Abstract:

The aim of this study was to clarify the effects of exogenous melatonin and ethylene in combination on the salt tolerance of alfalfa （Medicago sativa）. An experiment was conducted in which the alfalfa cultivar ‘Zhongmu No.1’ was treated with exogenous melatonin， ethephon， and melatonin+ethephon， and then the growth and physiological characteristics of alfalfa seedlings under salt stress were determined. Exogenous application of a range of concentrations of melatonin （0.01， 0.02， 0.05， 0.10 mmol·L^-1） and ethephon （0.05， 0.15， 0.25， 0.50 mmol·L^-1） as foliar sprays alleviated the reduction in the growth of alfalfa seedlings caused by salt stress （250 mmol·L^-1 NaCl）. The strongest alleviation effects were in the 0.05 mmol·L^-1 melatonin treatment and the 0.15 mmol·L^-1 ethephon treatment. The application of melatonin and ethephon alone to alfalfa seedlings under salt stress resulted in significantly increased plant height， leaf area， fresh weight， chlorophyll content， and carotenoid content of alfalfa seedlings， and significantly decreased relative electrical conductivity. We conducted further experiments using the above optimal treatment concentrations of melatonin and ethephon. Compared with the control group， salt-stressed alfalfa seedlings showed significantly inhibited growth. Compared with salt-stressed seedlings， those treated with exogenous hormones showed improved growth， especially those treated with melatonin and ethephon simultaneously. Compared with salt-stressed alfalfa seedlings， the seedlings treated with melatonin and ethephon simultaneously showed increased plant height， leaf area， and fresh weight （by 54.1%， 76.8%， and 32.1%， respectively）； increased tissue water content and chlorophyll content （by 46.2% and 47.8%， respectively）； decreased relative conductivity and malondialdehyde content （by 23.5% and 39.7%， respectively）； decreased hydrogen peroxide and superoxide anion contents （by 42.7% and 63.8%， respectively）； increased activities of superoxide dismutase， peroxidase， catalase， and ascorbate peroxidase （by 54.1%， 54.1%， 59.1%， and 62.0%， respectively）； higher reduced glutathione content （by 32.8%）； increased proline and soluble sugar contents （by 42.2% and 27.2%）， a higher potassium ion∶sodium ion （by 217.5%）， and increased contents of endogenous melatonin and ethylene （by 60.0% and 10.6%， respectively）. These comprehensive analyses showed that exogenous application of melatonin and ethylene significantly reduced the levels of membrane peroxidation and reactive oxygen species in alfalfa， increased the activity of antioxidant enzymes and the contents of osmoregulatory substances， regulated the ionic balance， and increased the contents of endogenous hormones. Together， these changes improved the salt tolerance of alfalfa seedlings. Thus， melatonin and ethylene treatments can improve the growth of alfalfa seedlings under salt stress.

Key words: Medicago sativa, salt stress, melatonin, ethylene, physiological response