Adel Zayed

Chromium in the environment: factors affecting biological remediation

Chromium, in the trivalent form (Cr(III)), is an important component of a balanced human and animal diet and its deficiency causes disturbance to the glucose and lipids metabolism in humans and animals. In contrast, hexavalent Cr (Cr(VI)) is highly toxic carcinogen and may cause death to animals and humans if ingested in large doses. Recently, concern about Cr as an environmental pollutant has been escalating due to its build up to toxic levels in the environment as a result of various industrial and agricultural activities. In this review, we present the state of knowledge about chromium mobility and distribution in the environment and the physiological responses of plants to Cr with the desire to understand how these processes influence our ability to use low cost, environmentally friendly biological remediation technologies to clean up Cr-contaminated soils, sediments, and waters. The use of biological remediation technologies such as bioremediation and phytoremediation for the cleanup of Cr-contaminated areas has received increasing interest from researchers worldwide. Several methods have been suggested and experimentally tested with varying degrees of success.

Accumulation and volatilization of different chemical species of selenium by plants

Abstract. Selenium (Se) removal from polluted waters and soils is especially complicated and highly expensive. Phytoremediation has been suggested as a low-cost, efficient technology for Se removal. Plants remove Se by uptake and accumulation in their tissues, and by volatilization into the atmosphere as a harmless gas. Unraveling the mechanisms of Se uptake and volatilization in plants may lead to ways of increasing the efficiency of the phytoremediation process. The objectives of this study were: (i) to determine the effect of different Se forms in the root substrate on the capacity of some plant species to take up and volatilize Se; (ii) to determine the chemical species of Se in different plant parts after the plants were supplied with various forms of Se; and (iii) to determine the influence of increasing sulfate levels on plant uptake, translocation, and volatilization of different Se species. Plants of broccoli (Brassica oleracea var. botrytis L.), Indian mustard (Brassica juncea L.), sugarbeet (Beta vulgaris L.) and rice (Oryza sativa L.) were grown hydroponically in growth chambers and treated for 1 week with 20u2009μM Se as Na2SeO4, Na2SeO3 or L-selenomethionine (SeMeth) and increasing sulfate levels. The data show that shoots of SeO4-supplied plants accumulated the greatest amount of Se, followed by those supplied with SeMeth then SeO3. In roots, the highest Se concentrations were attained when SeMeth was supplied, followed by SeO3, then SeO4. The rate of Se volatilization by plants followed the same pattern as that of Se accumulation in roots, but the differences were greater. Speciation analysis (X-ray absorption spectroscopy) showed that most of the Se taken up by SeO4-supplied plants remained unchanged, whereas plants supplied with SeO3 or SeMeth contained only SeMeth-like species. Increasing the sulfate level from 0.25u2009mM to 10u2009mM inhibited SeO3 and SeMeth uptake by 33% and 15–25%, respectively, as compared to an inhibition of 90% of SeO4 uptake. Similar results were observed with regard to sulfate effects on volatilization. We conclude that reduction from SeO4 to SeO3 appears to be a rate-limiting step in the production of volatile Se compounds by plants. Inhibitory effects of sulfate on the uptake and volatilization of Se may be reduced substantially if Se is supplied as, or converted to, SeO3 and/or SeMeth rather than SeO4.

Chromium accumulation, translocation and chemical speciation in vegetable crops

Abstract. Trivalent chromium (Cr3+) is essential for animal and human health, whereas hexavalent Cr (CrO42−) is a potent carcinogen and extremely toxic to animals and humans. Thus, the accumulated Cr in food plants may represent potential health hazards to animals and humans if the element is accumulated in the hexavalent form or in high concentrations. This study was conducted to determine the extent to which various vegetable crops absorb and accumulate Cr3+ and CrO42− into roots and shoots and to ascertain the different chemical forms of Cr in these tissues. Two greenhouse hydroponic experiments were performed using a recirculating-nutrient culture technique that allowed all plants to be equally supplied with Cr at all times. In the first experiment, 1u2009mg L−1 Cr was supplied to 11 vegetable plant species as Cr3+ or CrO42−, and the accumulation of Cr in roots and shoots was compared. The crops tested included cabbage (Brassica oleracea L. var. capitata L.), cauliflower (Brassica oleracea L. var. botrytis L.), celery (Apium graveolens L. var. dulce (Mill.) Pers.), chive (Allium schoenoprasum L.), collard (Brassica oleracea L. var. acephala DC.), garden pea (Pisum sativum L.), kale (Brassica oleracea L. var. acephala DC.), lettuce (Lactuca sativa L.), onion (Allium cepa L.), spinach (Spinacia oleracea L.), and strawberry (Fragariau2009×u2009ananassaDuch.). In the second experiment, X-ray absorption spectroscopy (XAS) analysis on Cr in plant tissues was performed in roots and shoots of various vegetable plants treated with CrO42− at either 2u2009mg Cr L−1 for 7u2009d or 10u2009mg Cr L−1 for 2, 4 or 7u2009d. The crops used in this experiment included beet (Beta vulgaris L. var. crassa (Alef.) J. Helm), broccoli (Brassica oleracea L. var. Italica Plenck), cantaloupe (Cucumis melo L. gp. Cantalupensis), cucumber (Cucumis sativus L.), lettuce, radish (Raphanus sativus L.), spinach, tomato (Lycopersicon lycopersicum (L.) Karsten), and turnip (Brassica rapa L. var. rapifera Bailey). The XAS speciation analysis indicates that CrO42− is converted in the root to Cr3+ by all plants tested. Translocation of both Cr forms from roots to shoots was extremely limited and accumulation of Cr by roots was 100-fold higher than that by shoots, regardless of the Cr species supplied. Highest Cr concentrations were detected in members of the Brassicaceae family such as cauliflower, kale, and cabbage. Based on our observations and previous findings by other researchers, a hypothesis for the differential accumulation and identical translocation patterns of the two Cr ions is proposed.

Trehalose-producing transgenic tobacco plants show improved growth performance under drought stress

Summary Trehalose plays a role in drought stress resistance in a variety of organisms, including the extremely drought-tolerant «resurrection plants». Transgenic tobacco plants that produce trehalose were engineered by introduction of the Escherichia coli ots A and ots B genes, encoding trehalose-6-P synthase and trehalose-6-P phosphatase, respectively. The introduction of these genes had a pronounced effect on plant morphology and growth performance under drought stress. The transgenic Ots plants had larger leaves and altered stem growth. When grown under drought stress imposed by limiting water supply, the two transgenic tobacco lines Ots2 and Ots 5 yielded total dry weights that were 28 % and 39 % higher than those of wild-type tobacco. These increases in dry weight were due mainly to increased leaf production: leaf dry weights were up to 85 % higher for the best trehalose accumulator, Ots 5. No significant differences were observed under well-watered conditions. Chlorophyll fluorescence analysis of drought-stressed plants showed a higher photochemical quenching (qQ) and a higher ratio of variable fluorescence over maximal fluorescence (Fv/Fm), indicating a more efficient photosynthesis. The Ots 5 plants showed more negative leaf osmotic potentials than wild-type plants, particularly under drought stress, as well as higher levels of nonstructural carbohydrates; Ots2 plants showed intermediate values. Detached leaves from young, well-watered Ots plants had a better capacity than wild-type leaves to retain water when air-dried. They had lower osmotic potentials than wild-type leaves, and higher levels of glucose, fructose and sucrose.

Selenium volatilization in roots and shoots: Effects of shoot removal and sulfate level

Summary Broccoli plants were grown hydroponically in growth chambers with 20 μM Se supplied as selenate. The separate contributions of root and shoot to the volatilization of Se by plants supplied with six different levels of sulfate (ranging from 0 to 10 mM) in half-Hoaglands nutrient solution were determined. Most of the Se volatilized by broccoli plants was from the roots which volatilized about 26 times faster than the rate of shoots. The removal of the shoot markedly increased the amount of Se volatilized over the following 72 h, the detopped root attaining rates that were 20 to 30 times the rate of the intact root. Comparable results were also obtained for five additional species; rice, cabbage, cauliflower, chinese mustard, and wild brown mustard (Brassica juncea). Part of the volatilization of Se by plants may involve microbes, i.e., bacteria. This is indicated by the fact that when prokaryotic antibiotics were added to the nutrient solution, the total rate of Se volatilization by root (broccoli) and nutrient solution was significantly decreased, much more than could be accounted for by the loss of microbial volatilization from the nutrient solution alone.

Selenium volatilization in Broccoli as influenced by sulfate supply

Summary The influence of sulfate supply on the rate of selenium volatilization in broccoli ( Brassica oleracea v. botrytis cv. Green Valiant) was investigated. Plants were cultured hydroponically in growth chambers. Sulfate was supplied at five different concentrations: 0.25, 0.5, 1.0, 5.0 and 10.0 mM in half-Hoaglands solution. All treatments received 20 µM Se as Na 2 ZSeO 4 . Measurements were made of the rate of Se volatilization per plant, Se and S concentrations in root, stem, and leaf blades, and of plant dry weight and leaf area. Each increase in sulfate level from 0.25 to 5 mM caused a progressive decrease in the daily rate of Se volatilization which decreased from 96.7 (at 0.25 mM) to 13.8 (at 10 mM) µg Se per square meter of leaf surface. The concentrations of Se in plant tissues (stem, leaf and root) responded differently to increased sulfate level than did Se volatilization rate: tissue Se concentrations did not change with increase in sulfate from 0.25 to 1 mM and decreased only at the higher sulfate levels, 5 and 10 mM. The step-wise decrease in Se volatilization with each increase in sulfate was however, correlated strongly with a step-wise decrease in the ratio of Se: S in plant tissues. These results suggest, that with increasing sulfate supply, sulfate in plant tissues increasingly competed with selenate for the enzymes of the S-assimilation pathway; this internal competition most likely led to a decreased production of selenoamino acids, especially selenomethionine (required for the production of the volatile form of Se, dimethylselenide), thereby reducing Se volatilization rate.

Accumulation of selenium by different plant species grown under increasing sodium and calcium chloride salinity

High levels of naturally occurring selenium (Se) are often found in conjunction with different forms of salinity in central California. Plants considered for use in phytoremediation of high Se levels must therefore be salt tolerant. Selenium accumulation was evaluated for the following species under increasing salt (NaCl and CaCl) conditions:Brassica napus L. (canola),Hibiscus cannibinus L. (kenaf),Festuca arundinacea L. (tall fescue), andLotus tenuis L. (birdsfoot trefoil). The experimental design was a complete randomized block with four salt treatments of <1, 5, 10, and 20 dS m-1, four plant species, three blocks, and six replicates per treatment. Ninety days after growing in the respective salt treated soil with a Se concentration of 2 mg Se kg-1 soil, added as Na2SeO4, all plant species were completely harvested. Among the species tested, shoot and root dry matter yield of kenaf was most significantly (p<0.001) affected by the highest salt treatment and tall fescue and canola were the least affected species. Generally there was a decrease in tissue accumulation of Se with increasing salt levels, except that low levels of salinity stimulated Se accumulation in canola. Canola leaf and root tissue accumulated the highest concentrations of Se (315 and 80 mg Se kg-1 DM) and tall fescue the least (35 and 7 mg Se kg-1 DM). Total soil Se concentrations all harvest were significantly (p<0.05) lower for all species at all salt treatments. Removal of Se from soil was greatest by canola followed by birdsfoot trefoil, kenaf and tall fescue. Among the four species, canola was the best candidate for removing Se under the tested salinity conditions. Kenaf may be effective because of its large biomass production, while tall fescue and birdsfoot trefoil may be effective because they can be repeatedly clipped as perennial crops.