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Do plants absorb toxins from the soil?

Do plants absorb toxins from the soil?


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Consider a plant like Aloe Vera that grows up in a toxic environment where the concentration of pesticides, and materials like lead, mercury, cadmium, arsenic etc is very high(e.g. Marshland dumping yard ). Would that mean that the extract from these plants would contain all these toxic elements.


Not "all of them". But yes, plants suck up water from the soil, with everything dissolved in this water - nutrients, heavy metals, poisons. And also they breathe air, and absorb stuff via this route.

There probably are some toxins which will not enter the plant, because their molecules are too large and/or fragile. For example, should a plant root come in contact with snake venom, I cannot imagine that any venom will end up stored in the plant leaves.

Plants also have their own metabolism, so they will change/deactivate some toxins. I've seen claims that some plants "purify" formaldehyde, although I don't trust the sources enough to be sure of that.

But the smaller the poison molecule, and the less similar to stuff which is usually digested in nature, the more likely that it will enter the plant and stick around instead of being broken down. The heavy metals you mentioned are prime candidates. If they are present in the groundwater - or also lead from air pollution, before we banned leaded gasoline - they end up in plants, including food plants. And mushrooms are even more at risk.

Growing food near waste dumps is a known problem in farming, and sometimes makes the news, for example here: http://bigstory.ap.org/article/mafia-toxic-waste-dumping-poisons-italy-farmlands


Houseplants as Biofilters: Do Indoor Plants Really Purify the Air?

Have you heard all the buzz about how indoor plants purify the air in your home? It’s true that plants are biofilters, a term often used for systems that use plants or microorganisms to clean air in order to combat pollution and the presence of harmful toxins. This technology is usually used on a large scale for wastewater treatment facilities and chemical plants, but any system that filters out toxins is a biological filter – and that includes plants, animals, insects, and even you! Does that mean that all the microbes, pollution, and viruses are filtered out of the air if you have some houseplants? There are many myths and claims out there about what houseplants can do for your air quality, so I did a little research on the truth about houseplants and air quality.


Does Cigarette Smoke Affect Plants?

Studies have already found that the smoke from forest fires negatively impacts trees that survive big blazes. The smoke seems to decrease a tree’s ability to photosynthesize and grow efficiently.

There have also been a few studies about how cigarette smoke affects the growth and health of indoor plants. One small study found that plants exposed to cigarette smoke for 30 minutes per day grew fewer leaves. Many of those leaves browned and dried out or dropped off sooner than leaves on plants in a control group.

The studies on plants and cigarettes are limited, but it does seem that at least concentrated doses of smoke can be damaging. These small studies confined the plants to small areas with lit cigarettes, so they do not exactly mimic what a real home with a smoker would be like.


You Asked: Can Indoor Plants Really Purify the Air?

P lants are indispensable to human life. Through photosynthesis, they convert the carbon dioxide we exhale into fresh oxygen, and they can also remove toxins from the air we breathe.

One famous NASA experiment, published in 1989, found that indoor plants can scrub the air of cancer-causing volatile organic compounds like formaldehyde and benzene. (Those NASA researchers were looking for ways to effectively detoxify the air of space station environments.) Later research has found that soil microorganisms in potted plants also play a part in cleaning indoor air.

Based on this research, some scientists say house plants are effective natural air purifiers. And the bigger and leafier the plant, the better. &ldquoThe amount of leaf surface area influences the rate of air purification,&rdquo says Bill Wolverton, a former NASA research scientist who conducted that 1989 plant study.

Wolverton says that, absent expensive testing, it&rsquos impossible to guess how many plants might be needed to clean a room of its contaminants. But he usually recommends at least two &ldquogood sized&rdquo plants per 100 square feet of interior space. &ldquoThe Boston fern is one of the most effective plants for removing airborne pollutants, but it is often difficult to grow indoors,&rdquo he says. &ldquoI usually recommend the golden pothos as my first choice, since it is a popular plant and easy to grow.&rdquo

But while Wolverton has long been a vocal advocate of indoor plants&mdashhe&rsquos written books on the topic, and now operates a consulting company that advocates for the use of plants to clean contaminated air&mdashother experts say the evidence that plants can effectively accomplish this feat is far from conclusive.

&ldquoThere are no definitive studies to show that having indoor plants can significantly increase the air quality in the home to improve health in a measurable way,&rdquo says Luz Claudio, a professor of environmental medicine and public health at the Icahn School of Medicine at Mount Sinai.

Claudio has reviewed the research on the air-quality benefits of indoor plants. She says there&rsquos no question that plants are capable of removing volatile chemical toxins from the air &ldquounder laboratory conditions.&rdquo But in the real world&mdashin your home, say, or in your office space&mdashthe notion that incorporating a few plants can purify your air doesn&rsquot have much hard science to back it up.

Most research efforts to date&mdashincluding the NASA study&mdashplaced indoor plants in small, sealed environments in order to assess how much air-scrubbing power they possessed. But those studies aren&rsquot really applicable to what happens in a house, says Stanley Kays, a professor emeritus of horticulture at the University of Georgia.

Kays coauthored a 2009 study on the air-cleaning powers of 28 different indoor plants. While many of those plants could remove toxins from the air, &ldquomoving from a sealed container to a more open environment changes the dynamics tremendously,&rdquo he says.

In many cases, the air in your home completely turns over&mdashthat is, swaps places with outdoor air&mdashonce every hour. &ldquoThere&rsquos a phenomenal amount of air coming in and going out in most houses,&rdquo Kays says. &ldquoFrom what I&rsquove seen, in most instances air exchange with the exterior has a far greater effect on indoor air quality than plants.&rdquo

Also, plants used in lab studies are grown in optimal conditions. They&rsquore exposed to ample light in order to maximize photosynthesis, which improves a plant&rsquos toxin-degrading abilities. &ldquoIn the home, this isn&rsquot the case at all,&rdquo Kays says. &ldquoThe amount of light in many parts of a house is often just barely sufficient for photosynthesis.&rdquo

He knows many people will be disappointed by what he has to say, and he wants to make it clear he believes house plants are not only pleasant living companions, but that they also provide a number of evidence-based health benefits. Studies have shown plants can knock out stress by calming the sympathetic nervous system, and can also make people feel happier. More research shows spending time around nature has a positive effect on a person&rsquos mood and energy levels.

&ldquoThere are some real plusses to having plants around,&rdquo Kays says. &ldquoBut at this time, it doesn&rsquot look like plants sitting passively in a house are effective enough to make a major contribution to purifying indoor air.&rdquo


Phytoremediation Plants Used to Clean Contaminated Soil

By Anita B. Stone – America’s priceless natural resource, land, has often been used as a natural, free dispose-all for toxic compounds. For many of us, it seemed to be a harmless practice, using the out of sight, out of mind idea. But, as a result, the damage to soil can be long term leaving areas of land that were once productive to lie fallow and become a wasteland. The surprising solution comes from phytoremediation plants — living green plants that can help clean and mitigate soil damage.

Just as there are best houseplants for clean air indoors, there are best plants that can be used outdoors for cleaner soil. Good soil lacks contaminants and provides trace minerals and key components for plant growth. But good soil is not always easy to find. And many contaminants can be expensive and require a great deal of time to remove from toxic soil. Good soil will result when phytoremediation plants clean contaminated soil. This problem is not just an occasional issue concerning a variety of news-worthy events. Homesteaders and farmers can face these same issues. For instance, disposing of petroleum products such as machine oil, asphalt, lead, tar or certain agricultural chemicals can pose problems. In order to reclaim the soil and get rid of contaminants, phytoremediation plants can be used to reduce these issues.

Phytoremediation plants refer to the use of living plants to reduce, degrade or remove toxic residue from the soil. Using green plants to decontaminate soil is a progressive and sustainable process, greatly reducing the need for heavy machinery or additional contaminants. Familiar plants such as alfalfa, sunflower, corn, date palms, certain mustards, even willow and poplar trees can be used to reclaim contaminated soil – a cheap, clean and sustainable process. The term, phytoremediation, can be best understood by breaking the word into two parts: “phyto” is the Greek word for plant. “Remediation” refers to a remedy, and in this case, a remedy for soil contamination whether it be located in the garden or across a large landscape area.

Here is where plants used in phytoremediation enter the area. These special plants are known as superplants, which readily absorb toxins from the very soil where they are growing. For phytoremediation plants to work effectively, the specific plant must be able to tolerate the toxic material it is absorbing from the soil. We cannot just plant any vegetation in contaminated soil and hope for the best. The history of the concept of phytoremediation plants is interesting and can be traced to earlier studies of the relationship between soil-plant systems and the nutritional quality of food.

In 1940, studies of compounds within edible plants and their ability to absorb additional nutrition from the soil became big news. Early research on soil contamination testing proved the ability of soil to increase a given plant’s nutrition beyond what was thought to be their ultimate level. Soil testing research led to further tests of a plant’s ability to absorb less desirable elements from the soil that is, toxins released through industrial waste, sewage and agricultural chemicals. Eventually, phytoremediation plants became an additional clean-up technique to remove harmful chemicals from the soil, such as cadmium, zinc, iron, and manganese. One plant used in phytoremediation for cleaner soil is Alpine Pennygrass because it was found to be able to remove 10 times more cadmium than any other known soil cleaning plant. Another plant used in phytoremediation for cleaner soil is Indian mustard, which removes lead, selenium, zinc, mercury, and copper from the soil.

In 1980, R.L. Chanely published a paper on the subject of what makes good soil and how to establish it through the use of phytoremediation plants. Plants such as mustard and canola thrive in contaminated soils, absorbing and therefore reducing the level of toxic accumulation. A native phytoremediation plant for cleaner soil, known as Indian Grass, has the ability to detoxify common agrochemical residues such as pesticides and herbicides. Indian Grass is one of nine members of grasses that assist in phytoremediation plants. When planted on farmland, the reduction of pesticides and herbicides is significant. This list also includes Buffalo grass and Western wheatgrass, both capable of absorbing hydrocarbons from the land.

Since any plant used as a phytoremediator must be able to tolerate any toxins it absorbs, researcher David W. Ow has been investigating which genes are key to increased plant tolerance. When identified, these genes can then be moved to other plant species to absorb high levels of certain metals. More research proves genetic movement. During testing into the nutritional value of broccoli, it was found that the plant worked well to deplete the soil of several metals. In California, some farmers who had been irrigating with recycled water discovered that their soil became overloaded with either selenium or boron.

Other plants used in phytoremediation for cleaner soil include species that reduce levels of organic compounds found in coal and tar, which are present in pitch, creosote, and asphalt. These include the very popular sunflower, which has the ability to absorb heavy metals, such as lead. Homesteaders, farmers, and agriculturalists have been practicing “intercropping” for several years. By simply employing the intercropping method, the above-mentioned plants can be effectively used as excellent choices. For example, sunflower plants were demonstrated to have removed 95 percent of uranium from a contaminated area in a 24-hour period. This highly successful crop is a powerful tool for the environment because of its ability to remove radioactive metals from superficial groundwater.

The willow is being used as a phytoremediation plant for cleaner soil. It not only beautifies the landscape but the roots have the capability of accumulating heavy metals in sites polluted with diesel fuel. A tree that is being studied for use as phytoremediation for cleaner soil is the poplar tree. Poplar trees have a root system that absorbs large quantities of water. Carbon tetrachloride, a well-known carcinogen, is easily absorbed by poplar tree roots. They can also degrade petroleum hydrocarbons like benzene or paint thinners that have accidentally spilled onto the soil. This has been a fantastic discovery. Besides their usefulness in controlling and absorbing toxic soil materials, poplar trees can be easily integrated into any type of landscape for aesthetic appeal.

With ongoing research and new toxin-absorbing plant life being discovered each year, we can expect phytoremediator choices for pollutant cleanup projects to increase. The process appears simple, but the research is slow, complicated and painstaking. But, compared to the process of soil removal, soil disposal, or physical extraction of contaminants, phytoremediation plants are a useful and working alternative that pinpoints toxic materials in the soil. We can remove quite a bit of soil contamination by using this process.

Some enthusiasts consider this process a low-cost “green” technology for soil cleanup, which can be used anywhere without specialized training or equipment. Planting a few additional plants, attractive to the landscape, can certainly enhance the soil on any land area. A variety of grasses, sunflowers, trees and other plants work in a positive way, helping farmers, homesteaders and agriculturalists rid levels of toxic materials found in our soil. These plants, themselves, are used in the restoration of healthy soils as they become their own ready-made storage containers for removal and subsequent treatment. The future of phytoremediation plants continues to move forward in creating clean soil. It is being used by industrial groups. With the help of farmers, homesteaders, and landowners, future research could create a system that will continually absorb contaminants, free up useless soil, and clean the environment on a continuous, constant and self-renewing basis.

Have you used phytoremediation plants to clean contaminated soil? If so, what plants did you use? Was the process successful? Let us know in the comments below.


Contents

The soil is home to a large proportion of the world's biodiversity. The links between soil organisms and soil functions are observed to be incredibly complex. The interconnectedness and complexity of this soil ‘food web’ means any appraisal of soil function must necessarily take into account interactions with the living communities that exist within the soil. We know that soil organisms break down organic matter, making nutrients available for uptake by plants and other organisms. The nutrients stored in the bodies of soil organisms prevent nutrient loss by leaching. Microbial exudates act to maintain soil structure, and earthworms are important in bioturbation. However, we find that we don't understand critical aspects about how these populations function and interact. The discovery of glomalin in 1995 indicates that we lack the knowledge to correctly answer some of the most basic questions about the biogeochemical cycle in soils. There is much work ahead to gain a better understanding of the ecological role of soil biological components in the biosphere.

In balanced soil, plants grow in an active and steady environment. The mineral content of the soil and its heartiful [word?] structure are important for their well-being, but it is the life in the earth that powers its cycles and provides its fertility. Without the activities of soil organisms, organic materials would accumulate and litter the soil surface, and there would be no food for plants. The soil biota includes:

  • Megafauna: size range - 20 mm upward, e.g. moles, rabbits, and rodents.
  • Macrofauna: size range - 2 to 20 mm, e.g. woodlice, earthworms, beetles, centipedes, slugs, snails, ants, and harvestmen. : size range - 100 micrometres to 2 mm, e.g. tardigrades, mites and springtails. and Microflora: size range - 1 to 100 micrometres, e.g. yeasts, bacteria (commonly actinobacteria), fungi, protozoa, roundworms, and rotifers.

Of these, bacteria and fungi play key roles in maintaining a healthy soil. They act as decomposers that break down organic materials to produce detritus and other breakdown products. Soil detritivores, like earthworms, ingest detritus and decompose it. Saprotrophs, well represented by fungi and bacteria, extract soluble nutrients from delitro. The ants (macrofaunas) help by breaking down in the same way but they also provide the motion part as they move in their armies. Also the rodents, wood-eaters help the soil to be more absorbent.

Soil biology involves work in the following areas:

    of biological processes and population dynamics
  • Soil biology, physics and chemistry: occurrence of physicochemical parameters and surface properties on biological processes and population behavior and molecular ecology: methodological development and contribution to study microbial and faunal populations diversity and population dynamics genetic transfers, influence of environmental factors and functioning processes: interactions between organisms and mineral or organic compounds involvement of such interactions in soil pathogenicity transformation of mineral and organic compounds, cycling of elements soil structuration

Complementary disciplinary approaches are necessarily utilized which involve molecular biology, genetics, ecophysiology, biogeography, ecology, soil processes, organic matter, nutrient dynamics [1] and landscape ecology.

Bacteria are single-cell organisms and the most numerous denizens of agriculture, with populations ranging from 100 million to 3 billion in a gram. They are capable of very rapid reproduction by binary fission (dividing into two) in favourable conditions. One bacterium is capable of producing 16 million more in just 24 hours. Most soil bacteria live close to plant roots and are often referred to as rhizobacteria. Bacteria live in soil water, including the film of moisture surrounding soil particles, and some are able to swim by means of flagella. The majority of the beneficial soil-dwelling bacteria need oxygen (and are thus termed aerobic bacteria), whilst those that do not require air are referred to as anaerobic, and tend to cause putrefaction of dead organic matter. Aerobic bacteria are most active in a soil that is moist (but not saturated, as this will deprive aerobic bacteria of the air that they require), and neutral soil pH, and where there is plenty of food (carbohydrates and micronutrients from organic matter) available. Hostile conditions will not completely kill bacteria rather, the bacteria will stop growing and get into a dormant stage, and those individuals with pro-adaptive mutations may compete better in the new conditions. Some gram-positive bacteria produce spores in order to wait for more favourable circumstances, and gram-negative bacteria get into a "nonculturable" stage. Bacteria are colonized by persistent viral agents (bacteriophages) that determine gene word order in bacterial host.

From the organic gardener's point of view, the important roles that bacteria play are:

Nitrification Edit

Nitrification is a vital part of the nitrogen cycle, wherein certain bacteria (which manufacture their own carbohydrate supply without using the process of photosynthesis) are able to transform nitrogen in the form of ammonium, which is produced by the decomposition of proteins, into nitrates, which are available to growing plants, and once again converted to proteins.

Nitrogen fixation Edit

In another part of the cycle, the process of nitrogen fixation constantly puts additional nitrogen into biological circulation. This is carried out by free-living nitrogen-fixing bacteria in the soil or water such as Azotobacter, or by those that live in close symbiosis with leguminous plants, such as rhizobia. These bacteria form colonies in nodules they create on the roots of peas, beans, and related species. These are able to convert nitrogen from the atmosphere into nitrogen-containing organic substances. [2]

Denitrification Edit

While nitrogen fixation converts nitrogen from the atmosphere into organic compounds, a series of processes called denitrification returns an approximately equal amount of nitrogen to the atmosphere. Denitrifying bacteria tend to be anaerobes, or facultatively anaerobes (can alter between the oxygen dependent and oxygen independent types of metabolisms), including Achromobacter and Pseudomonas. The purification process caused by oxygen-free conditions converts nitrates and nitrites in soil into nitrogen gas or into gaseous compounds such as nitrous oxide or nitric oxide. In excess, denitrification can lead to overall losses of available soil nitrogen and subsequent loss of soil fertility. However, fixed nitrogen may circulate many times between organisms and the soil before denitrification returns it to the atmosphere. The diagram above illustrates the nitrogen cycle.

Actinobacteria Edit

Actinobacteria are critical in the decomposition of organic matter and in humus formation. They specialize in breaking down cellulose and lignin along with the tough chitin found on the exoskeletons of insects. Their presence is responsible for the sweet "earthy" aroma associated with a good healthy soil. They require plenty of air and a pH between 6.0 and 7.5, but are more tolerant of dry conditions than most other bacteria and fungi. [3]

A gram of garden soil can contain around one million fungi, such as yeasts and moulds. Fungi have no chlorophyll, and are not able to photosynthesise. They cannot use atmospheric carbon dioxide as a source of carbon, therefore they are chemo-heterotrophic, meaning that, like animals, they require a chemical source of energy rather than being able to use light as an energy source, as well as organic substrates to get carbon for growth and development.

Many fungi are parasitic, often causing disease to their living host plant, although some have beneficial relationships with living plants, as illustrated below. In terms of soil and humus creation, the most important fungi tend to be saprotrophic that is, they live on dead or decaying organic matter, thus breaking it down and converting it to forms that are available to the higher plants. A succession of fungi species will colonise the dead matter, beginning with those that use sugars and starches, which are succeeded by those that are able to break down cellulose and lignins.

Fungi spread underground by sending long thin threads known as mycelium throughout the soil these threads can be observed throughout many soils and compost heaps. From the mycelia the fungi is able to throw up its fruiting bodies, the visible part above the soil (e.g., mushrooms, toadstools, and puffballs), which may contain millions of spores. When the fruiting body bursts, these spores are dispersed through the air to settle in fresh environments, and are able to lie dormant for up to years until the right conditions for their activation arise or the right food is made available.

Mycorrhizae Edit

Those fungi that are able to live symbiotically with living plants, creating a relationship that is beneficial to both, are known as Mycorrhizae (from myco meaning fungal and rhiza meaning root). Plant root hairs are invaded by the mycelia of the mycorrhiza, which lives partly in the soil and partly in the root, and may either cover the length of the root hair as a sheath or be concentrated around its tip. The mycorrhiza obtains the carbohydrates that it requires from the root, in return providing the plant with nutrients including nitrogen and moisture. Later the plant roots will also absorb the mycelium into its own tissues.

Beneficial mycorrhizal associations are to be found in many of our edible and flowering crops. Shewell Cooper suggests that these include at least 80% of the brassica and solanum families (including tomatoes and potatoes), as well as the majority of tree species, especially in forest and woodlands. Here the mycorrhizae create a fine underground mesh that extends greatly beyond the limits of the tree's roots, greatly increasing their feeding range and actually causing neighbouring trees to become physically interconnected. The benefits of mycorrhizal relations to their plant partners are not limited to nutrients, but can be essential for plant reproduction. In situations where little light is able to reach the forest floor, such as the North American pine forests, a young seedling cannot obtain sufficient light to photosynthesise for itself and will not grow properly in a sterile soil. But, if the ground is underlain by a mycorrhizal mat, then the developing seedling will throw down roots that can link with the fungal threads and through them obtain the nutrients it needs, often indirectly obtained from its parents or neighbouring trees.

David Attenborough points out the plant, fungi, animal relationship that creates a "Three way harmonious trio" to be found in forest ecosystems, wherein the plant/fungi symbiosis is enhanced by animals such as the wild boar, deer, mice, or flying squirrel, which feed upon the fungi's fruiting bodies, including truffles, and cause their further spread (Private Life Of Plants, 1995). A greater understanding of the complex relationships that pervade natural systems is one of the major justifications of the organic gardener, in refraining from the use of artificial chemicals and the damage these might cause. [ citation needed ]

Recent research has shown that arbuscular mycorrhizal fungi produce glomalin, a protein that binds soil particles and stores both carbon and nitrogen. These glomalin-related soil proteins are an important part of soil organic matter. [4]

Soil fauna affect soil formation and soil organic matter dynamisc on many spationtemporal scales. [5] Earthworms, ants and termites mix the soil as they burrow, significantly affecting soil formation. Earthworms ingest soil particles and organic residues, enhancing the availability of plant nutrients in the material that passes through and out of their bodies. By aerating and stirring the soil, and by increasing the stability of soil aggregates, these organisms help to assure the ready infiltration of water. These organisms in the soil also help improve Ph levels.

Ants and termites are often referred to as "Soil engineers" because, when they create their nests, there are several chemical and physical changes made to the soil. Among these changes are increasing the presence of the most essential elements like Carbon, Nitrogen, and phosphorus--elements needed for plant growth. [6] They also can gather soil particles from differing depths of soil and deposit them in other places, leading to the mixing of soil so it is richer with nutrients and other elements.

The soil is also important to many mammals. Gophers, moles, prairie dogs, and other burrowing animals rely on this soil for protection and food. The animals even give back to the soil as their burrowing allows more rain, snow and water from ice to enter the soil instead of creating erosion. [7]


These are the bad guys of which you should be wary. Cars don&rsquot just produce carbon dioxide and nitrous oxides, but also other types of particulate matter, including carbon that comes from fuel combustion of the engine.

Polycyclic Aromatic Hydrocarbons (PAH) constitute a small proportion of this particulate matter. These chemicals are known to cause certain types of cancer. However, they don&rsquot &ndash directly &ndash affect the plants. Instead, they mix up with dust and land on nearby vegetation, which means that plants and veggies get less sunlight than normal. This is certainly a bad thing.


Sunflowers in Chernobyl

The impact of sunflowers on polluted land was first discovered in the aftermath of the Chernobyl disaster. Despite the loss of life, plants continued to thrive in the nuclear wasteland and even grew afresh. Intrigued by this, scientists entered Chernobyl and planted new seeds.

To their surprise – and delight – they learned that sunflowers. In particular, they were capable of absorbing toxic heavy metals from the ground. Perhaps more importantly, they also sucked toxicity from the local ponds. Ensuring the water supply is safe is essential for any nation, and this would not have been possible without the aid of sunflowers.

The planting of sunflowers, along with mustard seeds, flax seeds, and soybeans (which are believed to be equally effective, if not as aesthetically pleasing), is now standard practice in the event of radiation. As a result, the sunflower has become a symbol of peace and a nuclear-free world.

Quite understandably, Ukrainian officials lost their stomachs for nuclear weapons after this historical tragedy. Before Chernobyl, the nation had an arsenal of some 1,900 nuclear weapons at their disposal. In the aftermath, all of these were dismantled in neighboring Russia.

By 1996, Ukraine was officially a nuclear-free country. To celebrate this landmark, ministers from Ukraine, Russia, and the USA convened at a disused missile base and planted sunflower seeds.

This new symbol of hope now grows on the area that once housed warheads, which would have been fired on America in the event of a nuclear conflict.


Types of Water Absorption in Plants

Plants typically absorb water by the following two methods:

Active Absorption of Water

This type of water absorption requires the expenditure of metabolic energy by the root cells to perform the metabolic activity like respiration. Active absorption in plant occurs in two ways, namely osmotic and non-osmotic absorption of water.

  1. Osmotic active absorption of water: In this type, the water absorption occurs through osmosis where the water moves into the root xylem across the concentration gradient of the root cell. The osmotic movement is due to the high concentration of solute in the cell sap and low concentration of the surrounding soil.
  2. Non-osmotic active absorption of water: Here, the water absorption occurs where the water enters the cell from the soil against the concentration gradient of the cell. This requires the expenditure of metabolic energy through the respiration process. Hence, as the rate of respiration increases, the rate of water absorption will also increase. Auxin is a growth regulatory hormone, which increases the rate of respiration in plants that, in turn, also increase the rate of water absorption.

Passive Absorption of Water

This type of water absorption does not require the use of metabolic energy. The absorption occurs by metabolic activity like transpiration. Passive absorption is the type where the water absorption is through the transpiration pull. This creates tension or force that helps in the movement of water upwards into the xylem sap. Higher is the transpiration rate, and higher is the absorption of water.

Role of Root Hairs in Water Absorption

A root contains some tubular, hair-like and unicellular structures called Root hairs. In the root system, the region from which the root hairs protrude out is termed as Root hair zone. The zone of root hair is the only region that participates in water absorption activity. Root hair zone is the water-permeable region. Root hairs are the outgrowths, which arise from the epidermal layer called the piliferous layer.

The cell wall of root hair consists of a double layer membrane. Pectin is present in the outer layer, and cellulose is present in the cell wall’s inner layer. Under the cell wall, there is a selectively permeable cytoplasmic-membrane. The cell or cytoplasmic membrane will allow specific substances to pass across the cell concentration gradient.

Root cells, nucleus, and vacuole or cell sap are present inside the cytoplasmic membrane. Soil aggregates contain small droplets of water carried away by the root hairs into the root xylem through different mechanisms, out of which osmosis is most common.


Scientists Using Plants to Clean Up Metals In Contaminated Soil

A forest once grew here in the bend of the Delaware River. Now a multibillion-dollar plant where the Du Pont Company manufactures 750 different chemicals sprawls under the span of the Delaware Memorial Bridge.

After 75 years of manufacturing toxic materials like tetraethyl lead, an anti-knock gasoline additive, at the Chambers Works, E. I. du Pont de Nemours & Company is trying to cleanse some of the contaminants from this now-barren land. Dr. Scott Cunningham, a Du Pont researcher, has an idea for reclaiming it with plants.

But Dr. Cunningham does not envision establishing another forest here. In order to remove substantial concentrations of lead from the ground, he is planting ragweed.

Dr. Cunningham is one of a handful of researchers around the world who are trying to use plants to clean contaminated soil. They are attempting to plant crops that will absorb metals, then harvest the plants and, it is hoped, process them to recycle the metals that are reclaimed.

The process, they say, offers cheaper, more environmentally sound possibilities for cleaning contaminated sites. Absorbing High Concentrations

"No one has successfully remediated a site with plants yet," Dr. Cunningham said. "But it just makes sense."

The researchers use varieties of plants, called "hyperaccumulators," that can build up in their cells higher concentrations of metal than exist in the soil where they are planted. They can be found thriving in areas that most plants, animals and humans would find uninhabitable.

Dr. Cunningham, for instance, tested the levels of lead in plants growing around a basin that used to contain the swill washed from water used in the tetraethyl lead manufacturing plants at the Chambers Works. Two types had large quantities of lead in their upper shoots, hemp dogbane and common ragweed.

Now Dr. Cunningham and his associates have planted a small plot in the defunct tetraethyl lead plant. Amid the exposed brick, pipes, railroad tracks and hard-packed gravel paths, the "garden" grows inside a fence marked with bright yellow tape.

Although they are trying many varieties of plants, the researchers say the ragweed and hemp dogbane are accumulating the most lead. Samples of the ragweed after four months have shown a concentration of 8,000 parts per million lead, although the plot's soil has only 1,000 parts per million. Cleaning Superfund Site

Another field project, the Woburn Market Garden Experiment at Rothamsted Experimental Station in Hartfordshire, England, has produced plants that take up 1 percent of their dried body weight in zinc. Researchers there, led by Dr. Steve McGrath, are also having success absorbing cadmium and nickel deposits, all left by earlier experiments that tested organic wastes as nutrients for plants.

Dr. McGrath has calculated that the zinc could be brought to acceptable levels in 13 croppings. That process could be speeded with manipulation of the soil, fertilizer and plant species, he said.

Perhaps the most widely publicized field experiment is one that Dr. Rufus L. Chaney, a research agronomist at the United States Department of Agriculture, began in 1991 with Mel Chin, a conceptual artist. The project, called "Revival Field," uses a variety of plants to clean a Superfund site in Minnesota.

The goal of all these projects is to produce a genetically altered plant and proper soil conditions to allow plants to amass 2 percent or more of their dried body weight in metals, Dr. Cunningham said. If the plants were large enough, a harvest could produce a substantial quantity of the metal, which could possibly be smelted from the plants.

The plants would need to be dried, like hay, burned and then smelted as a type of "bio-ore." This would avoid the need to return the metals to the ground.

If smelting were not practicable, the researchers say, the burned plants could still be placed in a landfill. The volume of waste from placing the ashed plants in a landfill would still be thousands of times less than that produced in current procedures for reclaiming contaminated soil, the researchers say. Few Alternatives for Cleanup

One of the reasons that this technology, called "green" remediation or phytoremediation, has attracted attention in the last few years is that there are few alternatives for cleaning metals from soil.

Bioremediation does not work, since the types of microbes that eat metals are very hard to remove from the soil once they are done.

So companies can vitrify the soil, pouring in a compound that traps the metals to keep them from spreading, or "wash" the soil with an acidic compound, which leaves metal-contaminated acid and soil with impaired abilities to support growth.

The third and most widely used method is to dig up the contaminated soil, mix it with cement and bury it.

"The only technology now is to dig the stuff up and bring it someplace less politically sensitive," said Dr. Paul Jackson, a biochemist at Los Alamos National Laboratory in New Mexico. " 'Suck, muck and truck,' they call it. That's not going to hack it for long."

More traditional forms of soil reclamation are also more expensive. Dwight Bedsole, business director for remediation, safety and environmental services at the Chambers Works, estimated that the company would spend $75 million to $100 million a year for the next five years for remediation at the site.

Dr. Cunningham said that if his research was successful, it could be used as an inexpensive way to slowly clean the land around small companies, urban roads or even farmhouses that used lead paint. It cannot be used to clean highly contaminated sites, however, like those with more than 1 percent lead in their soil. 'Magical Properties'

One of the biggest puzzles of phytoremediation research is why and how some plants accumulate such high levels of toxic metals. Although researchers have been studying plants' potential for reclamation for only a decade, evidence that plants absorb metals has been collected for hundreds of years.

Botanists, metallurgists and archeologists discovered early that the presence of certain plants indicated deposits of metals. Miners in Africa found copper, miners in Russia found uranium and miners in the United States found gold using this method. In addition, archeologists have used plants to pinpoint ancient mining sites and civilizations in Latin America.

Dr. Alan Baker, a geobotanist from the University of Sheffield in England, has traveled to remote climes to test and retrieve plants that survive in highly contaminated soils. He began his research 21 years ago in an effort to discover how the plants withstood such high-metal soils.

Most plants exclude the metals, storing them in their roots where they will not effect the mechanisms of the plant. But a very few accumulate the metals, detoxifying them and storing them in their leaves. Dr. Baker has given cuttings of these plants to Dr. McGrath, with whom he works on the Woburn Market Garden Experiment.

"These plants seem to have magical properties," Dr. Baker said. "There has got to be something we can do to exploit that. We have got to find a way to harness this ability and put it to use cleaning soils."

The same mechanisms that allow plants to take up metals from soil may be used to solve other environmental problems. Some researchers are trying to clean other types of waste, like radionuclides. And some are examining the possibility of breeding plants to exclude toxic wastes rather than take them up. Cleaning Nuclear Sludge

Dr. Jackson, for instance, has been using cells from Jimson weed to clean plutonium from nuclear sludge. He grows the cells, packs them in a resin that he places in a column, and then pumps the sludge through the column. The plutonium binds with the plant cells, removing the radionuclides from the liquid.

Dr. Jackson has also used plant cells in resin to clean metals like copper, selenium and uranium from water or other liquids. His research for the last 10 years has focused on the mechanism by which plants absorb metals and other toxics.

"Of course these metals combine with biological organisms," he said. "That's why they're toxic. If they didn't effect the environment, they would not be considered harmful. We are just taking advantage of that property."

Dr. Jackson is also looking for ways to alter plants so they do not take up toxic metals from the ground. Such alterations would be especially useful for crops like tobacco, tomatoes and potatoes that easily absorb cadmium.

At Rutgers University, Dr. Ilya Raskin is beginning another effort to exclude metals. He will be coordinating an effort to help clean up the waste from the accident at the Chernobyl nuclear power plant in Ukraine in 1986. One priority is to breed forage grasses that exclude all radionuclides, so locally grown meat and milk are not contaminated.

Dr. Raskin is also experimenting with the accumulating properties of mustards, which come from the same cabbage family as the plants Dr. McGrath is using in England. Dr. Raskin is testing the ability of mustards to take up radionuclides as well as chromium, a substantial pollution problem in New Jersey.



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