Abstract: 【Objective】 Rice is most sensitive to temperature from booting stage to heading and flowering stages, so cold damage during this stages severely limited rice growth and the further expansion of acreage in Northeast China. Study on the physiological mechanism of rice cold tolerance will provide an important safeguard for the rice production in Northeast China. The aim of this study was to understand the changes of reactive oxygen species metabolism, antioxidant enzyme activities and antioxidants contents in flag leaves of weedy and cultivated rice with different chilling sensitivity at flowing stage under cold water stress, and provide a theoretical basis for the breeding and cultivation of rice tolerance to cold. 【Method】 Two chilling-tolerant genotypes, weedy rice WR03-45 and cultivated rice Lijiangxintuanheigu, and two chilling-sensitive genotypes, weedy rice WR03-26 and cultivated rice Xiuzinuo, were used and grown in plastic pots. The time of booting stage of main spike was calculated according to length of the flag leaf collar. The tested materials were subjected to low temperature in cold water pool at booting stage, and the control was the tested materials without cold water treatment. In order to study  the effects of cold water stress on reactive oxygen species (ROS) metabolism, antioxidant enzyme activities and antioxidants contents of flag leaves at flowing stage, the flag leaves of tested materials were clipping and saved in ultra-low temperature freezer at -85 ℃, which were used to measure superoxide radical () generating rate, hydrogen peroxide (H₂O₂) content, malondialdehyde (MDA) content, activities of antioxidant enzymes of superoxide dismutase (SOD), guaiacol peroxidase (POD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR) and non-enzymatic antioxidants contents of ascorbate acid (AsA) and reduced glutathione (GSH). 【Result】 Under cold water stress, only CAT and GR activities in flag leaves of chilling-sensitive weedy rice WR03-26 increased significantly at flowering stage, and SOD, CAT and GR activities in flag leaves of chilling-sensitive cultivated rice Xiuzinuo increased significantly at flowering stage. There were no significant difference in contents of AsA and GSH and ratio of AsA/DHA and GSH/GSSG in flag leaves of WR03-26 and Xiuzinuo compared with their controls.  generating rate and MDA content increased significantly, and H₂O₂ content increased very significantly in flag leaves of WR03-26 and Xiuzinuo. SOD activity increased significantly, and POD, APX, CAT and GR activities increased very significantly in flag leaves of chilling-tolerant weedy rice WR03-45 at flowering stage. SOD, POD and APX activities increased significantly, and CAT and GR activities increased very significantly in flag leaves of chilling-tolerant cultivated rice Lijiangxintuanheigu at flowering stage. Antioxidants (AsA and GSH) contents and AsA/DHA ratio and GSH/GSSG ratio also increased significantly in flag leaves of WR03-45 and Lijiangxintuanheigu under cold water stress.  generating rate, H₂O₂ content and MDA content in flag leaves of WR03-45 and Lijiangxintuanheigu had no significant difference compared with their controls. 【Conclusion】 Under cold water stress, the chilling-tolerant rice could raise the antioxidant enzyme activities and antioxidants contents, decrease the levels of ROS and the membrane lipid peroxidation and improve the tolerance to cold water stress. Chilling tolerance in rice is associated with the capacity of antioxidative system removing ROS under chilling stress. Weedy rice WR03-45 is useful as a germplasm resource of strong chilling tolerance for improving chilling tolerance in cultivated rice.

Key words: rice (Oryza sativa L.), antioxidant defense system, cold water stress, reactive oxygen species