Objective  Regulating effect of melatonin application on the physiology of a tomato plant under aluminum stress was studied.

  Method  Al-tolerant Millennial tomato and Al-sensitive Provence tomato plants were grown in blank control, Al-added, melatonin-added (MT), or Al-and-melatonin-added (AMT) potting soils. Activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), contents of malondialdehyde (MDA), proline (Pro), ascorbic acid (AsA), glutathione (GSH), and chlorophyll as well as chlorophyll fluorescence parameters, Al-accumulation, and root activity in the plants were monitored. DNA damage occurred to the roots, stems, and leaves of the plants during the experimentation were recorded. Statistical analysis was performed on all collected data using the one-way ANOVA and Duncan test.

  Result  Under Al-stress, the growth and development of the tomato plants of either variety were severely inhibited, and the DNA in the organs, especially the roots, seriously damaged. Whereas the presence of melatonin in the soil (MT and/or AMT ) significantly alleviated the symptoms of Al-toxicity on the plants. For instance, with the addition, the main growth indicators, such as plant height, root length, and leaf area, were improved; the activity of antioxidant enzymes significantly heightened with an increasing trend initially and followed by a decline as treatment duration prolonged; the contents of GSH and AsA raised by 91.14% and 13.52% respectively; the reduction on MDA maximized at 38.39%; proline, which regulates the osmotic balance in tomato, increased to 144.81% and chlorophyll significantly; the fluorescence parameters improved; the photosynthetic capacity much restored; the root activity of Millennial tomato plant increased by 5.19%, and that of Provence tomato by 43.03%; the Al-accumulation in the roots, stems, and leaves significantly reduced; the DNA tailing in organs slowed; and the repairs on damaged DNA in cells observed.

  Conclusion  Applying exogenous melatonin in soil effectively activated various physiological responses, raised the activity of antioxidant enzymes, enhanced the plant photosynthesis, reduced damage on cellular DNA, and elevated the resistance to Al-toxicity of the tomato plants under the heavy metal stress.