Rice Nitrogen and Antioxidant Enzymes

Rice plants with consistent growth and development (sampling by population mean stem number) were selected at the panicle initiation and heading stage and 20 days after heading. The flag leaves (the first fully expanded leaf under the heart leaf before heading) were sampled and frozen in liquid nitrogen and stored at -80°C to analyse their nitrogen metabolic enzymes and antioxidant enzymes. Nitrate reductase (NR) activity was determined according to the in vitro method described by Li et al. (2000) . The enzyme activity was expressed in the number of micrograms of NaNO₂ produced per gram of sample per hour (μg/(h·g) (calculated as NaNO₂, the same below). Glutamine synthetase (GS) was determined according to the method described by Wang et al. (2005) . Glutamate dehydrogenase (GDH) was determined according to the method described by Masclaux et al. (2000) (link). The superoxide dismutase (SOD) activity and peroxidase (POD) activity were measured according to the method described by Qiu et al. (2010) (link). Catalase (CAT) activity was measured using the UV absorption method (Zeng et al., 1991 ).