DEVELOPMENT AND CHARACTERIZATION OF GENETIC RESOURCES FOR AGRONOMIC AND QUALITY TRAITS USING GENOMIC TOOLS
Location: Dale Bumpers National Rice Research Center
Title: Approaches to eliminating arsenic hazards in rice
Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: December 15, 2009
Publication Date: February 22, 2010
Citation: Loeppert, R.H., Pillai, T.R., Somenahally, A., Yan, W., Gentry, T.J., McClung, A.M. 2010. Approaches to eliminating arsenic hazards in rice [abstract]. Rice Technical Working Group Meeting Proceedings, February 22-25, 2010, Biloxi, Mississippi. p. 130-131.
Arsenic (As), which is toxic to plants and animals, is especially problematic to rice since the predominant As species are more soluble and more bioavailable under the reduced conditions of flooded rice culture. Two major problems have received international attention in efforts to minimize potential As hazard in rice: (i) the occurrence of As in grain and the related issues of food quality and (ii) As toxicity to rice and its implications to crop yield, food security, and agricultural sustainability. All soils contain As, with background concentrations generally ranging from 0.2 to 5 micro-g g-1. The primary sources of soil As are the weathering of naturally occurring soil parent materials and sediments, the use of As-contaminated irrigation water, and the use of arsenical pesticides and defoliants in agricultural. In this paper, we discuss our experiences during the past six years in the south central United States, where the occurrence of As and its subsequent toxicity result primarily from the historical use of arsenical pesticides and defoliants.
Arsenic in soil exists in two predominant oxidation states: AsIII and AsV. The primary species of interest are the inorganic (iAs) and mono- and di-methyl As forms. All forms of As are toxic to plants and animals, except for the dimethyl AsV species that has a relatively low toxicity. All forms of As are considerably more soluble and bioavailable under flooded than under dryland conditions. In spite of the fact that arsenicals are no longer used in production agriculture, soil As persists, which indicates the very low rates of net As loss. Though soil microbes are
involved in both methylation and demethylation processes, there is a net demethylation with the rate being highly dependent on soil and environmental conditions. A MMAsV demethylation half-life of approximately 0.5 to 1.0 year is reasonable to expect. There is considerable genetic variability (up to 4 fold) in As uptake and grain-As concentration of rice. The predominant As species in rice grain are iAsIII and dimethyl AsV. Rice grain in the south central United States tends to be considerably higher in the proportion of dimethyl AsV (DMAsV) to total grain As (TGAs) than the rice grain produced in south Asia. The reason for this difference (plant genetics vs. the occurrence of trace concentrations of methyl As in soil vs. crop management) remains a matter of intense interest. Arsenic can result in a reduction in rice-grain yields by the increased occurrence of straighthead, reduced tillering, and reduced root health. Though there is considerable genetic variability in relative susceptibility to straighthead, recent studies have indicated that there is poor correlation between this trait and grain-As concentration and speciation, indicating that different genes are likely involved in these respective processes. Several soil factors strongly impact As toxicity to rice, e.g., soil Fe-oxide mineralogy and content, soil silicate, phosphate and N status, and soil organic matter. The impact of organic matter is especially intriguing. Arsenic concentrations in grain are almost always increased, and tolerance to As is often reduced upon the addition of fresh organic matter to soil. The reduced tolerance to As might be attributable to decreased rhizosphere redox potentials and increasing As solubility, though it could also be strongly impacted by altered microbial populations and functions, which likely impact As speciation and local As dynamics. These complex processes deserve further study. Water management can have a profound impact on plant tolerance to As and the uptake of both iAs and methyl As species and tolerance to As toxicity. A primary impact of reduced water systems is on increased rhizosphere redox potentials and decreased As solubility; though, a second major factor is the altered microbial community population that can strongly impact As dynamics. Reduced water-use systems, e.g., intermittent flooding, can be used to greatly reduce As uptake and grain-As concentration. By means of combined plant selection and water-management strategies, the potential exists to virtually eliminate As as a concern in rice production.