The Interaction Of Chromium With Salinity And Aluminum And Alleviation Of Chromium Toxicity Through Chemical Approaches In Barley

Chromium (Cr) ranks 7th and 21st most abundant element on earth and crustal rock, respectively. Some fertilizers contain a considerable amount of Cr. Moreover, it is used on a large scale in many industries, and its malhandling results in contamination. Chromium contamination in environment has become a major issue in agricultural production. Plants often face multiple stresses, including salinity, heavy metals, drought, and temperature. The effect of these abiotic stresses may be far from additive. But, most researchers have focused on response of plants to single stress. Cr can occur concomitantly in soils with salinity and aluminum. Therefore, it is imperative to study the interactive effects of Cr and other abiotic stresses on plant growth and development.Several strategies have been proposed for the successful management of the Cr-contaminated agricultural soils. One of the major approaches is that the toxic effect of Cr can be alleviated by using proper amendments, such as suitable N form and silicon in Cr contaminated soils. Hydroponics experiments were conducted in green house of Zhejiang University, Hangzhou, China during 2007-2010, to study the interactive of chromium with salinity and aluminum and alleviation of Cr stress by using N fertilizer forms, silicon and hydrogen sulfide in barley. The salient findings are given as follows:(1) The effect of chromium and salinity stresses on the uptake and accumulation of mineral nutrients in two barley genotypes differing in salt tolerance. Salinity stress decreased K, Mg, Ca, Fe, Zn, Mn and Cu concentrations and accumulation in barley plants, and there was less decrease in CM72, a salt-tolerant genotype than in Gairdner, a salt-sensitive genotype. The effect of Cr on mineral concentration and accumulation varied with Cr level. At high Cr level (50μM), concentration and accumulation of all mineral elements were significantly reduced. But at low Cr level (10μM), nutrient concentration and accumulation were increased. Moreover, low Cr level alleviated the inhibiting effect of NaCl stress on nutrient uptake and accumulation. The combined stress of high Cr level and NaCl stress caused further reduction of the mineral concentration and accumulation as compared to two stresses alone. (2) The effect of chromium and salinity on growth and anti-oxidative enzymes in two barley genotypes differing in salt tolerance. Salinity stress decreased plant growth, photosynthetic rate, stomatal conductance, increased superoxide dismutase (SOD) and peroxidase (POD) activities, and MDA content in barley plants, with CM72, a salt-tolerant genotype being less affected than Gairdner, a salt-sensitive genotype. The effect of Cr on plant growth and anti-oxidative enzymes varied with Cr level. Low Cr level (10 uM) alleviated the inhibiting effect of NaCl stress on plant growth and oxidative stress. The combined stresses of high Cr level (50μM) and NaCl stress caused more severe oxidative stress, resulting in further reduction of plant growth parameters, photosynthetic rate and stomatal conductance as compared to either of the stress alone(3) The effect of pH, aluminum (Al) and chromium (Cr) on the uptake of mineral elements, Al and Cr in the two barley genotypes differing in Al tolerance. Al sensitive genotype Shang 70-119 had significantly higher Cr and Al concentrations in plants than an Al-tolerant genotype Gebeina. Roots had much higher Al and Cr concentrations than above-ground plant parts. Cr concentration was much higher in the solution with pH 4.0 than in that with pH 6.5. Al stress reduced P, Ca Mg, S, Cu, Mn, Zn and B concentrations in roots and restrained K and Fe from being translocated into shoots. Cr stress reduced P, K, Mg, S, Fe, Zn and Mn concentrations in roots at 6.5 pH and P, K, Ca, Mg, S, Zn and Mn concentrations in shoot at 4.0 pH. Translocation of all nutrients from roots to shoots was inhibited except Ca in 6.5 pH with Cr addition. Lower concentration of all nutrients was observed at pH 4.0 as compared to pH 6.5. Combined stress of Cr and Al, on the whole, caused further reduction in mineral concentration in all plant parts of the two barley genotypes as compared to Al or Cr stress alone. Moreover, the reduction was more pronounced in Al sensitive genotype Shang 70-119.(4) The effect of toxic metals aluminum (Al) and chromium (Cr) on growth, root morphology, photosynthetic parameters in two barley genotypes differing in Al tolerance. Al stress decreased plant growth, biomass, chlorophyll content and photosynthetic activities, including net photosynthetic rate (Pn), cellular CO2 concentration (Ci), stomatal conductance (Gs) and transpiration rate (Tr) in barley plants, with Gebeina, a Al-tolerant genotype being less affected than Shang 70-119, a Al sensitive genotype. Cr stress also caused a marked reduction in growth and photosynthetic traits in barley plants. Greater reduction was observed at lower pH level (4.0) as compared to higher pH level (6.5). Combined stress of Cr and Al, on the whole caused further reduction in growth and photosynthetic characters of the two barley genotypes as compared to two stresses alone. Significant more reduction was observed in Al sensitive genotype Shang 70-119 than Al-tolerant genotype Gebeina.(5) The effects of single and binary metal stress viz. aluminum and chromium on the root dehydrogenase activity, oxidative stress and antioxidative enzymes in the two barley cultivars differing in Al tolerance. Al or Cr stress decreased plant growth, root dehydrogenase activity and caused oxidative damage as characterized by increased MDA and H2O2 contents. Under Al or Cr stress, the activities of anti-oxidative enzymes, including superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), gluthione reductase (GR) and catalase (CAT) were dramatically increased in plant tissues. Gebeina, an Al-tolerant cultivar had relatively less oxidative damage as compared to Shang 70-119, an Al-sensitive genotype. The extent of oxidative damage induced by Cr varied with pH in culture solution, with lower pH (4.0) being more severe than higher pH (6.5). Binary metal stress of Cr and Al caused further decrease in plant growth, root dehydrogenase activity and increase in MDA and H2O2 contents as well as anti-oxidative enzyme activity. There was also a marked difference between the two barley cultivars in the increased extent of these anti-oxidative enzymes under Cr or Al stress.(6) The effect of nitrogen fertilizer forms on growth, physiological, biochemical changes and Cr and N uptake in barley plants. The treatments consisted of three Cr levels (0,75 and 100μM) and three N forms ((NH4)2SO4, urea and Ca(NO3)2). N forms in the absence of Cr had significant effect on plant growth, biomass, physiological and biochemical changes. Ca(NO3)2 fed plants had higher plant growth and biomass, followed by urea and (NH4)2SO4, respectively. Cr stress caused significant reduction in growth and biomass in all three N forms. Ca(NO3)2 significantly alleviated Cr toxicity as compared to urea and NH4)2SO4. Ca(NO3)2 fed plants had higher plant height, dry biomass, tillers per plant, net photosynthetic rate (Pn), cellular CO2 concentration (Ci), stomatal conductance (Gs) and transpiration rate (Tr), PAR utilization efficiency (PARUE), photosynthetic efficiency (Fv/Fm), chlorophyll a, b, total chlorophyll, carotenoids, root dehydrogenase activity than urea and (NH4)2SO4 fed plants. Cr toxicity caused oxidative stress in all three forms of N. But Ca(NO3)2 fed plants had less oxidative stress as indicated by less electrolyte leakage (EL), lower MDA, H2O2 and superoxide anion (O2) contents than urea and (NH4)2SO4 fed plants. Moreover, Ca(NO3)2 fed plants had higher soluble protein, proline, free amino acid contents, activities of anti-oxidative enzymes, including SOD, POD, APX, CAT and GR and contents of non-enzymatic antioxidants, including GSH and ASC than urea and (NH4)2SO4 fed plants. Ca(NO3)2 fed plants had also higher N concentration and lower Cr concentrations in all three parts of plants than urea and (NH4)2SO4 fed plants. The current results indicated that Ca(NO3)2 is a better fertilizer for the soils contaminated with Cr.(7) The alleviating effects of silicon on growth inhibition, photosynthesis, ultrastructural changes and uptake of Cr in barley plants. The treatments consisted of three silicon (0,1 and 2 mM) and two Cr (0 and 100μM) levels. The results showed that the application of Si at both levels enhanced plant growth and photosynthetic parameters relative to the control. Addition of Si into the medium solution demonstrated the distinct effect of alleviating Cr toxicity, reflected by significant increase in growth parameters and photosynthetic parameters, such as SPAD value, net photosynthetic rate (Pn), cellular CO2 concentration (Ci), stomatal conductance (Gs) and transpiration rate (Tr), and photosynthetic efficiency (Fv/Fm), with 2 mM Si having greater effect than lm M Si. Moreover, Cr application caused leaf ultrastructural disorders, such as uneven swelling of chloroplast, increased amount of Plastoglobuli, disintegrated and disappeared thylakoid membranes, increased size and number of starch granules in leaves. Cr toxicity also caused ultrastructural modification in roots, including increased vacuolar size, presence of Cr metal in cell walls and vacuoles, disruption and disappearance of nucleus. Exogenous Si alleviated these ultrastructural disorders both in roots and leaves. Apparently, Silicon and Cr behaved antagonistically, indicating that Si could be a candidate for Cr detoxification in crops under Cr-contaminated soil.(8) The alleviating effects of hydrogen sulfide (H2S) on growth inhibition, photosynthetic parameters, ultrastructural changes and uptake of Cr in barley plants. The treatments consisted of three H2S donor sodium hydrosulfide (NaHS) (0,100 and 200μM) and two Cr (0 and 100μM) levels. The results showed that H2S application increased plant growth and photosynthesis slightly as compared to control. H2S alleviated the inhibition in plant growth and photosynthesis by Cr stress, with higher H2S level having more beneficial effect. H2S also reduced Cr concentration in all three plant parts, and higher H2S level had greater effect. Exogenous application of NaHS alleviated cell disorders both in root and leaf cells. On the basis of these results it may be concluded that H2S is a promising gas molecule, which can improve plant survival under chromium stress.