[Screening and Identification of Arsenic-resistant Endophytic Bacteria from Different Rice Tissues (Oryza sativa L.) in Different Growth Stages].

Soil microorganisms are critical for arsenic transport and transformation in the soil-plant system. Endophytes are a very rich microbial resource, but to date no study has reported the responses of endophytes to arsenic pollution. In this study, the cultivable endophytic bacteria were isolated from the root, stem, leaf, and panicle of rice (Oryza sativa L. cv. Yongyou-538) at the seedling stage, tillering stage, flowering stage, and productive stage. All isolates were first screened by 1 mmol ·L-1 arsenite[As (Ⅲ)] and 10 mmol ·L-1 arsenate[As (Ⅴ)], and further screened by 2 mmol ·L-1 As (Ⅲ) and 20 mmol ·L-1 As (Ⅴ). From this study, a total of 126 strains of rice endophytes were obtained, belonging to 13 genera. Among them, 37 strains (8 genus) were isolated from the rice seedling stage, 25 strains (5 genus) were isolated from the rice tillering stage, 24 strains (8 genus) from the rice flowering stage, and 30 strains (8 genus) from the rice productive stage. The dominant genera were Bacillus sp., Psoudomonas sp., and Acinetobacter sp. After twice screening, 20 strains were found to be resistant to 1 mmol ·L-1 As (Ⅲ) and 10 mmol ·L-1 As (Ⅴ), including 16 strains that were resistant to 2 mmol ·L-1 As (Ⅲ) and 13 strains were resistant to 20 mmol ·L-1 As (Ⅴ). Importantly, the strain CS1 was found to be the most resistant bacteria to both As (Ⅲ) and As (Ⅴ) among all the isolated strains.

[Effects of CaCO3 Application on Soil Microbial Nitrogen Cycle in an Acid Soil].

Soil nitrification and denitrification are important steps in closing the nitrogen cycle. Understanding the effects of CaCO3 application on the physicochemical properties and nitrogen cycle in acid soil would provide some theoretical and technical information for stable and sustainable utilization of this agricultural soil. In this study, a field trial was conducted in Xiangtan city, Hunan province. In this field, the soil pH was 5.54, and CaCO3 was applied at four levels: 0, 2.25, 4.5, and 7.5 t·hm-2. After one season of rice cultivation, the soil was sampled to determine dissolved organic carbon (DOC), NH4+-N, and NO3--N levels, and the potential nitrification rate (PNR) and denitrifying enzyme activity (DEA) were measured. The results showed that CaCO3 application improved the content of soil DOC (762.10-868.58 mg·kg-1) and PNR [0.59-0.82 µg·(g·h)-1]. However, excessive application of CaCO3 (7.5 t·hm-2) revealed an obvious inhibition on the activity of soil nitrification. Furthermore, the result of Pearson correlation analysis indicated that soil nitrification was positively correlated with soil DOC and negatively correlated with NH4+-N content, whereas denitrification had a significant positive correlation with NO3--N content and soil DOC, but a negative correlation with water content.

[Effects of Straw Incorporation on Cadmium Accumulation and Subcellular Distribution in Rice].

Cadmium (Cd) is classified as a Group-1 human carcinogen and rice consumption constitutes a major source of dietary intake of Cd for populations whose staple food is rice. Straw incorporation is widely performed in Cd-contaminated paddy fields, which may significantly affect the bioavailability of Cd in soil and the distribution of Cd in rice plants, consequently altering Cd accumulation in rice grains. In this study, both pot and field trials were conducted to investigate the effects of different amounts of straw incorporation (0.0%, 1.0%, 2.5%, and 5.0%) on Cd sub-cellular distribution in rice plants and Cd accumulation in rice grains. The results showed that Cd was mainly sequestered in cell wall, accounting for 86%-95% and 30%-51% of total cadmium in root and shoot cells, respectively. In shoot cells, about 35%-61% of Cd was distributed in cellular soluble fractions. When rice straw was incorporated at 1.0% and 2.5% levels, Cd sequestration in the cell wall significantly increased and Cd translocation from roots to shoots significantly decreased. However, when rice straw was incorporated at the 5% level, Cd sequestration in root cell walls significantly decreased and Cd translocation from roots to shoots significantly increased at the tillering stage. At the filling stage, 5% rice straw incorporation still significantly increased Cd sequestration in root cell walls and Cd translocation from roots to shoots did not significantly change. The rice straw and rape straw used for the field trail contained high concentrations of Cd (0.49 and 0.67 mg·kg-1, respectively). Rape straw incorporation alone or together with lime did not significantly affect Cd accumulation in brown rice or rice straw. Rice straw incorporation alone did not significantly affect Cd accumulation in brown rice or rice straw, while incorporation with lime significantly decreased Cd accumulation in both brown rice and rice straw. Biochar application can also significantly reduce Cd accumulation in rice and when biochar was added together with lime, the reduction in Cd accumulation in rice was more significant. Therefore, at Cd-contaminated paddy fields, rice straw or rape straw is not suggested to be returned directly; incorporation with lime would be better for reducing Cd accumulation in rice grains. The results of this study will provide theoretical and practical guidance for the safe production of rice and for straw recycling at Cd-polluted paddy fields.

Observation of large group index enhancement in Doppler-broadened rubidium vapor.

We report experimental observation of large group index across the Lamb dips of ground hyperfine states in Doppler-broadened 87Rb vapor. By sweeping the laser frequency through each hyperfine transition we measure the saturated absorption and optical phase shift using a phase-locked Mach-Zehnder interferometer. Our measurements provide a direct demonstration of the theoretical prediction by Agarwal et al. [G. S. Agarwal and T. N. Dey, Phys. Rev. A 68, 063816, (2003)] for the first time. An enhancement factor as large as 1005 in group index was observed for Rb vapor at temperature of 85 °C. The experimental data are in good agreement with the theory.