Тяжелые металлы в окружающей среде
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| What are heavy metal hyperaccumulator plants?
The term ―hyperaccumulator‖ was coined for plants that actively take up large amounts of one or more heavy metals from the soil. Moreover, the heavy metals are not retained in the roots but are translocated to the shoot and accumulated in aboveground organs, especially leaves, at concen- trations 100–1000-fold higher than those found in non-hyperaccumulating species. They show no symptoms of phytotoxicity. Although a distinct feature, hyperaccumulation also relies on hyper- tolerance, an essential key property allowing plants to avoid heavy metal poisoning, to which hy- peraccumulator plants are as sensitive as non-hyperaccumulators. About 450 angiosperm species have been identified so far as heavy metal (As, Cd, Co, Cu, Mn, Ni, Pb, Sb, Se, Tl, Zn,) hyperaccumulators, accounting for less than 0.2% of all known species. However, new reports indicate that these numbers are somewhat underestimates, so that it is con- ceivable that many yet unidentified hyperaccumulators may exist in nature. On the other hand, some species classified as hyperaccumulators based on field samples might be deleted from the list if this trait is unconfirmed by experimentation under controlled conditions. For instance, the finding that in a number of cuprophytes the Cu and Co hyperaccumulation by field samples was actually due to leaf surface contamination has led to a critical re-examination of the Cu/Co hyperaccumulators. The hyperaccumulator species are distributed in a wide range of distantly related families, showing that the hyperaccumulation trait has evolved independently more than once under the spur of selective ecological factors. The evolutionary reasons that gave rise to hyperaccumulating plants are unknown and still under debate. Heavy metal hyperacumulators do occur on metal-rich soils in both tropical and temperate zones. They are found in vegetations from regions of South Africa, New Caledonia, Latin America, North America, and Europe. Initially the term hyperaccumulator referred to plants able to accumulate more than 1 mg g −1 Ni (dry weight) in the shoot, an exceptionally high heavy metal concentration considering that in vegetative organs of most plants Ni toxicity starts from 10 to 15 g g −1 . Threshold values were suc- cessively provided to define the hyperaccumulation of each other heavy metal, based on its specific phytotoxicity. According to such a criterion hyperaccumulators are plants that, when growing on native soils, concentrate >10 mg g −1 (1%) Mn or Zn, >1 mg g −1 (0.1%) As, Co, Cr, Cu, Ni, Pb, Sb, Se or Tl, and >0.1 mg g −1 (0.01%) Cd in the aerial organs, without suffering phytotoxic damage. Most hyperaccumulators are endemic to metalliferous soils behaving as ―strict metallo- phytes‖, whereas some ―facultative metallophytes‖ can live also on non-metalliferous ones, alt- hough are more prevalent on metal-enriched habitats. Why did plants evolve hyperaccumulation of heavy metals? 60 61 monosaccharides and dissolved organic carbon in comparison with those of non-mycorrhizal plants. These exudates can chelate Cd 2+ and Pb 2+ and therefore, contribute to detoxification of the heavy metals and improve the performance of the seedlings. If this first line of defense fails, most of the heavy metals that do enter the plant are then kept in root cells, where they are detoxified by complexation with amino acids, organic acids or metal- binding peptides and/or sequestered into vacuoles. This greatly restricts translocation to the above- ground organs thus protecting the leaf tissues, and particularly the metabolically active photosyn- thetic cells from heavy metal damage. A further defense mechanism generally adopted by heavy metal-exposed plants is enhance- ment of cell antioxidant systems, which counteracts oxidative stress. Different pathways could lead to upregulating genes in the antioxidant defense systems, some of which include the utilization of plant hormones. In addition to the five classical plant hormones, i.e., gibberellins (GAs), cytokinins (CKs), auxins, abscisic acid (ABA), and ethylene, jasmonate (JA), brassinosteroids (BR), and sali- cylic acid (SA) are also well known for regulating many physiological processes and heavy metal stress tolerance. Furthermore, it is also expected that some more growth hormones are yet to be dis- covered in future. жүктеу/скачать 6.49 Mb. Достарыңызбен бөлісу:
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