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Metal-Munching Plants

Years ago, prospectors looking for promising sites to mine silver or gold often noticed that certain plants grew on old mine tailings. They reasoned that these plants might be indicators of the precious metals entrapped in the soil. It turns out that these prospectors had it right.

Certain plants not only grow in heavy metal-laden soils but are able to extract these metals through their root systems and accumulate them in their tissues without being damaged. Plants absorb metals because they require certain ones such as zinc and copper as components of their proteins and enzymes for normal growth and development. In certain soils, plant mineral uptake makes no distinction between heavy metals such as cadmium or selenium and these required elements. In either case, these metals are absorbed by the plant's extensive root system, which may extend a meter or more in depth. Scientists have coined a new term to describe this process—phytore-mediation—the use of plants to facilitate the removal of toxic compounds from ground water and soil. The plants that "munch metal" so well are called "hyperaccumulat-ing" plants.

What makes a good hyperaccumulator? Researchers are attempting to answer this question, but so far, nobody knows for sure. One clue may be metal-binding polypep-tides called phytochelatins that sequester and detoxify heavy metals in plant tissue. A survey of numerous plants has shown that phytochelatins are produced when these plants are exposed to heavy metals in soil. Interestingly, a wide range of plants from the most advanced flowering plants (even orchids) to red, green, and brown algae produce these detoxifying polypeptides. One possible method of accumulating heavy metals is for the plant to transport them into the cell's vacuoles, a sort of waste disposal dump.

Phytoremediation is attracting the increased attention not only of scientists, but also of regulators who see it as a low-cost alternative for cleaning up contaminated sites across the country. The conventional process of soil excavation and reburial in a landfill is very expensive, typically costing about $1.5 million per acre, depending on the pollutant. The price tag opens the door for many alternative ideas. These plants can be harvested and disposed of—and in certain instances, the metals can even be recovered by sending the plants to a smelter. But can hyperaccumulators effectively get the job done?

Alpine pennycress (Thlaspi caerulescens) is a remarkable hyperaccumulator, able to accumulate 4% of its dry body weight in zinc. This translates into 10 metric tons of zinc per hectare. The problem, however, is that Thlaspi is a small and slow-growing plant. For phytoremediation to become practical, plants with metal uptake rates comparable to Thlaspi but with faster growth rates and larger tissue mass must be found. Screening of other plants is being done in several laboratories around the world. One plant that has been identified so far is Indian mustard, Brassica juncea, a relative of some highly nutritious vegetables— cabbage, cauliflower, broccoli, collard, kale, and mustard greens. Indian mustard can accumulate 3.5% of its dry body weight in lead. It also can absorb cadmium, chromium, nickel, selenium, zinc, and copper. Cattails (Typha spp.), which also are known to accumulate heavy metals, are already in use in some areas in the final stages of the treament of human sewage.

Despite the advantages of phytoremediation, one drawback is that multiple crops must be planted over several years to reduce contamination to acceptable levels, while removal of the soil provides an immediate resolution. Additionally, there is concern about increasing the accumulation of these metals in the food chain as wildlife and insects eat the plants, accumulating toxicity in their bodies. In this way, toxic metals could work their way up the food chain and pose a new set of problems.

Phytoremediation is being put to a big test at Chernobyl, the site of the largest environmental disaster in modern history. In 1986, the meltdown of the nuclear reactor left radioactive wastes scattered over the Ukrainian countryside. A pilot study there suggests that Indian mustard could remove radioactive strontium from the soil over a 5-year period. While scientists continue to test this promising natural process, environmental cleanups of the future could be as simple as letting the flowers grow.

D. C. Scheirer

Roots and Soil 83

The ancient Incas of Peru knew that water would rise just so far in some areas. Where the water table was close to the surface, they removed the upper 0.6 meter (2 feet) of soil and planted their crops down in the hollowed-out areas so that the roots would be able to reach the capillary water. They knew that in some areas having sandy soils and low annual precipitation, soils could be compacted by a heavy roller to create finer capillaries to raise water from below. This technique is effective only if the available water is within 1.5 to 3.0 meters (5 to 10 feet) of the surface and if the soils do not contain much silt or clay.

After rain or irrigation, water in the soil drains away by gravity. The water remaining after such draining is referred to as the field capacity of the soil. Field capacity is mainly governed by the texture of the soil, but the structure and organic content also influence it to a certain extent. Plants readily absorb water from the soil when it is at, or near, field capacity. As the soil dries, the film of water around each soil particle becomes thinner and more tightly bound to the soil particle and less likely to enter the root. If water is not added to the soil, eventually a point is reached at which the rate of absorption of water by the plant is insufficient for its needs, and the plant wilts permanently. The soil is then said to be at the permanent wilting point. In clay soils, the permanent wilting point is reached when the water content drops below 15%, while in sandy soils, the permanent wilting point may be as low as 4%. Available water is soil water between field capacity and the permanent wilting point.

Soil pH

The pH (acidity or alkalinity) of a soil affects both the soil and the plants growing in it in various ways. Cranberries, for example, thrive under acidic conditions, but a soil that is unusually acid or alkaline may be toxic to the roots of other plants, and mycorrhizae do not survive in soils having pH extremes. These conditions, however, do not normally directly affect plants nearly as much as they affect nutrient availability. For example, alkalinity causes minerals, such as copper, iron, and manganese, to become less available to plants, while acidity, if high enough, inhibits the growth of nitrogen-fixing bacteria. Acid soils tend to be common in areas of high precipitation where significant amounts of bases are leached from the topsoil.

It is a common agricultural practice to counteract soil acidity by adding compounds of calcium or magnesium in a process known as liming. Alkaline soils can be made more acid by the addition of sulfur, which is converted by bacteria to sulfuric acid. The addition of some nitrogenous fertilizers may have the same effect.

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