The aim of this work was to characterize the phot1 mutant of rice during early seedling growth in various light conditions. We isolated the rice T-DNA insertion mutant phot1a-1 and compared it to the Tos17 insertion mutant phot1a-2. When phot1a mutants were grown under WL (100) and BL (40 miccromol m(-2) s(-1)), they demonstrated a considerable reduction in photosynthetic capacity, which included decreased leaf CO(2) uptake and plant growth. Pigment analysis showed no significant difference between wild-type and mutants in the Chl a:b ratios, whereas in the latter, total concentration was reduced (a 2-fold decrease). Carotenoid contents of the mutants were also decreased considerably, implying the involvement of phot1a in pigment degradation. Deletion of phot1a showed higher contents of H(2)O(2) in leaves. Chloroplastic APX and SOD activities were lower in the mutants whereas the activities of cytosolic enzymes were increased. Immunoblotting indicated reduced accumulation of photosystem proteins (D1, D2, CP43, Lhca2, and PsaC) relative to the other light-harvesting complexes in the mutant. We conclude that the defect of Os Phot1a affects degradation of chlorophylls and carotenoids, and under photosynthetically active photon fluxes, mutation of phot1a results in loss of photosynthetic capacity owing to the damage of photosystems caused by elevated H(2)O(2) accumulation, leading to a reduction in plant growth.