Cultivar differences in inorganic arsenic accumulation in the grain as influenced by irrigation management and soil amendments

Authors: McClung, Anna; Green, Carrie; Codling, Eton; Chaney, Rufus

Submitted to: Rice Technical Working Group Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 12/4/2019
Publication Date: 1/6/2021
Citation: Mcclung, A.M., Green, C.E., Codling, E.E., Chaney, R.L. 2021. Cultivar differences in inorganic arsenic accumulation in the grain as influenced by irrigation management and soil amendments. Rice Technical Working Group Meeting Proceedings. p. 88-89.

Interpretive Summary:

Technical Abstract: Arsenic is a naturally occurring element found in the soil, however exposure to high levels can result in serious health effects in humans. Numerous reports have demonstrated that rice which is generally grown in flooded paddies where anerobic conditions in the soil can result in arsenic being made available for plant uptake can result in high levels of arsenic in the grain. Although milling and cooking rice in excess water are methods which reduce arsenic in the grain prior to consumption, other methods which reduce the accumulation of arsenic in the grain are more desirable. Irrigation practices like alternate wetting and drying (AWD) which result in rice fields being subjected to repeated anerobic and aerobic conditions have been shown to effectively reduce arsenic uptake in rice plants. In addition, research has shown that rice cultivars differ in the amount of arsenic that is accumulated in the grain. This study was conducted to determine differences among US rice varieties for grain arsenic accumulation in response to two irrigation management systems using native soil and fields amended with monosodium methyl arsenate (MSMA), an arsenic-based broad spectrum herbicide. The study was conducted across two years at Stuttgart, AR and included 48 southern US cultivars, three commercial hybrids, and three international cultivars arranged using an RCB design with three replications. Adjacent experiments were conducted using conventional flood management and AWD on native and MSMA amended soils. Plots were evaluated for days to heading, plant height, days to maturity, grain yield, and straighthead rating (scale of 0 resistant to 8 very susceptible). Rough rice samples were dehulled and approximately 25 g of brown rice were ground to a fine powder using a UDY Cyclone Sample Mill (0.8mm mesh). A sample size of 0.7g was weighed out and mixed with 10 ml of 0.28 M HNO3; digested in a preheated DigiPrep Hotblock at 95°C for 90 minutes, filtered and brought to a 20 ml final volume with 0.28 M HNO3. Inorganic arsenic (iAs) in the extractions was measured using a modified ICP-OES hydride generation method. Initial analysis for iAs was performed only on the native soil treatment and results are reported here. Analysis of variance performed by irrigation treatment demonstrated that the variety effect was 2 to 16 fold greater than the year x variety interaction, thus the data were pooled across years. Varieties were significantly different for all traits measured. In general, the flooded treatment as compared to the AWD resulted in plants being about 2 d earlier, 5-7 cm taller, and yields being 11-14% greater. However, the study conducted using the native soil showed no difference in iAs in brown rice between the two irrigation treatments. In addition, the range in iAs among the varieties was 2.4 fold under the flood+native soil treatments with Zao 402 having the highest (257 ppb) and Saber the lowest (108 ppb). These results were similar to the AWD+native soil treatments where there was a 1.7 fold difference in iAs with Jefferson being the lowest (122 ppb) and CL XL745 being the highest (203 ppb). Correlations between iAs with heading (r= -0.39), with height (r= 0.37) and with grain yield (r=0.50) when grown under flood+native soil conditions indicate rapidly growing varieties with high yield potential tend to accumulate greater amounts of iAs in the grain. Of the five indica cultivars included in the study, four were ranked in the top nine for mean level of iAs, regardless of irrigation treatment. However, significant variation among the cultivars indicated that varieties having both high yield and low arsenic accumulation could be identified under native soil conditions, regardless of irrigation treatment. These results demonstrate that there is significant variability among US cultivars for iAs in brown rice suggesting genetics can