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Root system morphology (RSM) in plants plays a key role in acquiring nutrients from the soil and is also altered by abiotic or biotic factors including soil microorganisms and signal molecules. The present study was made to evaluate the effects of an arbuscular mycorrhizal fungus (AMF, <i>Glomus versiforme</i>) and exogenous peroxide hydrogen (H₂O₂, 0, 1 and 100 &#956;M) on root length, lateral root number and activities of polyamine-metabolized enzymes in trifoliate orange (<i>Poncirus trifoliata</i>) seedlings. After 5 months of inoculation with AMF, root mycorrhizal colonization was significantly increased by application of 1 &#956;M H₂O₂, but markedly restrained by 100 &#956;M H₂O_2. Inoculation with AMF significantly increased the taproot length and the number of second- and third-order lateral roots under 1 and 100 &#956;M H₂O₂application. The AMF infection significantly increased 0-1 cm classed root length and total root length, regardless of H₂O₂ concentration. In general, inoculation with AMF increased arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) activity of roots under 0, 1 and 100 &#956;M H₂O₂, increased diamine oxidase (DAO) activity of roots under 0 &#956;M H₂O₂ and decreased DAO activity of roots under 1 and 100 &#956;M H₂O₂. Root polyamine oxidase (PAO) activity was similar between AMF and non-AMF seedlings, irrespectively of H₂O₂concentration. Results suggest that lower concentration of H₂O₂(1 &#956;M) might be regarded as a signal to stimulate mycorrhizal and lateral root development through increase of ADC and ODC and decrease of DAO, while high concentration of ₂O₂ (100 &#956;M) as a toxic compound of reactive oxygen species restricted AMF colonization.