Plants such as alpine pennycress (Thlaspi caerulescens) are hyperaccumulators, meaning that they concentrate certain elements in their bodies at levels 100 times (or more) greater than normal. The hyperaccumulation of metals also benefits the plants by helping them to evade "weak-stomached" predators, including caterpillars, fungi, bacteria, and humans; high-metal plants can poison unwelcome guests. The saltcedar gets its common name because it accumulates salt and then gets rid of it by pumping the salt out onto its leaves. The salt helps keep some herbivores from eating saltcedar leaves. An example of a metal concentrated by hyperaccumu-lators is lead (Pb), which was once an antiknock component of most gasolines and was deposited along highways from automobile exhaust. Plants growing near these highways often absorbed this lead, as did some grasses growing near lead mines. Radioactive strontium (90Sr) is abundant in plants growing near nuclear test sites and sites of nuclear accidents such as Chernobyl, Russia.
Gold (Au) often occurs in plants such as Phacelia sericea that grow near gold mines. These plants have been used by geologists to locate deposits of gold in soil. Some researchers are experimenting with different species such as Brassica juncea, a member of the mustard family that grows very quickly on mine wastes and is a hyperaccumu-lator of gold. By growing these plants in soil containing minute quantities of gold, investigators were able to obtain almost 1 milligram of gold per gram of dry plant tissue. Investigators hope to increase this gold yield and thereby use such plants to "mine" for gold.
Selenium may account for more than 1% of the dry weight of locoweed (Astragalus). If locoweed is eaten by livestock, the large amounts of selenium in these plants affect the animals' nervous systems, causing them to stagger about as if intoxicated (fig. 7.31). This illness, called "blind staggers," often kills livestock and has even caused ranchers to abandon their ranches. Similarly, bees that harvest pollen from locoweed often produce honey containing large amounts of selenium.
The use of hyperaccumulators to remove toxic substances from soil is part of a growing practice called phy-toremediation (see fig. 2.12). For example, at one site, a field of Brassica juncea decreased the concentration of selenium by more than half, down to a depth of 1 meter. Phytoreme-diation is expected to save some of the billions of dollars the United States has budgeted for soil cleanups over the next few years (costs range from $190,000 to $2,800,000 per hectare). Phytoremediation might be an economically important way of dealing with an environmental problem in such areas as military sites laced with the heavy metals lead and cadmium and municipal waste dumps, where copper and mercury are problems. What might be an evolutionary disadvantage for a plant to become a hyperaccumulator?
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