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Chromium Morpho-Phytotoxicity
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This entry is adapted from the peer-reviewed paper 10.3390/plants9050564

Chromium (Cr) is considered as one of the chronic pollutants that cause damage to all living forms, including plants. Various industries release an excessive amount of Cr into the environment. The increasing accumulation of Cr in agricultural land causes a significant decrease in the yield and quality of economically important crops. The Cr-induced biochemical, molecule, cytotoxic, genotoxic, and hormonal impairments cause the inhibition of plant growth and development.

Chromium Dichromate Arabidopsis Rice Abiotic stress Plant Plant Physiology Morph toxicity

1. Introduction

Chromium (Cr) is considered one of the major carcinogens, and is categorized 7th among the top 20 hazardous pollutants by the Environmental Protection Agency, United States of America (EPA, US) [1][2][3]. Cr(VI) and Cr(III) are the most stable form of Cr in the environment. On the bases of bioavailability in soil and translocation to different plant parts, Cr(VI) is reported to be more toxic than Cr(III) [3][4][5]. The industrial process coupled with anthropogenic and natural processes have resulted in increased accumulation of Cr in both terrestrial and aquatic ecosystems [3][4][6]. Chromium in soil and water directly affects human, animal, and plant physiology, and may accumulate within food chains, which can be a serious health threat to the secondary (herbivores) and tertiary (carnivores and omnivores) consumers [3][7][8].

2. Chromium-Mediated Control of Seed Germination

The first phenotypic and physiological change mediated by Cr in plants is seed germination, which is very important for the continuity of the plant life cycle [9]. Endogenous and exogenous stimuli mediated genetic and epigenetic changes were reported to be involved in the regulation of seed germination, and plant biochemical, molecular and ultrastructural changes [9][10][11]. Chromium-induced inhibition of seed germination in various plant species have been reported, and the germination rate depends on Cr(VI) concentration and type of plant species as shown in (Table 1). Chromium stress affects the activities of both alpha and beta-amylase, which are the sources of energy provided to the emerging embryos. In summary, Cr reduces the activity of amylase, leading to the reduced sugar availability for energy production, and inhibits the rate of plant seed germination [12].
Table 1. Chromium-induced seed germination inhibition in various plant species.
Table
Plant Species Common Name Chromium Concentration Medium Time of Exposure (Days) Seed Germination (%) References
Avena sativa Oat 500 mg/kg Cr(VI)
2000 mg/kg Cr(III)		 Soil 7 ≈82
≈95		 [13]
Beta vulgaris Swiss chard 50 µM Cr(III) Distilled water 12 71 [14]
Brassica juncea Mustard 300 µM Cr(VI) ½-strength Hoagland 3 80.8 [15]
Brassica oleracea Cabbage 300 mg/kg Cr(VI) Distilled water 3 ≈65 [16]
Cajanus cajan Pigeon Pea 100 ppm Distilled water 3 93 [17]
Cucumis sativus Cucumber 300 mg/kg Cr(VI) Distilled water 3 ≈96 [16]
Glycine max Soybean 200 mg/L Cr(VI) Hydroponic - 72.6 [18]
Lactuca sativa Lettuce 300 mg/kg Cr(VI) Distilled water 3 ≈50 [16]
Lactuca sativa Lettuce 50 µM Cr(III) Distilled water 12 94 [14]
Oryza sativa Rice 100 µM Cr(VI) Distilled water 4 ≈50 [19]
Sorghum bicolor Sorghum 500 mg/kg Cr(VI)
2000 mg/kg Cr(III)		 Soil 7 ≈60
≈10		 [13]
Spinacia oleracea Spinach 50 µM Cr(III) Distilled water 15 64 [14]
Triticum aestivum Wheat 100 ppm
300 mg/kg Cr(VI)
500 mg/kg Cr(VI)
2000 mg/kg Cr(III)		 Distilled water
Distilled water
Soil		 0.17
3
7		 63
≈90
≈70
≈25		 [20]
[16]
[13]
Zea mays Corn 300 mg/kg Cr(VI) Distilled water 3 ≈99 [16]

3. Chromium-Induced Modulation of the Root Growth and Development

The plant root is the first organ that encounters soil pollutants, Cr is one of the most important soil pollutants, which affects root growth and development [11][21]. Chromium-induced reduction in the root growth mainly depends on the plant species, Cr-type and its concentration as shown in the (Table 2) Chromium is also involved in the regulations of secondary root growth and number, lateral root development, root hair, and formation of adventitious roots [12][22][23]. The reduced root length with a brownish appearance and reduced root hair number have been observed in Zea mays, exposed to high Cr(VI) levels [23]. The root growth inhibition mediated by Cr(VI), maybe due to the inhibition of cell division and reduction in the cell size of the elongation zone [21]. The reductions of mitotic cell division in Amaranthus viridis and Arabidopsis thaliana, have been reported, which is associated with the reduction in cell cycle-related genes and alterations in the cellular ultrastructure [3][21].
Table 2. Chromium-induced reduction in root growth as compared to control of various plant species.
Table
Plant Species Common Name Chromium Concentration Medium Time of Exposure (Days) Root Growth (%) References
Arabidopsis thaliana Arabidopsis 200 µM Cr(VI) ½ MS 1 92.8 [21]
Avena sativa Oat 500 mg/kg Cr(VI)
2000 mg/kg Cr(III)		 Soil 7 ≈40
≈55		 [13]
Brassica campestris Cabbage 1 mg/L Cr(VI) ½-strength Hoagland 21 ≈35 FW [24]
Brassica juncea Mustard 300 µM Cr(VI) ½-strength Hoagland 15 43.7 [15]
Brassica napus Oilseed Rape 400 μM Cr(VI) Hoagland’s 6 ≈50 [25]
Brassica oleracea Cabbage 300 mg/kg Cr(VI) Distilled water 3 ≈25 [16]
Cajanus cajan Pigeon Pea 100 ppm Distilled water 10 32 [17]
Cucumis sativus Cucumber 300 mg/kg Cr(VI) Distilled water 3 ≈15 [16]
Lactuca sativa Lettuce 300 mg/kg Cr(VI) Distilled water 3 <10 [16]
Oryza sativa Rice 80 µM Cr(VI) ¼ -strength Kimura B 7 78 [26]
Sorghum bicolor Sorghum 500 mg/kg Cr(VI)
2000 mg/kg Cr(III)		 Soil 7 ≈10
≈30		 [13]
Triticum aestivum Wheat 500 µM Cr(VI)
10 mg/kg Cr(VI)
300 mg/kg Cr(VI)
500 mg/kg Cr(VI)
2000 mg/kg Cr(III)		 Sand
Quartz sand
Distilled water
Soil		 -
7
3
7		 ≈57
≈20
< 10
≈10
≈45		 [27]
[28]
[16]
[13]
Zea mays Corn 300 mg/kg Cr(VI)
173 µM Cr(VI)		 Distilled water
Hydroponic		 3
7		 ≈43%
≈70%		 [16]
[23]

4. Chromium-Mediated Changes in Total Biomass Production in Plants

The biomass production is considered proportional to yield, which is greatly compromised in the plants exposed to Cr, indicating that Cr is reducing plant biomass as well as the yield of the important crops worldwide [29][30][31][32]. Numerous, species were investigated and reported to exhibit reduced biomass production under high Cr(VI) levels, and the toxicity varies based on the different plant species, and concentration and type of Cr(VI) used as shown in (Table 3). Several factors such as reduction/imbalance in the uptake/translocation of water and nutrients, cell division and division rate inhibition, selective inorganic nutrient uptake inefficiency, increased ROS accumulation, essential nutrient substitution from ligand and plant key molecules, and Cr-induced ROS mediated alteration and damages to plastids, pigment contents, mitochondria, lipids, RNA, and DNA are involved in the Cr-decreased growth, development, and yield in plants at molecular, cellular, tissue, and organ levels are involved in the alteration in the plant biomass production [3][29][30][31][33][34][35][36]. The degree of severity of these factors depends on the type of Cr and plant species [3]. The hyper heavy metal accumulator plants such as Brassica juncea and Alyssum maritime are were reported to be potentially more tolerant and can survive a range of high Cr concentrations [4][37].
Table 3. Chromium-meditated reduction in the total plant biomass as compared to control in various plant species.
Table
Plant Species Common Name Chromium Concentration Medium Time of Exposure (Days) Total Biomass Production (%) References
Amaranthus viridis and Amaranthus cruentus Green and Blood Amaranth 50 μM ½-strength Hoagland 7 > 50 FW
≈80 FW		 [38]
Arabidopsis thaliana Arabidopsis 800 μM Cr(VI) ½-strength MS 2 50 FW
75 DW		 [29]
Brassica juncea Mustard 300 μM Cr(VI) Semi-hydroponic medium 5 80–89 growth [39]
Brassica juncea Mustard 100 µM Cr(VI) Soil 20 > 50 FW and DW [40]
Brassica napus Oilseed Rape 400 μM Cr(VI) Hoagland 6 67 DW [25]
Brassica napus Rapeseed 500 μM Cr Soil 56 30.6 FW
28 DW		 [41]
Citrus reticulata Kinnow Mandarin 750 μM Cr(VI) Soil 120 63 DW [42]
Cyperus alternifolius and Coix lacryma-jobi Umbrella Palm and Adlay Millet 40 mg/L Cr(VI) Soil 120 77 DW
44 DW		 [43]
Hordeum vulgare Barley 100 μM Cr(VI) Quartz sand 60 ≈23.7DW [44]
Lemna minor Duckweed 500 μM Cr(VI) SIS growth medium 7 60 [45]
Oryza sativa Rice 80 µM Cr(VI) Hydroponic 7 58 [26]
Parthenium hysterophorus Solanum nigrum Santa Maria
Black Nightshade		 500 µM Cr(VI) Soil 21 65.5 FW
64.DW
110 FW
106 DW		 [46]
Solanum melongena Eggplant 25 µM Cr(VI) ½-strength Hoagland 7 87 FW
83 DW		 [32]
Triticum aestivum Wheat 500 µM Cr(VI) Sand
Quartz sand		 7 ≈65% [27]
Zea mays Corn 173 µM Cr(VI) Hydroponic 7 ≈85 FW [23]

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