Se(IV) phytotoxicity for monocotyledonae cereals (Hordeum vulgare L., Triticum aestivum L.) and dicotyledonae crops (Sinapis alba L., Brassica napus L.)
Introduction
The importance of selenium (Se) as a naturally occurring potentially toxic trace element in various natural and human-affected environments has received considerable publicity and scientific attention during the past century [1], [2], [3]. Although naturally occurring, Se accounts for a large proportion of contamination problems (existing and potential). A number of anthropogenic activities also generate Se-laden wastes, including petroleum refining, mining, fossil fuel combustion, and a wide variety of industrial processes. Consequently, one primary focus of researchers is the consideration of different methods for Se removal, immobilization in soil/water systems or accumulation in biota. Understanding of the possible oxidation and coordination states under various conditions is essential since these factors control Se mobility, bioavailability, and toxicity [4]. Two selenium inorganic forms naturally occur most often: Se(IV) and Se(VI). While algae prefer selenium in the form of selenites, terrestrial plants favor selenates. Se(IV) can be harmful to plants even if the concentration is quite low [5]. Soluble, toxic oxyanionic forms, including selenate (SeO42−, Se6+) and selenite (SeO32−, Se4+), comprise most of the Se found in agricultural drainage waters, as well as in industrial water streams [6]. Many non-biological techniques and biological treatment options have been described, e.g., Se anion exchange, sorption, immobilization, and accumulation [7], [8], [9], [10].
The essentiality of selenium for animals and bacteria is frequently discussed; however, its physiological role in plants still remains controversial [11]. Plants differ in their ability to accumulate Se in their tissues [12]. Although trace amounts of selenium are tolerable, Se is more toxic at higher concentrations than arsenic or mercury [13]. Low Se concentrations inhibit lipid peroxidation in Lolium perenne, and this decrease coincides with growth enhancement. At high concentrations, Se acts as a prooxidant and leads to drastic reductions in yield [14]. In non-tolerant plant species, Se compounds may impair germination and growth and lead to chlorosis [13]. Kabata-Pendias and Pendias [11] found that increasing concentrations of selenium reduce the absorption of heavy metals (mainly Mn, Zn, Cu, Fe and Cd). The reduction of heavy metal absorption depends on the ratios of Se and individual element. Most of the toxic effects of Se are related to its chemical similarities to sulphur. Most enzymes involved in sulphur metabolism also catalyze analogous reactions with the corresponding Se substrates [15]. Although previously cited literature indicates a long-standing appreciation of the need for control of Se valence and coordination in the environment, relatively little direct information regarding Se phytotoxic effects is available. In view of the aforementioned considerations, a study of Se(IV) phytotoxicity to geographically widely raised agricultural plants, represented by monocotyledonae cereals like Hordeum vulgare L. and Triticum aestivum L. and the dicotyledonae crops S. alba L. and Brassica napus L., was initiated. Adverse effects were found, such as root and shoot growth inhibition, relationships between fresh mass (FM) and dry mass (DM) production and changes in water content (WC) and photosynthetic pigment levels (chlorophyll a, chlorophyll b and total carotenoids). Observations were completed by Se quantification in the roots and shoots of studied plants.
Section snippets
Plant materials and chemicals
Seeds of white mustard (S. alba L.), oilseed rape (B. napus L.), common wheat (T. aestivum L.) and common barley (H. vulgare L.) used in the tests were obtained from Chepo, s.r.o. (Unhošť–Fialka, Czech Republic). Selenium(IV), SeO2 of analytical grade p.a., was obtained from Lachema, Brno, Czech Republic.
Growth inhibition tests
For seed cultivation, 21 cm × 15.5 cm vertical cultivation containers (Phytotoxkit, MicroBioTests Inc., Nazareth, Belgium) with cellulose and filter paper soaked with 24 ml of freshly prepared
Plant growth inhibition
The deleterious effects of Se(IV) were expressed as root and shoot growth inhibition in terms of regression analysis-calculated IC50 values and their 95% confidence intervals (CI) (Table 1). On the basis of these values, dicotyledonae plants were revealed to be more sensitive to Se(IV) than monocotyledonae plants. The roots and shoots of young S. alba seedlings were most sensitive to selenium, as their IC50 values reached only 13.7 and 25.8 mg Se(IV) l−1, respectively. T. aestivum seedlings were
Discussion
Selenium fertilization of vegetable crops has been used to increase dietary selenium levels in humans and other animals [19]. Growing plants enriched with selenium could be an effective way to reduce dietary deficiencies and increase health benefits [20], [21]. However, higher selenium concentrations in selenium-enriched media can influence the germination and growth of plants. Peng et al. [22] found that soils with more than 16.0 mg Se kg−1 reduced the germination of wheat (T. aestivum L.) seeds,
Conclusion
Se(IV) was most toxic to S. alba, where it inhibited root and shoot growth still at concentrations 14 and 26 mg Se(IV) l−1, respectively. Furthermore, in S. alba it also inhibited chlorophyll a and b production and increased total carotenoids level relative to the chlorophylls. Increased carotenoids level could be explained as a response to Se-mediated oxidation stress in seedlings. Addition of Se reduced FM production and decreased water content as in the roots as in the shoots of this plant.
Acknowledgement
This study was supported by the Grant VEGA 1/4361/07 and Grant PriF UK 9/2008 from the Faculty of Natural Sciences, Comenius University in Bratislava.
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