Abstract: Photoperiod-sensitive genic male sterile ricc-Nongken 58S and wild type Nongken 58 (Oryza sativa L. subsp, japonica) were treated with long day (LD) cycles and short day (SD) cycles, respectively, from the secondary rachis branch primodia of young panical differentiation for 10 days. LD treatments inhibited the activities of ascorbate peroxidase (AsA POD) of Nongken 58S and Nongken 58, compared with SD treatments. The LD inhibitory effect of the AsA POD activity in Nongken 585 was more than that in Nongken 58. The contents of ascorbate (ASA) and MDA gradually increased as the AsA POD activities. decreased. There was a negative correlation between the AsA POD activity and the AsA or MDA content. The time of LD inhibition of the AsA POD activities coincided with that of LD inhibition of young panical development. It was suggested that the changes, which trigger other biochemical chains, of the AsA POD activities in leaves might affect the young panical development.