September 19, 2012

In the United States, there has been evidence for over a century of a relationship between arsenic and a physiological disease in rice known as straighthead. Straighthead causes the seedhead of the plant to be empty or partially empty, and can result in complete yield loss in farmers’ fields. Although it is not common in U.S. rice fields, straighthead has been reported in rice growing areas around the world.

Arsenic occurs naturally in soils and has also been used as a pesticide in U.S. agriculture—more commonly in the past than today. Also, the herbicide MSMA contains arsenic.

Historically, all rice in the United States has been grown in fields that are flooded throughout  the growing season. However, it is known that a way to prevent straighthead from occurring is to drain rice fields prior to flowering to the point that the ground is dry, and then reflood.

Thus, although the relationship between arsenic, flooded fields, and straighthead has not been well understood, use of resistant varieties and draining rice fields are methods growers have used to mitigate this problem for decades. In addition, for many years ARS researchers have helped to identify these straighthead resistant cultivars for breeders and growers to use.

Although high levels of naturally occurring arsenic in soil and water have been observed in some rice-producing areas like Bangladesh, this has never been a concern for U.S. rice production.  However, a 2005 report by Williams et al. indicated that U.S. rice had high levels of total arsenic in the grain, as compared to rice  grown in  some other countries. Because of this, ARS has collaborated with other researchers to better understand the interrelationship of genetic variability, cultural management practices, soil chemistry, straighthead symptoms, and arsenic accumulation in rice.

Pillai et al. 2010 evaluated approximately  25 rice cultivars from the United States and other rice-producing countries  grown in soil that had not been treated with arsenic (called “native soil”), and found significant differences among cultivars for accumulation of inorganic (considered more toxic) and organic forms of arsenic. These results indicated that there is a genetic component to accumulation and/or transformation of arsenic in its inorganic and organic forms in the rice plant.

In 2011, Hua et al. compared the impact of using flooded versus intermittent flooding—in which the soil was periodically allowed to dry—on availability of arsenic when the herbicide MSMA was applied. The study showed that under flooded conditions, the soil lacked oxygen,  and this resulted in the release of iron and arsenic into the soil solution, making them more available for plant uptake. They concluded that the combination of water management practices that allow periodic aeration of the soil and use of cultivars that are low accumulators of arsenic can reduce arsenic in the grain.

Somenahally et al. (2011) demonstrated that irrigation practices change microbial communities in flooded rice fields.  Under MSMA-treated soils where arsenic levels were high, fields that were maintained under a permanent flood had high concentrations of iron-reducing bacteria (bacteria that can make iron and arsenic already present in the soil more soluble, and therefore more available for uptake by the plant), as compared to soil that was only intermittently flooded. These bacteria resulted in greater iron  and arsenic release into the soil solution under flooded conditions.

Thus, using irrigation practices that allow intermittent aeration of the soil shifts microbial communities to those that can bind arsenic, reducing its availability for uptake by the plant. In a subsequent report by the same group, they demonstrated that this also resulted in lower arsenic accumulation in the rice grain.

Research by Norton et al. 2011 demonstrated that there is significant genetic variability for grain arsenic accumulation among a set of several hundred global cultivars. They found that aerobic soil conditions resulted in a 10-fold lower amount of arsenic uptake among the varieties as compared to a flooded field. In addition, a comparison of more recently released U.S. cultivars versus historical U.S. rice varieties showed that newer varieties accumulated significantly less grain arsenic than previously used cultivars. It is not clear if this is related to the use of varieties that have reduced height (semidwarfs with less biomass in which to store arsenic), are earlier maturing (which would give the plant less time to accumulate arsenic), or are more straighthead-resistant.

Current studies are underway by ARS to evaluate some 50 U.S. cultivars under high arsenic (MSMA-treated) and native soils and using permanently flooded  and intermittently flooded fields.  We hope to determine if there are current commercially acceptable cultivars that have relatively lower grain arsenic contents even under soil and water management conditions where arsenic is readily available. In addition, this study will demonstrate to what extent that water management and cultivar choice can mitigate grain arsenic in rice.

Research with colleagues at the University of Arkansas is underway to evaluate a range of irrigation practices to determine their impact upon yield, milling quality, and grain arsenic accumulation. These results will be used to determine if it is economically feasible to use less irrigation water (intermittent flooding) and still maintain high yields and good milling quality while reducing arsenic in the grain.

Results from an initial laboratory study conducted by ARS researchers in Fayetteville, AR, have shown that adding aluminum sulfate to arsenic-treated soil can reduce arsenic solubility by 70 percent. A follow-up study is now underway in the field to evaluate the impact of different rates of aluminum sulfate on arsenic accumulation in rice plant leaves and grain when grown on a soil where MSMA has been applied.

Although research has been conducted by ARS that has identified genetic markers that are associated with straighthead resistance, work is now underway to determine genetic markers associated with reduced grain arsenic accumulation (Zhang et al., submitted) and to see if these chromosomal regions are different than the location of straighthead resistance genes.

Citations

Hua,B., W. Yan, J. Wang, B. Deng, and J. Yang. 2011. Arsenic accumulation in rice grains: effect of cultivars and water management practices. Environ. Eng. Sci. 28: 591-596.

Norton, G.J., Pinson, S.R., Alexander, J., Mckay, S., Hansen, H., Duan, G., Islam, M., Islam, S., Stroud, J.L., Zhao, F., Mcgrath, S.P., Zhu, Y., Lahner, B., Yakubova, E., Guerinot, M., Tarpley, L., Eizenga, G.C., Salt, D.E., Meharg, A.A., Price, A.H. 2012. Variation in grain arsenic assessed in a diverse panel of rice grown in multiple sites. New Phytologist. 193: 650–664.

Pillai, T., W. Yan, H. Agrama, W.D. James, A. Ibrahim, A. McClung, T. Gentry, and R.H. Loeppert. 2010.Total grain arsenic and arsenic species concentrations in diverse rice cultivars under flooded conditions. Crop Sci. 50: 2065-2075.

Somenahally, A.C., E.B. Hollister, R.H. Loeppert, W. Yan, and T. Gentry. 2011. Microbial communities in rice rhizosphere altered by intermittent and continuous flooding in fields with long-term arsenic application. Soil Bio. & Biochem. 43: 1220-1228.

Somenahally, A.C., E.B. Hollister, W. Yan, T. Gentry, and R.H. Loeppert,. 2011. Water management impacts on arsenic speciation and iron-reducing bacteria in contrasting rice-rhizosphere compartments. Environ. Sci. Technol. 45: 8328-8335.

Zhang, M., S.R.M. Pinson, L. Tarpley, M.L. Guerinot, B. Lahner, E. Yakubova, I. Baxter, D.E. Salt. Mapping and validation of quantitative trait loci associated with concentration of 16 elements in unmilled rice grains.   Theoretical and Applied Genetics submitted

 (bold italics indicate ARS researchers)