For the financial support we are grateful to the National Natural Science Foundation of China (No. 21007014, 21107024 ), the Start Foundation of Hunan Agricultural University (No. 10YJ01 ), the National Science and Technology Major Projects ( 2009ZX07212-001-05 ), the National Environmental Protection Public Welfare Program (No. 201009047 ) and Key Laboratory of Production Environment and Agro-product Safety of Ministry of Agriculture, Tianjin Key Laboratory of Agro-environment and Food Safety ( 2010-KJ-KF-03 ) and Scientific Research Fund of Hunan Provincial Education Department ( 11C0650 ).