Abstract: The results from present research indicated that the activities of superoxide dismutase (SOD) and catalase (CAT) in flag leaves of hybrid rice decreased as the plant aged, exposed to different nitrogen rates, while the MDA contents increased. Peroxidase (POD) showed its different trend. The POD activities slightly decreased first, then dramatically increased from heading to ripening stages. It was appeared there was significantly positive correlation between SOD and CAT and significantly negative correlation between the activities of the two enzymes and MDA concentrations. The dynamic property of POD activities was more complicate. The nitrogen rates had influences on the cell defense enzyme systems, but had different effects and attributions to the different enzymes. Finally, the relationships between defense enzyme systems and heterotic manifestation in rice were discussed.

Key words: sewage treatment, emergy analysis, ecological footprint, sustainability.