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? The discovery of a class of plants that concentrate exceptionally high amounts of normally toxic heavy metals in leaves has attracted considerable interest, and challenged biologists to find reasons for this unusual behavior by providing answers to the question: why do some plants do it? In other words: what functions does hyperaccumulation perform in these plants and what are the benefits and the adaptive values of metal hyperaccumulation? A variety of hypotheses have been proposed to explain the role of high elemental concentra- tions in leaves, namely: metal tolerance/disposal, drought resistance, interference with neighboring plants, and defense against natural enemies. According to the tolerance/disposal hypothesis, the pe- culiar hyperaccumulation pattern would allow plants to take heavy metals away from the roots by sequestering them in tolerant leaf tissues. This eliminates them from the plant body by shedding the high-metal aerial organ. Another postulated explanation is that large amounts of heavy metals might increase plant drought resistance, with a water-conserving role in the cell walls or acting as osmo- lytes inside the cells. These hypotheses, however, are hardly supported by experimental evidence. The interference hypothesis, also termed ―elemental allelopathy‖, suggests, instead, that perennial hyperaccumulator plants may interfere with neighbouring plants through enrichment of metal in the surface soil under their canopies. This gives rise to a high-metal leaf litter that prevents the estab- lishment of less metal tolerant species. The hypothesis of elemental allelopathy does not have satis- factory experimental verification yet. The hypothesis which has attracted most attention suggests that the high heavy metal con- centrations in aerial tissues may function as a self-defense strategy evolved in hyperaccumulator plants against some natural enemies, such as herbivores and pathogens. This ―elemental defense‖ hypothesis has been widely tested, gaining much supporting evidence, although some tests have led to contradictory responses. However, since the metal treatment will strongly affect the plants‘ metabolome, it might be that herbivores do not directly perceive metals in their food, but rather metal-induced metabolites. Chemical defense of plants from enemy attack can also involve a variety of organic (secondary metabolite) compounds. However, elemental defense offers some advantages over organic defense: the toxic elements are not synthesized by the plant but taken up from the soil thus making the elemental defense less metabolically expensive than the organic one; the inorganic elements cannot be biochemically degraded by most herbivores, so that this counter defense mecha- nism of the enemies is prevented. Joint defensive effects may actually exist between elemental and organic plant compounds, which may act in concert with each other and enhance plant defense overall. This new joint effects hypothesis may justify the simultaneous presence of elemental and organic defenses as well as the hyperaccumulation of more than one heavy metal in the same plant. This new interesting idea needs to be further supported by more research. жүктеу/скачать 6.49 Mb. Достарыңызбен бөлісу:
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