Mulberry ( Morus alba L.) seedlings were used to test hydrogen peroxide (H2O2 ) as a potential systemic messenger in photosynthetic induction. The upper leaf of mulberry was dark-adapted for 45 min and then illumi-nated with photosynthetically active radiation (PAR) of 1000 μmol m ?2 s ?1 . Photosynthetic induction and H2O2 con-tent in the lower leaf was measured. The results show that pre-illumination of the upper leaf promoted photosynthetic induction and increased endogenous H2O2 in the lower leaf. Without pre-illuminating upper leaf, exogenous H2O2 treat-ment on the lower leaf promoted photosynthetic induction. The application of diphenyleneiodonium and trichloroacetic acid on petioles of the upper leaf inhibited H2O2 increase in the lower leaf, indicating that H2O2 transport was from upper leaves to lower leaves through the phloem. The results show that H2O2 might serve as a signal messenger to promote rapid induction of photosynthesis of leaves in lower parts of the canopy and enable plants to use light energy more efficiently.