Research has shown that certain minerals are required by plants for normal growth and development. The soil is the source of these minerals, which are absorbed by the plant with the water from the soil. Even nitrogen, which is a gas in its elemental state, is normally absorbed from the soil as nitrate ions. Some soils are notoriously deficient in micro nutrients and are therefore unable to support most plant life. So-called serpentine soils, for example, are deficient in calcium, and only plants able to tolerate low levels of this mineral can survive. In modern agriculture, mineral depletion of soils is a major concern, since harvesting crops interrupts the recycling of nutrients back to the soil.
Mineral deficiencies can often be detected by specific symptoms such as chlorosis (loss of chlorophyll resulting in yellow or white leaf tissue), necrosis (isolated dead patches), anthocyanin formation (development of deep red pigmentation of leaves or stem), stunted growth, and development of woody tissue in an herbaceous plant. Soils are most commonly deficient in nitrogen and phosphorus. Nitrogen-deficient plants exhibit many of the symptoms just described. Leaves develop chlorosis; stems are short and slender, and anthocyanin discoloration occurs on stems, petioles, and lower leaf surfaces. Phosphorus-deficient plants are often stunted, with leaves turning a characteristic dark green, often with the accumulation of anthocyanin. Typically, older leaves are affected first as the phosphorus is mobilized to young growing tissue. Iron deficiency is characterized by chlorosis between veins in young leaves.
Much of the research on nutrient deficiencies is based on growing plants hydroponically, that is, in soilless liquid nutrient solutions. This technique allows researchers to create solutions that selectively omit certain nutrients and then observe the resulting effects on the plants. Hydroponics has applications beyond basic research, since it facilitates the growing of greenhouse vegetables during winter. Aeroponics, a technique in which plants are suspended and the roots misted with a nutrient solution, is another method for growing plants without soil.
While mineral deficiencies can limit the growth of plants, an overabundance of certain minerals can be toxic and can also limit growth. Saline soils, which have high concentrations of sodium chloride 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 heavy metals such as lead, cadmium, mercury, and aluminum; however, even copper and zinc, which are essential elements, can become toxic in high concentrations. Although most plants cannot survive 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 hyperaccumulators, can concentrate 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. Hyperaccumulators run the entire range of the plant world. They may be herbs, shrubs, or trees. Many members of the mustard family, spurge family, legume family, and grass family are top hyperaccumulators. Many are found in tropical and subtropical areas of the world, where accumulation of high concentrations of metals may afford some protection against plant-eating insects and microbial pathogens.
Only recently have investigators considered using these plants to clean up soil and waste sites that have been contaminated by toxic levels of heavy metals–an environmentally friendly approach known as phytoremediation. This scenario begins with the planting of hyperaccumulating species in the target area, such as an abandoned mine or an irrigation pond contaminated by runoff. Toxic minerals would first be absorbed by roots but later relocated to the stem and leaves. A harvest of the shoots would remove the toxic compounds off site to be burned or composted to recover the metal for industrial uses. After several years of cultivation and harvest, the site would be restored at a cost much lower than the price of excavation and reburial, the standard practice for remediation of contaminated soils. For examples, in field trials, the plant alpine pennycress removed zinc and cadmium from soils near a zinc smelter, and Indian mustard, native to Pakistan and India, has been effective in reducing levels of selenium salts by 50 percent in contaminated soils.
Paragraph 1: Research has shown that certain minerals are required by plants for normal growth and development. The soil is the source of these minerals, which are absorbed by the plant with the water from the soil. Even nitrogen, which is a gas in its elemental state, is normally absorbed from the soil as nitrate ions. Some soils are notoriously deficient in micro nutrients and are therefore unable to support most plant life. So-called serpentine soils, for example, are deficient in calcium, and only plants able to tolerate low levels of this mineral can survive. In modern agriculture, mineral depletion of soils is a major concern, since harvesting crops interrupts the recycling of nutrients back to the soil.
1. According to paragraph 1, what is true of plants that can grow in serpentine soil?
○ They absorb micronutrients unusually well.
○ They require far less calcium than most plants do.
○ They are able to absorb nitrogen in its elemental state.
○ They are typically crops raised for food.
Paragraph 2: Mineral deficiencies can often be detected by specific symptoms such as chlorosis (loss of chlorophyll resulting in yellow or white leaf tissue), necrosis (isolated dead patches), anthocyanin formation (development of deep red pigmentation of leaves or stem), stunted growth, and development of woody tissue in an herbaceous plant. Soils are most commonly deficient in nitrogen and phosphorus. Nitrogen-deficient plants exhibit many of the symptoms just described. Leaves develop chlorosis; stems are short and slender, and anthocyanin discoloration occurs on stems, petioles, and lower leaf surfaces. Phosphorus-deficient plants are often stunted, with leaves turning a characteristic dark green, often with the accumulation of anthocyanin. Typically, older leaves are affected first as the phosphorus is mobilized to young growing tissue. Iron deficiency is characterized by chlorosis between veins in young leaves.
2. The word “exhibit” in the passage is closest in meaning to
○ fight off
○ show
○ cause
○ spread
3. According to paragraph 2, which of the following symptoms occurs in phosphorus-deficient plants but not in plants deficient in nitrogen or iron?
○ Chlorosis on leaves
○ Change in leaf pigmentation to a dark shade of green
○ Short, stunted appearance of stems
○ Reddish pigmentation on the leaves or stem
4. According to paragraph 2, a symptom of iron deficiency is the presence in young leaves of
○ deep red discoloration between the veins
○ white or yellow tissue between the veins
○ dead spots between the veins
○ characteristic dark green veins
Paragraph 3: Much of the research on nutrient deficiencies is based on growing plants hydroponically, that is, in soilless liquid nutrient solutions. This technique allows researchers to create solutions that selectively omit certain nutrients and then observe the resulting effects on the plants. Hydroponics has applications beyond basic research, since it facilitates the growing of greenhouse vegetables during winter. Aeroponics, a technique in which plants are suspended and the roots misted with a nutrient solution, is another method for growing plants without soil.
5. The word “facilitates” in the passage is closest in meaning to
○ slows down
○ affects
○ makes easier
○ focuses on
6. According to paragraph 3, what is the advantage of hydroponics for research on nutrient deficiencies in plants?
○ It allows researchers to control what nutrients a plant receives.
○ It allows researchers to observe the growth of a large number of plants simultaneously.
○ It is possible to directly observe the roots of plants.
○ It is unnecessary to keep misting plants with nutrient solutions.
7. The word “suspended” in the passage is closest in meaning to
○ grown
○ protected
○ spread out
○ hung
Paragraph 5: Scientists have known for some time that certain plants, called hyperaccumulators, can concentrate 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. Hyperaccumulators run the entire range of the plant world. They may be herbs, shrubs, or trees. Many members of the mustard family, spurge family, legume family, and grass family are top hyperaccumulators. Many are found in tropical and subtropical areas of the world, where accumulation of high concentrations of metals may afford some protection against plant-eating insects and microbial pathogens.
8. Why does the author mention “herbs”, “shrubs”, and “trees”?
○ To provide examples of plant types that cannot tolerate high levels of harmful minerals.
○ To show why so many plants are hyperaccumulators.
○ To help explain why hyperaccumulators can be found in so many different places.
○ To emphasize that hyperaccumulators occur in a wide range of plant types.
9. The word “afford” in the passage is closest in meaning to
○ offer
○ prevent
○ increase
○ remove
Paragraph 6: Only recently have investigators considered using these plants to clean up soil and waste sites that have been contaminated by toxic levels of heavy metals–an environmentally friendly approach known as phytoremediation. This scenario begins with the planting of hyperaccumulating species in the target area, such as an abandoned mine or an irrigation pond contaminated by runoff. Toxic minerals would first be absorbed by roots but later relocated to the stem and leaves. A harvest of the shoots would remove the toxic compounds off site to be burned or composted to recover the metal for industrial uses. After several years of cultivation and harvest, the site would be restored at a cost much lower than the price of excavation and reburial, the standard practice for remediation of contaminated soils. For examples, in field trials, the plant alpine pennycress removed zinc and cadmium from soils near a zinc smelter, and Indian mustard, native to Pakistan and India, has been effective in reducing levels of selenium salts by 50 percent in contaminated soils.
10. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.
○ Before considering phytoremediation, hyperaccumulating species of plants local to the target area must be identified.
○ The investigation begins with an evaluation of toxic sites in the target area to determine the extent of contamination.
○ The first step in phytoremediation is the planting of hyperaccumulating plants in the area to be cleaned up.
○ Mines and irrigation ponds can be kept from becoming contaminated by planting hyperaccumulating species in targeted areas.
11. It can be inferred from paragraph 6 that compared with standard practices for remediation of contaminated soils, phytoremediation
○ does not allow for the use of the removed minerals for industrial purposes
○ can be faster to implement
○ is equally friendly to the environment
○ is less suitable for soils that need to be used within a short period of time
12. Why does the author mention “Indian mustard”?
○ To warn about possible risks involved in phytoremediation
○ To help illustrate the potential of phytoremediation
○ To show that hyperaccumulating plants grow in many regions of the world
○ To explain how zinc contamination can be reduced
Paragraph 5: Scientists have known for some time that certain plants, called hyperaccumulators, can concentrate 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. ■Hyperaccumulators run the entire range of the plant world. ■They may be herbs, shrubs, or trees. ■Many members of the mustard family, spurge family, legume family, and grass family are top hyperaccumulators. Many are found in tropical and subtropical areas of the world, where accumulation of high concentrations of metals may afford some protection against plant-eating insects and microbial pathogens.
13. Look at the four squares [■] that indicate where the following sentence could be added to the passage.
Certain minerals are more likely to be accumulated in large quantities than others.
Where could the sentence best fit?
14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Plants need to absorb certain minerals from the soil in adequate quantities for normal growth and development.
●
●
●
Answer Choices
○Some plants are able to accumulate extremely high levels of certain minerals and thus can be used to clean up soils contaminated with toxic levels of these minerals.
○Though beneficial in lower levels, high levels of salts, other minerals, and heavy metals can be harmful to plants.
○When plants do not absorb sufficient amounts of essential minerals, characteristic abnormalities result.
○B(yǎng)ecause high concentrations of sodium chloride and other salts limit growth in most plants, much research
has been done in an effort to develop salt-tolerant agricultural crops.
○Some plants can tolerate comparatively low levels of certain minerals, but such plants are of little use for recycling nutrients back into depleted soils.
○Mineral deficiencies in many plants can be cured by misting their roots with a nutrient solution or by transferring the plants to a soilless nutrient solution.
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14. Some plants are able to
Though beneficial in lower…
When plants do not…
研究表明，某些礦物質(zhì)是植物正常生長發(fā)育所必需的。土壤是這些礦物質(zhì)的來源，它們通過水分被植物從土壤中吸收。即使是元素狀態(tài)為氣體的氮，也通常作為硝酸根離子從土壤中被吸收。眾所周知，一些土壤缺乏微量營養(yǎng)素，因此大多數(shù)植物不能生長。例如所謂的蛇紋巖土壤，由于缺乏鈣，只有那些能忍受如此低含量的鈣的植物才能夠存活。在現(xiàn)代農(nóng)業(yè)，土壤礦物質(zhì)枯竭是一個(gè)大問題，因?yàn)槭崭钋f稼切斷了養(yǎng)分返回土壤的循環(huán)。
礦物質(zhì)缺乏通�？捎商囟ǖ陌Y狀檢測(cè)出來，如褪綠（葉綠素?fù)p失導(dǎo)致黃葉或白葉的現(xiàn)象）、壞疽（孤立的壞死斑）、花青素的形成（形成深紅色葉片和莖色素沉積）、發(fā)育不良以及草本植物長木質(zhì)組織。土壤最常缺乏的是氮和磷。氮缺乏植物表現(xiàn)出了剛才描述的許多癥狀：葉片黃化、莖短而細(xì)以及發(fā)生在莖、葉柄以及下葉表面的花青素變色。磷缺乏的植物往往發(fā)育不良，葉片變成特殊的深綠色，經(jīng)常伴隨著花青素的積累。由于磷流向新生的組織，通常較老的葉片首先受到影響。鐵缺乏癥的特點(diǎn)是嫩葉的葉脈之間萎黃。
大多數(shù)關(guān)于營養(yǎng)素缺乏癥的研究都基于水培法，即在無土營養(yǎng)液中培養(yǎng)。這項(xiàng)技術(shù)允許研究人員創(chuàng)造缺乏某種營養(yǎng)素的溶液，然后觀察對(duì)植物生長造成的影響。水培法的應(yīng)用已經(jīng)超越了基礎(chǔ)研究，因?yàn)樗龠M(jìn)了溫室蔬菜在冬季的生長�？諝馀囵B(yǎng)法，一種把植物懸掛起來，將其根部噴上營養(yǎng)液的技術(shù)，是另外一種無土栽培的方法。
雖然缺乏礦物質(zhì)會(huì)抑制植物生長，但某些礦物質(zhì)過量可能會(huì)有毒，同樣也會(huì)抑制植物生長。含有高濃度的氯化鈉和其他鹽類的鹽堿土壤抑制植物生長，于是研究繼續(xù)集中開發(fā)耐鹽農(nóng)作物品種。著重研究重金屬的毒性作用，如鉛、鎘、汞、鋁；然而即使是銅和鋅這樣的必需元素，如果濃度過高也會(huì)產(chǎn)生毒性。雖然大多數(shù)植物無法在這種土壤生存，某些植物卻能夠忍耐如此高含量的礦物質(zhì)。
科學(xué)家早前就了解到，某些所謂的富集植物能夠比普通植物多集中100倍甚至更多的礦物質(zhì)。一項(xiàng)對(duì)已知富集植物的調(diào)查表明，它們中75%積聚了鎳，而鈷、銅、鋅、錳、鉛和鎘則是其他選擇性聚集的礦物質(zhì)。富集植物存在于整個(gè)世界范圍，它們可能是草本植物、灌木或樹。芥屬、大戟屬、豆科和禾本科植物中的許多成員都是靠前的富集植物。許多富集植物被發(fā)現(xiàn)于熱帶和亞熱帶，金屬可以為植物提供保護(hù)，對(duì)抗植食昆蟲和細(xì)菌病原體。
直到最近研究者才考慮用這些植物來清理已經(jīng)被有毒重金屬污染的土壤和廢棄物物處理點(diǎn)——一種被稱為植物修復(fù)法的修復(fù)方法。這套方案首先從在目標(biāo)區(qū)域種植超積累物種開始，如在廢棄礦井和被徑流污染的灌溉池塘。有毒礦物質(zhì)首先被根吸收，隨后被運(yùn)送至莖和葉。收割下來的枝葉將被焚燒以移除有毒化合物或被制成混合肥料回收金屬用于工業(yè)。經(jīng)過幾年的種植和收割，該污染點(diǎn)將被修復(fù)，而其造價(jià)遠(yuǎn)比修復(fù)污染土壤的標(biāo)準(zhǔn)做法——挖掘和填埋來得低。舉例來說，在實(shí)地試驗(yàn)中，高山菥蓂從靠近一個(gè)鋅冶煉廠的土壤中去除了鋅和鎘，原產(chǎn)自巴基斯坦和印度的印度芥菜可以將染土壤中硒的水平有效地降低了50%。
