Abstract
We have already shown that chlorophyll biosynthesis in cucumber plants grown under low nitrogen conditions was promoted and suppressed by the application of abscisic acid (ABA) and cytokinin, respectively. Furthermore, the amount of O_2- and H_2O_2 increased in cotyledons and first leaves of cucumber plants treated with kinetin, whereas it decreased in the presence of ABA. The activities of reactive oxygen species (ROS) degrading enzymes such as ascorbate peroxidases (APX), catalase (CAT), glutathione reductase (GR) and superoxide dismutase (SOD) were determined in cucumber plants grown under low nitrogen conditions in the presence of ABA or kinetin. The activity of APX and SOD in first leaves of cucumber plants increased remarkably but that of GR increased slightly in the presence of ABA, although almost no difference in activities of these enzymes in cotyledons was observed in both treatments. No significant difference in activities of these enzymes was found in first leaves treated with kinetin except the activity of SOD slightly increased in first leaves of cucumber plants treated with 10^<-5> M kinetin. These results suggest that ABA increased activities of ROS-degrading enzymes and suppressed leaf yellowing in cucumber plants grown under low nitrogen conditions. Activities of these enzymes were not suppressed by kinetin, although ROS still accumulated. The total amounts of ROS production might be exceeded ones degraded by ROS-degrading enzymes.
