Abstract:During the period of senescence of eucalyptuses (Eucalyptus grandis×E.urophylla) cultivars ‘3229’ and ‘U6’ phellogen,a comparative study on changes of antioxidant enzymes,chlorophyll content,the contents of malondialdehyde (MDA),hydrogen peroxide (H2O2),activities of lipoxygenase (LOX) and DNA dynamic changes were conducted.Data showed that chlorophyll content decreased,but the contents of MDA and H2O2 and activity of LOX increased in both cultivars ‘3229’ and ‘U6’.In ‘3229’,the sample collected at the ripe stage of new phellogen formation (NPF) showed significantly high superoxide dismutase (SOD) activity compared to that collected at the early and detached stage of NPF,while the latter two had no significant difference in enzyme activity.In ‘U6’,however,no significant difference was found among the samplings for different batches,although the sample of the ripe stage of NPF displayed the highest SOD activity.Activity of ascorbate peroxidase (APX) significantly increased from the early to ripe stage of NPF and thereafter,declined.In ‘3229’,peroxidase (POD) activity significantly increased between the ripe and detached stage of NPF,but in ‘U6’,it did between the early and ripe stage of NPF.DNA ladders were also observed in the eucalyptuses phellogen.The results suggested that senescence in eucalyptuses phellogen could involve programmed cell death (PCD),reduced antioxidant activity and increased H2O2.