Study On The Speciation And Transformation Of Arsenic In Rice

It is very important to guarantee the security of rice which is one of the most primary food in China. At present, the contamination of arsenic(As) is the main problem in rice. Because of the differences of arsenic toxicity between speciations, the security evaluation of limited arsenic content was not only to investigate the total arsenic and inorganic arsenic, but also to consider small organic arsenic speciation contents in rice. In recent years, there have been more and more studies for the metabolism of arsenic speciation in rice. But most of these studies have focused on the grain ripening rice or the distribution of As with a single point in the plants. Few studies were focused on the accumulation of arsenic in rice grain and the analysis of mercapto compounds in rice under arsenic stress. In this paper, two typical rice cultivars Oryza sativa L. japonica. cv. Nipponbare and Oryza sativa L.indica. cv. 93-11 were chosed as experimental materials. In order to explore the mechanism of As tolerance, the different temporal and spatial distributions of arsenic species and mercapto compounds were studied under arsenic stress in rice. The results are as follows:1. Established the method for the simultaneous determination of As(V), As(III), As B, MMA and DMA in rice by liquid chromatography-inductively coupled plasma-mass spectrometry(LC-ICP-MS). The extraction reagent was 0.3 mol/L nitric acid with heat-assistant condition for 1.5 h at 95 ℃. Then, five arsenic species were separated by an anion exchange column(Dionex Ion Pac AS19, 250 mm × 4 mm) and detected by inductively coupled plasma-mass spectrometry(ICP-MS). The results showed that the method was much more precise for the risking assessment of the rice. This method was simple, accurate and durable for the determination of arsenic species in rice.2. As(III) was the main arsenic speciation in grains under As(III) and As(V) treatment, while the main arsenic speciation was DMA under DMA treatment. The content of DMA counts for 90% of the total arsenic. The results showed a significant positive correlation with DMA in grain under the treatment time of DMA. Compared with inorganic arsenic, DMA was more easily accumulated in grain and husk. The spatial distribution of As in rice plant was: root > stem > leaf > shell kernel and the root was the main parts of stranded arsenite.3. Arsenic speciation in the xylem and phloem sap depends on the supplying arsenic speciation. The contents of As is: DMA > As(III) > As(V) in xylem and phloem sap, while the content of DMA counts for 48%~65% of the total arsenic. From the different organization to the grain, the transloction efficiency for As(III) and As(V) were all below 1, while the transloction efficiency for DMA were all above 1. In addition, under the same concentration of As treatment, compared to other tissues and organs, the accumulation of DMA, As(III) and As(V) in flag leaf was not gap, but the transloction efficiency for DMA from flag leaf to grain was significantly greater than that of inorganic arsenic.Rice can load DMA into grains efficiently through the phloem.4. Under As(III) and DMA stress, roots and leaves of rice seedlings could produce GSH, PC2, PC3 and PC4. The yield of GSH in roots was lower than the leaves under the treatment of low concentration of arsenic, but the content tended to decrease, then increased with the concentration increased. The results showed a significant positive correlation with PC2, PC3 and PC4 in root, especially the content of PC2 under the concentrations of As(III) treatment. With DMA and As(III) treated leaves and DNA treated roots, the correlation between PC2, PC3 and PC4 and concentrations of As was weak.