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托福阅读例文翻译解析：矿物质和植物

2015-01-15 17:39:34来源：网络

　　While mineral deficiencies can limit the growth of plants, an overabundance of certain mineralscan be toxic and can also limit growth. Saline soils, which have high concentrations of sodiumchloride and other salts, limit plant growth, and research continues to focus on developing salt-tolerant varieties of agricultural crops. Research has focused on the toxic effects of heavymetals such as lead, cadmium, mercury, and aluminum; however, even copper and zinc, whichare essential elements, can become toxic in high concentrations. Although most plants cannotsurvive in these soils, certain plants have the ability to tolerate high levels of these minerals.

　　Scientists have known for some time that certain plants, called hyper accumulators, canconcentrate minerals at levels a hundredfold or greater than normal. A survey of known hyperaccumulators identified that 75 percent of them amassed nickel, cobalt, copper, zinc,manganese, lead, and cadmium are other minerals of choice. Hyper accumulators run the entirerange of the plant world. They may be herbs, shrubs, or trees. Many members of the mustardfamily, spurge family, legume family, and grass family are top hyper accumulators. Many arefound in tropical and subtropical areas of the world, where accumulation of highconcentrations of metals may afford some protection against plant-eating insects and microbialpathogens.

　　Only recently have investigators considered using these plants to clean up soil and waste sitesthat have been contaminated by toxic levels of heavy metals – an environmentally friendlyapproach known as phytoremediation. This scenario begins with the planting of hyperaccumulating species in the target area, such as an abandoned mine or an irrigation pondcontaminated by runoff. Toxic minerals would first be absorbed by roots but later relocatedto the stem and leaves. A harvest of the shoots would remove the toxic compounds off siteto be burned or composted to recover the metal for industrial uses. After several years ofcultivation and harvest, the site would be restored at a cost much lower than the price ofexcavation and reburial, the standard practice for remediation of contaminated soils. Forexamples, in field trials, the plant alpine pennycress removed zinc and cadmium from soils neara zinc smelter, and Indian mustard, native to Pakistan and India, has been effective in reducinglevels of selenium salts by 50 percent in contaminated soils.


本文关键字: 托福阅读例文翻译解析 矿物质和植物

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