth’s crust [2]. The trivalent [chromite; Cr(III)] and the hexavalent [chromate; Cr(VI)] are the most stable naturally found Cr species [3]. Hexavalent form of Cr is a potentially strong oxidizing agent, and higher water solubility, mobility, and bioavailability make it the most toxic form of Cr as compared to other Cr species [4]. The oxygenated environment can convert Cr(III) into Cr(VI), the factors that are involved in maintaining the proper ratio of these Cr forms are oxygen concentration, pH, complexing factors, and reducing agents [5].

Chromium extraction from the mines has been excessively increased due to its increasing use in various industries [2]. Kazakhstan, South Africa, China, and India are the world-leading Cr using countries [2,6,7]. Leather tanning, metallurgy, electroplating, alloying, ceramic glazes, wood preservation, water corrosion inhibition, refractory bricks, pressure-treated lumber, textile dyes, and mordant, pigments and paints production, and paper and pulp production industries contribute to the hyperaccumulation of Cr in the environment. Furthermore, anthropogenic activities, such as the dumping Cr-contaminated liquids and solids wastes, are the reason for the hyperaccumulation of Cr in the environment [8,9,10,11]. The emission of Cr from the cooling towers of the industries and the dust rising from the roads and roadsides are considered to be the most important Cr sources [12,13].

Increased Cr accumulation in the agricultural land causes damage the plant growth and development at the organ, cellular, or even genetic level [14]. Cr-induced phytotoxicity is mostly mediated via induced reactive oxygen species (ROS), which cause the cellular and extracellular damage in plants [8]. In the current study, we reviewed Cr-induced ROS, associated cellular, and ultra-structural damages in plants

2. Chromium-Induced Oxidative Stress in Plants

Plants that are exposed to unfavorable conditions produce reactive oxygen species (ROS) as a defense mechanism [15,16]. The hyperaccumulation of ROS generates endogenous stress that can damage plant growth and development [8]. Hydrogen peroxide (H₂O₂), superoxide anion (O₂⁻), singlet oxygen (¹O₂), hydroxyl ion (HO⁻), peroxyl (RO⁻), alkoxyl (RO⁻), and organic hydroperoxide (ROOH) are the various ROS that are found in plants [2,17,18]. Reactive oxygen species are produced in the mitochondria, peroxisome, and chloroplast as a byproduct of various biochemical reactions [18,19,20,21]. Plants mechanisms that are in the regulation of ROS level include ROS biosynthesis, enzymatic, and/or non-enzymatic ROS scavenging [8]. Heavy metals, such as lead (Pb), cadmium (Cd), aluminum (Al), nickel (Ni), and Cr, are reported for the enhancement in ROS productions and accumulation [8,19,22]. Various plant species that are exposed to toxic Cr level or industrial wastes containing the toxic level of Cr, showed induced ROS accumulation, as summarized in

Table 1. Accumulations and investigations of various ROS species in numerous plant species exposed to Cr(VI) and/or Cr(III). Superoxide (O₂⁻), hydrogen peroxide (H₂O_2), hydroxyl ion (HO⁻), and singlet oxygen (¹O₂).

Plant Species	Common Name	ROS Types	Cr(VI) Concentration	References
Arabidopsis thaliana	Arabidopsis	O2−, H2O2	100–400 µM	[8,23]
Helianthus annuus	Sunflower	O2−, OH−, H2O2	20 mg/L & 20 mg/Kg	[24,25,26]
Zea mays	Maize	O2−, H2O2, OH−	100–300 µM & 100–300 mg/Kg	[27,28,29,30,31,32]
Brassica juncea	Indian mustard	1O2, O2−, H2O2, OH−	300 µM	[17,33]
Glycine max	Soybean	H2O2	400 mg/kg & 500 mg/kg Cr(III)	[22]
Oryza sativa	Rice	O2−, H2O2	80–200 µM	[34,35,36,37]
Amaranthus viridis & Amaranthus cruentus	Green & Blood amaranth	O2−, H2O2	50 µM	[38]
Chenopodium quinoa	Quinoa	H2O2	5 mM Cr(III)	[39]
Cucumis sativus	Cucumber	O2−, H2O2	200 µM	[40]
Brassica napus	oilseed rape	O2−, H2O2, OH−	400 μM	[41,42]
Brassica campestris	Cabbage	O2−	1 mg/L	[43]
Pisum sativum	Pea	O2−, H2O2	100 μM	[44]
Allium cepa	Onion	O2−, H2O2, OH−	200 µM	[45]
Matricaria chamomilla	Chamomile	H2O2	120 µM Cr(III)	[46]
Lens culinaris	Lentil	H2O	250 µM	[47]
Raphanus sativus	Radish	O2−, H2O2	1.2 mM	[48]
Pistia Stratiotes	Lettuce	H2O2	10 mM	[49]

.

Chromium-induced ROS accumulation mediates various physiological, biochemical, molecular, and developmental changes in plants [41]. These alterations in the physiological and biochemical process may be provoked by directly interacting with enzymes, lipids, proteins, and genetic material (DNA and/or RNA), or by Cr-induced ROS accumulation [8,50,51]. Cr direct interaction or Cr-induced ROS both mediated membrane damage, degradation and deactivation of genetic material, proteins, and enzymes, which resulted in the growth inhibition by the suppression cell division or activation programmed cell death [8,52,53].

Chromium-induced ROS mediates ultra-structural alteration in various plant tissues and irreversibly degrades biomolecules, except for DNA, cysteine, and methionine, which can be restored, in a dose-dependent and tissue-specific manner [23,45,49,54]. Reactive oxygen species are produced during the reduction reaction of Cr(VI) to Cr(III) and Fenton reaction. The catalytic power of Cr(III) is greater than iron (Fe), copper (Cu), cobalt (Co), manganese (Mn), and zinc (Zn) in the Fenton reaction [2,45,54,55]. The Cr involvement in such reactions is not well studied and some other intermediates and factors may also be involved in the Cr-induced ROS generation [8]. ROS mediated various physiological, biochemical, molecular, and ultrastructural changes, as shown in Figure 1.

3. Chromium-Mediated Alteration in the Enzymatic Antioxidant System

Plants have developed a complex and well-organized enzymatic antioxidant system to deal with access ROS, produced by various endogenous and exogenous stimuli, including toxic Cr levels [8]. Superoxide (O₂⁻) is converted to H₂O₂ by superoxide dismutase (SOD). H₂O₂ is converted by ascorbate peroxidase (APX), peroxidase (POD), and catalase (CAT) to H₂O [8,56]. Furthermore, to minimize the Cr, cadmium (Cd), bisphenol A (BPA), and other abiotic stresses mediated oxidative stress, plants use the enzymatic antioxidant system, which includes, SOD, APX, POD, CAT, glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione peroxidase (GPX), and glutathione S-transferase (GST) [8,17,21,50,57,58]. Previous studies have reported that Cr-induces the alteration in the production and accumulation of enzymatic antioxidant system for the regulation and scavenging Cr-induced ROS have been summarized in

Table 2. Chromium-modulated antioxidant enzymes in various plant species. Ascorbate peroxidase (APX), catalase (CAT, dehydroascorbate reductase (DHAR), glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferase (GST), monodehydroascorbate reductase (MDHAR), peroxidase (POD), and superoxide dismutase (SOD).

Plant Species	Common Name	Enzymes	Cr(VI)	References
Helianthus annuus	Sunflower	CAT, SOD, POD, APX	20 mg/kg	[25,26]
Triticum aestivum Hordeum vulgare	Wheat & Barley	CAT, APX	22 mg/kg	[59]
Brassica oleracea	Cauliflower	CAT, SOD, POD	200 μM	[60]
Pennisetum alopecuroides	Fountain Grass	CAT, SOD, POD	1500 mg/kg	[61]
Sorghum bicolor	Sorghum	CAT, SOD, APX, GR, GST	64 ppm	[62]
Brassica juncea	Indian Mustard	GR, GPX, CAT, SOD, POD, APX, MDHAR, DHAR	300–500 μM	[17,63]
Solanum melongena	Eggplant	APX, GST, GR	25 µM	[64]
Amaranthus viridis & Amaranthus cruentus	Green & Blood Amaranth	CAT, SOD, POD, GST	50 μM	[38]
Zea mays	Maize	APX, CAT, SOD, POD	100–250 μM	[65,66]
Hibiscus cannabinus	Kenaf	CAT, SOD, POD	1.5 Mm Cr(III)	[67]
Oryza sativa	Rice	APX, CAT, SOD, POD, GR	20–100 μM	[68,69]
Vigna radiate	Mung Bean	CAT, SOD, POD	500 μM	[70]
Brassica chinensis	Pakchoi	CAT, SOD, POD	100 μM & 200 mg/kg	[71,72]
Setaria italic	Foxtail Millet	CAT, SOD, POD, APX	1000 μM	[73]
Solanum nigrum & Parthenium hysterophorus	Black Nightshade & Santa-maria	SOD, POD	500 μM Cr(III)	[74]
Brassica rapa	Turnip	SOD, APX	250 µM	[75]
Brassica napus	Rapeseed	CAT, SOD, POD, APX	500 μM	[76]
Brassica campestris	Cabbage	SOD, POD	1 mg/L	[43]
Gossypium hirsutum	Cotton	CAT, SOD, POD, APX	100 μM	[77]
Corchorus olitorius	Tossa Jute	CAT, SOD, POD, APX, GR	400 mg/kg	[78]
Brassica napus	Canola	CAT, SOD, POD, APX	50 μM	[79]
Raphanus sativus	Radish	CAT, SOD, POD	8 mM	[80]
Hordeum vulgare	Barley	CAT, SOD, POD, APX	225 μM	[81]

.

4. Chromium-Induced Lipid Peroxidation

Lipid peroxidation is initiated by the increased ROS accumulation through the decomposition of membrane lipids and proteins, and it is one of the primary reasons for abiotic stress-induced cell damages [82]. Chromium stress has been reported for the induced ROS production, and it has been also reported for biological membrane damage [2]. One of the lipid peroxidation products, called malondialdehyde (MDA), which is considered as an oxidative damage indicator, has been greatly studied in the heavy metals mediated damage of biological membrane, including Cr [8,82]. Chromium-induced ROS mediated lipid peroxidation in various plant species, including economically important crops, has been summarized in

Table 3. Chromium-induced lipid peroxidation indicators in various plant species. Thio-barbituric acid reactive substances (TBARS) and malondialdehyde (MDA).

Plant Species	Common Name	LPO	Cr(VI)	References
Arabidopsis thaliana	Arabidopsis	MDA	400 µM	[8]
Zea mays	Maize	MDA	100–300 µM	[27,28,31,32,65]
Triticum aestivum Hordeum vulgare	Wheat & Barley	MDA	22 mg/kg	[59]
Solanum lycopersicum	Tomatoes	MDA	24.66 mg/k	[83]
Oryza sativa	Rice	MDA, TBARS	20–200 µM & 20 mg/L	[35,36,69,82,84,85]
Limnobium laevigatum	Floating Plant	MDA	70 µg/L Cr(III)	[86]
Citrus reticulata Blanco	Kinnow	MDA	750 µM	[87]
Sorghum bicolor	Sorghum	MDA	64 ppm	[62]
Helianthus annuus	Sunflower	MDA	20 mg/kg	[25]
Brassica juncea	Indian Mustard	MDA	100–500 μM & 100 mg/Kg	[17,63,88,89]
Solanum melongena	Eggplant	MDA	25 µM	[64]
Tradescantia pallida	Rose	MDA	20 mg/L	[90]
Amaranthus viridis & Amaranthus cruentus	Green & Blood Amaranth	MDA	50 μM	[38]
Pteris vittata	Chinese Brake	TBARS	5 mM	[91]
Chenopodium quinoa	Quinoa	MDA	5 mM Cr(III)	[39]
Saccharum spp. Hybrid	Sugarcane	MDA	50 ppm	[92]
Cucumis sativus	Cucumber	MDA	200 µM	[40]
Pisum sativum	Pea	MDA	100 μM	[44]
Brassica rapa	Turnip	MDA	250 µM Cr(III)	[75]
Brassica napus	Canola	MDA	50–100 μM	[79,93,94]
Brassica oleracea	Cauliflower	MDA	250 μM	[95]
Salvinia minima	Floating Fern	MDA	20 mg/L	[96]
Tradescantia pallida	Wandering Jew	TBARS	20 mg/L	[97]
Gossypium hirsutum	Cotton	MDA	100 μM	[77]
Triticum aestivum	Wheat	TBARS	200 μM	[98]
Allium cepa	Onion	MDA	200 μM	[45]
Raphanus sativus	Radish	MDA	125 m	[80]
Miscanthus sinensis	Chinese Reed	MDA	1000 μM	[99]
Brassica napus	Rapeseed	TBARS	480 μM Cr(III)	[100]

.

5. Chromium-INDUCED DNA DAMAGE and Genotoxicity

Genotoxicity is one of the most serious threats of heavy metals toxicity to living organisms [101,102]. DNA damage can have serious consequences, such as deregulation or mutagenesis of the cell replication process, leading to tumor formation, and ultimately cell death [101,103,104]. Heavy metals cause DNA damage either by direct interaction with DNA or by induced ROS accumulation, which is considered to be one of the main internal causes of DNA damage (Figure 1). Heavy metals not only induce DNA damage, but also interrupt DNA damage repair mechanisms [101,102].

In contrast to other heavy metals, which are directly interacting with DNA, Cr-induces ROS mediated genotoxicity [105]. Chromium-induced genotoxicity and carcinogenic effects are greatly investigated in yeast and animal cells. Its carcinogenic effects have been also reported in the workers, working in the Cr mines and Cr consuming industries [101,104,105,106]. In vivo and in vitro investigations revealed that Cr(VI) produces various types of structural alterations in genetic materials, including inter-DNA strand cross-links, DNA chromosomal protein cross-links, and nucleotide strand breaks [105,107,108].

Chromium-DNA adducts (association of Cr with phosphodiester backbone of DNA), which are mainly reported in mammalian cells, being considered the primary cause of Cr(VI) induced mutagenicity [105,109]. Cr(VI)-mediated genotoxicity has been reported in humans, rats, fish, fish cell lines, yeast, and bacteria [105,108,110,111,112,113]. Some studies have reported that Cr(III) is also interacting with DNA to form a covalent bond with the phosphate backbone [105]. Cr(III) also interacts with the DNA base pairs’ stacking mode, which leads to DNA lesion, cleavage, and the DNA single/double-strand breakage [105,108]. Cr(VI)-induced ROS mediates these various DNA degradations [45]. The current study reviews chromium-induced chromosomal fragmentation and bridging, alteration in DNA methylation, DNA mutation, increase in percent tail DNA, tail moment, and percent DNA damage in tail length, chromosome aberrations or micronuclei formations, DNA inter/intrastrand crosslinks, protein-DNA crosslinks, DNA-single/double-strand breaks, DNA adducts, DNA transcription, and replication dysfunction, abnormal DNA repair mechanisms, changes in signaling pathways for survival, genomic instability, oxidized bases, instability of microsatellites, and genetic/epigenetic alteration in different plant species t (

Table 4. Chromium-induced genotoxicity in various plant species.

Plant Species	Common Name	Genotoxicity	Cr- Type	References
Glycine max	Soybean	DNA damage	Cr(VI)/(III)	[22]
Vicia faba	Faba Bean	Micronucleus, Chromosomal fragmentation & bridging, Increase in % tail DNA, tail moment and Tail length	Tannery solid waste & Cr(VI)	[124,125,126,127]
Allium cepa	Onion	DNA damage, Chromosomal Aberrations, Micronuclei, Chromosomal fragmentation & bridging	Tannery solid waste, Tannery effluent & Cr(VI)	[45,125,127,128,129]
Hordeum vulgare	Barley	Chromosomal aberrations	Cr(VI)	[130]
Vicia sativa	Vetch	Chromosomal aberration, Chromosomal fragmentation & bridging	Wastes, Cr(VI)/(III)	[125,127,131]
Raphanus sativus	Radish	Chromosomal aberration	Cr(VI)/(III)	[125]
Zea mays	Maize	Chromosomal aberration	Cr(VI)/(III)	[125]
Brassica napus	Oilseed Rape	Methylation changes, mutation	Cr(VI)	[127,132]
Arabidopsis thaliana	Arabidopsis	DNA mutation	Cr(VI)	[127,133]

).

6. Chromium-Induced Ultrastructural Changes

6.1. Cr-Induced Necrosis and Cellular Injury

Chromium-induced cytotoxicity affects essential micronutrient absorption, lipid peroxidation, cell cycle arrest and ultimate cell death in plants [8,22,56,87]. Toxic Cr levels also mediate the stomatal abnormalities, such as the decreased size of stomatal aperture, swelling of guard cells, changes in membrane permeability level, ion flux, and osmotic pressure [22,87,114,115]. These stomatal aberrations significantly influence the a, b, and total chlorophyll contents, stomatal conductance, photosynthetic rate, respiration, and transpiration rate [87,114,115]. Trichomes, which are unicellular outgrowths on the leaf, play a defensive role in plants under stress conditions [116,117]. Metal ions’ active transport regulates the number and distribution of trichomes, and an increased trichome number has been noticed in the plants exposed to toxic Cr(VI) levels [22].

Exposure to high Cr-concentration causes mitochondrial damages, such as outer membrane rupture, swelling, deformed or altered internal cristae, dense electron accumulated materials, and spherical morphology [23,118,119]. It has been also reported that mitochondria were underdeveloped in the Brassica napus seedlings that were exposed to 400 μM Cr as compared seedlings exposed to control conditions [41,120]. The ultrastructural investigations also revealed that Cr(VI) stress alters plastid structure, more specifically, chloroplast, with a spherical and contracted morphology [120,121,122,123]. The irregular shape and size of the chloroplast with contained large plastoglobuli and starch grains were reported in Spirodela poyrhiza seedlings that were exposed to high Cr(VI)-level [23]. Cell membrane injury, disruption of cytoplasm, and vacuole upon Cr exposure are frequently reported [23,120,121].

Table 5. Chromium-induced ultra-structure variation in numerous plant species. Epi-C-wax (epicuticular wax), TRICH (trichome), CW (cell wall), MITO (mitochondria), CM (cell membrane), THY (thylakoid), THY-O (thylakoid orientation), PG (plastoglobuli), SG (starch grains), GB (Golgi bodies), ER (endoplasmic reticulum), CHLP (chloroplast), I-cristae (interior- Cristae), T-nuclei (tubular nuclei), T-stroma (translucent stroma), ML (middle lamella), NM (nuclear membrane), and PT (Plant tissue used).

Plant Species	Common Names	PT	Effect	Cr-Type	References
Glycine max	Soybean	L	Loss of Epi-C-wax increased TRICH-number	Cr(VI)/(III)	[22]
Brassica napus	Oilseed rape	L & R	Alteration in CW, MITO, CM, THY, PG, SG, GB, ER, Irregular nucleus, THY disappeared, Increased SG number/size.	Cr(VI)	[41,42,120,134]
Triticum aestivum Hordeum vulgare	Wheat & Barley	L	Damaged CHLP, THY; Increased PG, Swollen MITO; altered I-cristae	Cr(VI)	[59]
Nicotiana tabacum	Tobacco	L & R	CW/CM not distinguishable, Disarranged CHLP structure, Undeveloped nucleus, damaged NM, Swelled/distorted THY, Damaged CHLP, MITO, Altered THY-O, Increased PG, Large SG	Cr(VI)	[122,123]
Oryza sativa	Rice	L	Swollen CHLP, grana/stroma/lamellae, Reduced grana/CHLP, Increased SG, Matrix zone expanded.	Cr(VI)	[35,135]
Arabidopsis thaliana	Arabidopsis	R	T-nuclei, GB disintegrated, spherical MITO, plastids; T-stroma; damaged MIOT, plastids; increased SG, amorphous material deposition in CW, ML, vacuoles, collapsed vacuoles, cytoplasm contained opaque lipid,	Cr(VI)	[23,47,136]
Eichhornia crassipes	Water Hyacinth	L	Damaged THY, MITO, CHLP (structure/distribution), grana	Cr(VI)	[137]
Salvinia minima	Floating Fern	L	Damaged CHLP, grana, THY, increased number/size of SG; large PG	Cr(VI)	[96]
Taraxacum officinale	Dandelion	C	Altered MITO with no/reduced I-cristae	Cr(VI)	[138]
Hordeum vulgare	Barley	L & R	Swollen CHLP, increased PG, Disintegrated/disappeared THY, MITO, Increased SG size/number, Increased vacuolar size, Cr-presence in CW, Vacuoles, Nucleus disruption/disappearance	Cr(VI)	[139]
Solanum lycopersicum	Tomatoes	P	Abnormal shaped reduced grana/CHLP; altered THY, MITO; reduced cristae numbers	Cr(III)	[140]
Potamogeton crispus	Curled Pondweed	L	Swollen CHLP, CHLP- envelop breakage, decreasing cristae, MITO vacuolization	Cr(VI)	[141]

summarizes the ultrastructural changes reported in the different plant species exposed to Cr-stress.

6.2. Electron-Dense Material Deposition in the Subcellular Compartments

Plants restrict the accumulation of heavy metals in the less sensitive organelles to avoid damage to the more sensitive organelles at the cellular level [16,142,143]. The precipitation of electron-dense granules in subcellular compartments, especially in the cell wall, is the first line cellular defense mechanism, against toxic heavy metals [23,144,145]. The electron-dense deposition in the interspace between the cell wall and cell membrane, vacuoles, plastids, between the cisternae of endoplasmic reticulum, and cytoplasm in the seedlings of Arabidopsis that were exposed to Cr(VI) have been previously reported [23,136]. The deposition of electron-dense material in the pectic middle lamella instead of cellulosic/hemicellulosic components of Arabidopsis root tip cells has been also reported [23].

There is a prominent difference in the degree of Cr(VI)-induced damages among the different cellular compartments of plants [23,47,136]. The cellular compartments, such as mitochondria, plastids, Golgi bodies, and vacuoles, were severe; cytoplasm, cell membrane, endoplasmic reticulum (ER) were mild; cell wall and nuclei were moderately damaged in the seedlings of Arabidopsis that were exposed to high Cr(VI) levels [23], as shown in Table 5.

7. Chromium-Mediated Changes in Photosynthesis and Photosynthetic Apparatus

Various heavy metals that influence plant biochemical, physiological, and metabolic processes affect photosynthesis and photosynthetic apparatus, leading to reduced plant growth and yield [3,8,19,23]. The effect of Cr on the photosynthesis and photosynthetic apparatus has been greatly studied in different plant species, and it mainly influences the enzymatic activities, electron transport chain, CO₂ fixation, photosynthetic phosphorylation, and structure of plastids [35,65,146,147]. In various plant species Cr-reduced chlorophyll contents, carotenoids, and photosynthetic activities have been greatly investigated, as summarized in

Table 6. Chromium-induced alteration in photosynthesis and photosynthetic apparatus in various plant species. Chl a (Chlorophyll a), Chl b (Chlorophyll b), Chl t (total chlorophyll), Chl f (chlorophyll fluorescence), Trmmol (transpiration rate), Cond (stomatal conductance), photo (photosynthetic rate), PSII (photosystem II), Ci (intercellular CO₂), Φ_PSII (effective quantum of yield of photosystem II), qP (photochemical quenching), NPQ (non-photochemical quenching), P_N (net CO₂ assimilation rate), ETR (electron transportation rate), pigment (photosynthetic pigments).

Plant Species	Common Name	Alteration in Photosynthetic Parameters	Cr(VI)	References
Arabidopsis thaliana & Brassica juncea	Arabidopsis & Indian Mustard	Reduced chl a, b, and t Reduced chl a, Reduced Chl t, Carotenoids, and net photo, b, and t, Gas exchange	400 µM 100–300 µM & 100 mg/Kg	[8] & [58,88,89,157]
Helianthus annuus	Sunflower	Reduced chl a, b, t, gas exchange, and carotenoid levels	Tannery effluent & 20 mg/kg	[26,158]
Citrus reticulate	Kinnow Mandarin	Decreased chl t, photosynthetic activity, Trmmol, Cond, and water use efficiency	0.75 mM	[87]
Cyperus alternifolius & Coix lacryma-jobi	Umbrella Palm & Adlay Millet	Inhibition in photosynthetic capacities	40 mg/L	[159]
Solanum melongena	Eggplant	Reduced pigments, photo, photochemistry of PSII	25 μM	[64]
Oryza sativa	Rice	Reduced Chl a, b, and carotenoids, Reduced Fv/Fm	80–200 µM	[34,35]
Zea mays	Maize	Reduced carotenoids, chl a, b, and t, Photo, Trmmol, Ci, Water use efficiency and intrinsic, Alteration in Fv/Fm, Fv/F0, Fm/F0, and qP	Tannery effluent & 150–250 μM	[29,147,160]
Amaranthus viridis & Amaranthus cruentus	Green & Blood Amaranth	Inhibition photochemistry of PSII	50 μM	[38]
Nicotiana tabacum	Tobacco	Reduced Chl a, b, carotenoids, photo, gas exchange, Fv/Fm fluorescence	50 μM	[122]
Sesbania grandiflora	Hummingbird Tree	Reduced Chl t	1.92 mM/Kg	[161]
Lactuca sativa	Lettuce	Decreased levels Chl a, ΦPSII, qp, NPQ, PN and RuBisCO activity	200 mg/L	[162]
Triticum aestivum	Wheat	Decline active reaction centers of PSII, ETR, and PSII heterogeneity	300 μM	[163]
Humulus scandens	Asian Hop	Decreased chl f parameters, chl t, and PSII reaction	300 mg/kg Cr(III)	[164]
Cucumis sativus	Cucumber	Decline in Fm, Fv, Fv/Fm, Fm/F0, and Fv/F0	200 µM	[40]
Lemna minor	Duckweed	Decreased in Fv/Fm, chl b	6 mg/L	[165]
Pisum sativum	Pea	Decreased pigments and Fv/Fm, Fv/F0 and qP, and NPQ increased	100 μM	[44]
Raphanus sativus, Solanum lycopersicum & Spinacia oleracea	Radish, Tomato & Spinach	Reduced photosynthetic activity and Chl t	100 mg/kg	[166]
Brassica napus	Rapeseed	Reduced chl t, and carotenoid	500 μM	[76]
Solanum lycopersicum & Solanum melongena	Tomato & Eggplant	Reduced pigments	7.5 ppm	[155]

. The structural changes in the chloroplast could be one of the factors that are involved in the defective photosynthesis [2]. Chromium-induced chloroplast ultrastructural changes mediate the suppression of photosynthesis in various plant species, as summarized in Table 5. Chromium-reduced alterations in the volume and auto-fluorescence of chloroplast [127], altered thylakoid arrangement, chloroplast membrane distortion, and negatively affected light/dark reactions have also been reported [2,22,96,148]. Electron transport chain inhibition might be due to the Cr-induced redox changes in the Fe and Cu carriers or binding of Cr to cytochrome groups to inhibit its oxidative activity [149,150,151].

Furthermore, high Cr-level mediates ROS accumulation, which is an alternative sink for the electron, being involved in the suppression of photosynthesis [8,127,152]. Heavy metals-induced ROS modulated alteration in the photosynthesis and photosynthetic machinery is intensely studied [60,76]. Destabilization and degradation of antenna complex proteins, Mg⁺ substitutions with H⁺ ion, and thylakoid membrane damage are the main steps in ROS assisted leaf pigment-protein structure and function retardation [2,153]. The Cr(VI)-induced degradation of a chlorophyll biosynthesis key enzyme delta-aminolaevulinic acid dehydratase, and its competing capability with Mg and Fe translocation to leaves are involved in the decreased photosynthetic pigments and photosynthesis [81,154]. High Cr-level in the soil greatly influences macro/micronutrient uptake. As Cr has no specific uptake channels, it is competing with essential elements for the uptake channels [155,156].

8. Strategies to Overcome Cr-Uptake and Phytotoxicity

Chromium (III) has an essential role in the human metabolic process [102]. However, none of the Cr species have been reported to be essential in plants, thus there is no specialized mechanism for Cr-uptake in plants [23]. In plants, Cr-uptake, which depends on the Cr-type and plant species, is carried out through essential nutrients uptake channels [167]. Plants uptake Cr(III) by passive mechanism, while the uptake of Cr(VI), which has a structural resemblance with sulfate and phosphate, takes palace by the active mechanism through sulfate and phosphate channels [2,28,167]. The restriction of Cr(VI)-uptake and no change in Cr(III)-uptake by the treatment of exogenous metabolic inhibitors confirmed the active and passive uptake mechanisms of these Cr species, respectively [2,89]. The molecular mechanism for Cr uptake and translocation is elusive and further studies are required.

Heavy metal ATPase (HMA), cation diffusion facilitator (CDF), superfamily of ATP binding cassette (ABC), natural resistance-associated macrophage protein (NRAMP), and ZRT IRT-like proteins (ZIP) are some of the gene families that are involved in the transportation of metals and heavy metals in plants [18]. Further investigations regarding the possible role of these gene families in Cr-uptake and translocation will increase our understanding of the Cr-transportation mechanism in plants. Some of the studies reported that Cr is sharing the iron, sulfate, and phosphate transport pathways in plants [55]. Thus, plants that are exposed to a toxic level of Cr-concentrations are also experiencing starvation of essential elements [168,169]. As Cr-competes with some essential metals for the uptake, these essential elements enriched environment can reduce Cr-uptake, transport, and toxicity in plants.

Iron enriched growth medium significantly reduced Cr(VI)-uptake and translocation in plants [170]. The pretreatment of seeds with salicylic acid, application of auxin and ethylene inhibitors to growth media, treatment of polyamine-brassinosteroid, 24-epibrassinolide, and plant growth-promoting bacteria reduce Cr-uptake, translocation, and toxicity [8,30,42,48,68,123,171]. The natural selection of Cr-tolerant varieties, conventional breeding, and targeted genes mutation can be used for the control of Cr-phytotoxicity and damage to yield of economically important crops.

9. Conclusions

Plants exposed to toxic Cr-level mediate high ROS accumulation by either oxidation and interconversion of one Cr form to other or by the inhibition enzymatic antioxidant system. Cr-induced ROS mediates DNA damage and genotoxicity, cytotoxicity, ultrastructural damages, and alteration in photosynthesis and photosynthetic apparatus. These alterations include necrosis, programmed cell death, cell cycle arrest, and suppression of cell division that ultimately reduce plant growth, development, and yield, as shown in Figure 1.