在過去這幾年裡，瑪麗·洛伊·格里諾特(Mary Lou Guerinot)一直在監測得克薩斯東南部的試驗田，了解大米作物從土壤里吸收金屬和其他讓人擔心的元素的情況。
已經有很多報告指出，從麥片棒到嬰兒食品的各類大米製品中檢出了砷，這令消費者已經產生了警惕。一些食品加工企業已開始主動篩查產品砷含量，而像美國食品藥品監督局(Food and Drug Administration)這類機構現在建議人們要確保所吃的穀物品種多樣化，好「將過量食用單一食品導致的潛在負面健康影響最小化」。
「水稻作物的問題在於，它往往會在稻米，而非葉片或其他部位積聚砷，」普度大學(Purdue University)植物生物學家喬迪·班克斯(Jody Banks)研究的領域是植物中的砷積聚，她說：「而且吸附的速度極快。」
不過，較低濃度的砷已在世界各國，包括美國的土壤中檢出。據美國地質調查局(United States Geological Survey)的研究，密西西比河泛濫平原沖刷的肥沃土壤，其砷含量可高達路易斯安那、密西西比和阿肯色州其他地區的五倍。
The Trouble With Rice
May 05, 2014
As a plant, rice is particularly prone to absorbing certain toxic metals from the soil.
For the past few years, Mary Lou Guerinot has been keeping watch over experimental fields in southeast Texas, monitoring rice plants as they suck metals and other troublesome elements from the soil.
If the fields are flooded in the traditional paddy method, she has found, the rice handily takes up arsenic. But if the water is reduced in an effort to limit arsenic, the plant instead absorbs cadmium — also a dangerous element.
“It’s almost either-or, day-and-night as to whether we see arsenic or cadmium in the rice,” said Dr. Guerinot, a molecular geneticist and professor of biology at Dartmouth College.
The levels of arsenic and cadmium at the study site are not high enough to provoke alarm, she emphasized. Still, it is dawning on scientists like her that rice, one of the most widely consumed foods in the world, is also one of nature’s great scavengers of metallic compounds.
Consumers have already become alarmed over reports of rice-borne arsenic in everything from cereal bars to baby food. Some food manufacturers have stepped up screening for arsenic in their products, and agencies such as the Food and Drug Administration now recommend that people eat a variety of grains to “minimize potential adverse health consequences from eating an excess of any one food.”
But it’s not just arsenic and cadmium, which are present in soil both as naturally occurring elements and as industrial byproducts. Recent studies have shown that rice is custom-built to pull a number of metals from the soil, among them mercury and even tungsten. The findings have led to a new push by scientists and growers to make the grain less susceptible to metal contamination.
The highest levels often occur in brown rice, because elements like arsenic accumulate in bran and husk, which are polished off in the processing of white rice. The Department of Agriculture estimates that on average arsenic levels are 10 times as high in rice bran as in polished rice.
Although these are mostly tiny amounts — in the part per billion range — chronic exposure to arsenic, even at very low levels, can affect health. The F.D.A. is now considering whether a safety level should be set for arsenic in rice.
“Rice is a problem because it’s such a widely consumed grain,” said Rufus Chaney, a senior research agronomist with the U.S.D.A.’s Agricultural Research Service, who is leading a investigation of metal uptake by food crops. “But it’s also a fascinating plant.”
Like people, plants have systems for taking up and absorbing necessary nutrients. In plants, these “transporter” systems work to pull minerals such as iron, calcium, zinc and manganese from the soil.
The rice plant has a well-designed system for taking up silicon compounds, or silicate, which help strengthen the plant and give stiffness and shape to its stems. Tissues generally referred to as phloem move such water-soluble nutrients throughout the plant.
But that delivery system also inclines the plant to vacuum up arsenic compounds, which are unfortunately similar in structure to silicate. And the traditional methods of growing rice, which often involve flooding a field, encourage formation of a soluble arsenic compound, arsenite, that is readily transported by the rice plant.
“The issue with the rice plant is that it tends to store the arsenic in the grain, rather than in the leaves or elsewhere,” said Jody Banks, a plant biologist at Purdue University, who studies arsenic uptake in plants. “It moves there quite easily.”
The highest concentrations of arsenic in rice-growing regions are mostly found in parts of Asia — including Bangladesh and India — where the underlying arsenic-rich bedrock contaminates groundwater used for both drinking and irrigation of rice fields.
But arsenic at lower levels is found in all soils, including American fields. The fertile soils fanning out across the Mississippi River floodplain are up to five times as high in arsenic as other parts of Louisiana, Mississippi and Arkansas, according to studies done by the United States Geological Survey.
It’s for that reason, as well as for water conservation, that scientists have experimented with reducing the amount of water used for rice fields. But as Dr. Guerinot has found, that makes cadmium more available to the plant instead.
Other plants also take up cadmium, Dr. Chaney noted, usually by the channels normally used to acquire zinc from the soil. But the rice plant, curiously, absorbs nearly all of its cadmium through a manganese transport system. And this route — discovered by a determined group of Japanese researchers — brings a new set of complications.
While zinc is relatively common in soil, soluble manganese is less readily found. So cadmium has little competition in the rice plant’s transport system — meaning that it is accumulated with apparent enthusiasm.
The association between cadmium in rice and human disease goes back decades. Most scientists cite the identification of itai-itai (ouch-ouch) disease in Japan during the 1960s as the first recognition of this problem. The name comes from the painful effects of bone fractures, one of many health problems related to cadmium exposure.
Researchers eventually discovered that cadmium pollution from mines and other industry had spread into rice farming areas in Japan, causing the grain to be loaded with the toxic metal. A host of similar problems have occurred in China, setting off an uproar over tainted rice last year.
Scientists say that the cadmium occurring naturally in American soil is not high enough to cause acute disease. Still, because rice is such an important food crop, scientists are searching for ways to block its metal-acquiring tendencies.
There are efforts to breed rice plants that transfer more zinc and iron into the grain, which would both increase nutritional quality and reduce toxicity. There are also programs, including the experiment in Texas, that try to breed improved rice cultivars less prone to absorb toxic minerals.
And researchers have explored the idea of genetic engineering to make the plant’s transport systems more precise so that cadmium or arsenic is filtered out.
Finally, they are looking into using other plants to reduce the toxic elements in the soils themselves, a process called phytoextraction. Dr. Banks, for instance, is studying a fern that deftly pulls arsenic from the soil and stores it in the fronds.
The plant, known as a Chinese brake or ladder fern, is so talented in this regard that the Chinese have approached American scientists about the feasibility of using it to clean up contaminated soils. Of course the ferns eventually have to be incinerated or taken to a toxic disposal site.
“You definitely wouldn’t want to eat them,” said Dr. Banks.