Effects of exogenous melatonin on plant growth, root hormones and photosynthetic characteristics of trifoliate orange subjected to salt stress

Soil salinity has negative effects on crop growth and production, and melatonin (MT) plays an important role in regulating plant salt stress. However, it is not clear whether exogenous melatonin mitigates the negative effect on citrus plants subjected to salt stress. This study aimed to explore the response of exogenous melatonin (0, 50, 100, 150 µmol/L) on plant growth, root hormone levels and the photosynthetic system of trifoliate orange (Citrus (Poncirus) trifoliata L.) seedlings exposed to 0 or 150 mmol/L NaCl for 4 weeks. The 150 mmol/L NaCl treatment significantly increased root zeatin riboside, gibberellin and brassinolide levels, while dramatically reducing plant growth, root auxin levels, leaf photosynthesis and fluorescence indexes of seedlings. However, melatonin treatment partially ameliorated reductions in plant height and dry matter accumulation caused by salt stress. Melatonin (100 µmol/L) appears to interact with IAA but not the other hormones studied. Furthermore, the effects of NaCl stress on the net photosynthetic rate, stomatal conductance, maximum photochemical efficiency, PSII effective photon yield, photochemical quenching and other indicators of seedlings leaves were also partially alleviated and the damage of NaCl stress was also reduced when seedlings were treated with melatonin. This suggests that 100 µmol/L melatonin may be an effective treatment.