Chromium

In the past decades the increased use of chromium (Cr) in several anthropogenic activities and consequent contamination of soil and water have become an increasing concern. Cr exists in several oxidation states but the most stable and common forms are Cr(0), Cr(III) and Cr(VI) species. Cr toxicity in plants depends on its valence state. Cr(VI) as being highly mobile is toxic, while Cr(III) as less mobile is less toxic. Cr is taken up by plants through carriers of essential ions such as sulphate. Cr uptake, translocation, and accumulation depend on its speciation, which also conditions its toxicity to plants. Symptoms of Cr toxicity in plants are diverse and include decrease of seed germination, reduction of growth, decrease of yield, inhibition of enzymatic activities, impairment of photosynthesis, nutrient and oxidative imbalances, and mutagenesis.

Physiological responses of plants to chromium

Although stimulating effects to the additions of small amounts of Cr on plant growth and yield have been observed by several researchers, there is no conclusive evidence yet of an essential role of Cr in plant metabolism. Using highly purified cultures and sensitive analytical techniques, Huffman and Allaway clearly and conclusively demonstrated the nonessentiality of Cr to plants. if Cr is required for normal plant growth, the required concentrations in plant tissues ought to be much lower than the levels of any known essential nutrient. This is because they could not reduce the Cr concentration in their nutrient solution below 0.38 × 10-6 mM as well as the entry of trace amounts of atmospheric Cr to the experiment. The stimulation of plant growth in response to Cr addition may simply be due to a limited substitution of Cr(VI) for molybdate. The interaction between Cr (III) and Fe, P and Mn was extensively studied in Fe-sufficient and Fe-deficient bean plants. The fact that low levels of Cr(III) significantly enhanced the growth of both Fe-control and Fe-deficient plants and reduced chlorosis in the latter, this was neither correlated to changes of Mn, P, or Fe tissue concentrations nor to Cr-induced alterations of the Fe/Mn and P/Fe ratios. Ultrastructural studies did not show any consistent correlations between changes in chloroplast structure and growth stimulation caused by Cr. The positive effect of Cr(III) on plant growth was suggested to be related to an influence on the levels of cytokinin or other plant growth hormones, interaction with nucleic acids, competitive inhibition by Cr on the binding of spermine to DNA, or Cr-induced increase in the levels of the polyamine spermine.

Source: Chromium as an Environmental Pollutant: Insights on Induced Plant Toxicity, Helena Oliveira, Journal of Botany, 2012

Effect of Chromium on Plant Physiology

Process	Crop / Plant	Effects	References
Photosynthesis	Wheat, Peas, Rice, Maize, Beans, Sunflower	Eletron transport inhibition, Calvin cycle enzyme inactivation, reduced CO2 fixation, Chloroplast disorganisation	Davis et al. (2002), Zaid (2001), Shankar (2003)
Water Relation	Bush beans, Sunflower, Mung bean	Decreased water potential, increased transpiration rate, reduced diffusive resistance, wilting, Reduction in tracheary vessel diameter)	Davis et al. (2003)
Enzymes and compounds	Nymphaea and various cereals and legumes	Uptake of N,P, K,Fe,Mg,Mn,Mo,Zn,Cu,Ca,B affected inhibition of assimilatory enzymes, increase activity of ROS scavenging enzymes, Changes in glutathione pool, No. of production of phytochelatin	Khan et al. (2000), Vajpayee et al. (2000), Panda and Patra (2000), Barton et al. (2000), Samantaray, 2002, Shankar (2003), Jain et al. (2000), Toppi et al. (2000)

Source: Chromium toxicity in plants, Arun K.Shankar et al. 2005, Journal of Environmental International

Nutritional and toxic effects of chromium

Toxicity of chromium to plant

Visual symptoms of Cr toxicity in plants are stunted growth, a poorly developed root system, curled and discolored leaves leaf chlorosis, narrow leaves,chlorotic bands on cereals,yield reduction, and some plants may exhibit brownish-red leaves containing small necrotic areas or purpling of basal tissues. Immediate wilting and plant death has also been reported as a result of exposure to very high levels of Cr. There are conflicting views on whether Cr toxicity is first observed on the shoot or root system. Cr concentrations increased in all plant parts compared to control plants, but only in the leaves,was there a corresponding reduction in biomass. The amount translocated to the leaves, however, was very small and most of the Cr remained in the roots. In many observation the following sequence of sensitivity of the symptoms of Cr toxicity: induction of stress compounds (e.g., putrescine, chitinase) > root growth > visible damage symptoms > leaf growth, leaf water content. The visual symptoms of shoot growth inhibition by Cr may actually result from less visible damage to the root system. In Studies clearly demonstrated that despite the fact that shoot percentage dry weight and fresh weight were the most affected by Cr, the major effect of Cr toxicity in sugar beet plants was damage to the fibrous root system. Based on this fact, a mechanism of Cr toxicity in sugar beet plants was proposed as follows: high levels of Cr exert a severe effect on fibrous root growth and function. As a result of root damage, water and nutrient uptake is diminished (this is evidenced by wilting and reduced shoot moisture content and by visual symptoms of mineral deficiencies (e.g., Fe deficiency chlorosis) in leaves. Reduced leaf water content most probably results in reduced leaf expansion. Reduction in leaf expansion may also occur as a result of lower photosynthetic rates due to Fe deficiency.

Source: Chromium in the environment: factors affecting biological remediation, Adel M. Zayed & Norman Terry, Plant and Science Journal , 2003

Chromium in the Environment

As chromium compounds were used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at abandoned industrial sites, now needing environmental cleanup and remediation per the treatment of brownfield land. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications. World Health Organization recommended maximum allowable concentration in drinking water for Cr (VI) is 0.05 mg/l.

The development of biotechnology for water purification from toxic hexavalent chromium by duckweed plants (Lemna minor L.)

Duckweed cultivation in the presence of Cr (VI)

Lemna plants growth under Cr (VI) induced stress

Reduction of Cr (VI) to Cr (III) in plant cells and medium

• Detoxicated water may be returned to environment
• Chromium (III) hydroxide Cr(OH)3 is used as a pigment, as a mordant, and as a catalyst for organic reactions.
• Duckweed is acceptable for obtaining biofuels

Source:http://icbge.org.ua/eng/The_development_of_biotechnology_for_water_purification_from_toxic_hexavalent_
chromium_by_duckweed_plants_%28Lemna _minor_L.%29

Enhancing phytoremediation of chromium-stressed soils through plant-growth-promoting bacteria

Chromium, specifically hexavalent chromium is one of the most toxic pollutants that are released into soils by various anthropogenic activities. It has numerous adverse effects not only on plant system but also on beneficial soil microorganisms which are the indicators of soil fertility and health. Recent emergence of phytoremediation as an environmental friendly and economical approach to decontaminate the chromium stressed soils has received wider attention. But major drawback of this process is that it takes long time. Application of multifunctional plant-growth-promoting bacteria (PGPB) exhibiting chromium resistance and reducing traits when used as bioinoculants with phytoremediating plants, has resulted in a better plant growth and chromium remediating efficiency in a short time span. PGPB improve chromium uptake by modifying root architecture, secreting metal sequestering molecules in rhizosphere and alleviating chromium induced phytotoxicity. The purpose of this review is to highlight the plant-beneficial traits of PGPB to accelerate plant-growth and concurrently ameliorate phytoremediation of chromium contaminated soils.

Source:http://www.scoop.it/t/plant-roots-by-christophe-jacquet/p/4038856234/2015/03/10/enhancing-phytoremediation-of-chromium-stressed-soils-through-