Abstract:

The aim of this study was to investigate the effects of an arbuscular mycorrhizal fungus （AMF） and melatonin treatments on the photosynthetic parameters and antioxidant system of Medicago sativa （alfalfa） plants under salt stress. Melatonin was applied at a range of concentrations （0， 50， 100， 150， and 200 μmol·L^-1） to roots of M. sativa under salt stress （100 mmol·L^-1 NaCl）. A total of 10 treatments were inoculated with an AMF （Glomus versiforme）， whereas the control plants were left uninoculated. It was found that this AMF formed a stable symbiotic relationship with alfalfa， with a maximum mycorrhizal infection rate of 65.2%. Compared with the control group， the group inoculated with AMF and treated with melatonin at 100 μmol·L^-1 showed increased total biomass （by 5.4%） and chlorophyll a and chlorophyll b contents （by 7.3% and 45.3%， respectively）， a decreased root to shoot ratio （by 39.5%）， and increased net photosynthetic rate， transpiration rate， intercellular CO₂ concentration， and stomatal conductance （by 23.3%， 28.3%， 17.6%， and 14.1%， respectively）. Compared with the control group， the group inoculated with AMF and treated with melatonin at 100 μmol·L^-1 showed increased maximum photochemical efficiency and PS Ⅱ potential activity （by 19.6% and 22.9%， respectively）； increased nitrogen， phosphorus， and potassium contents in the leaves （to 1.4-times， 1.1-times， and 1.2-times that in the control， respectively）， and decreased sodium content （by 11.1%）. Comparisons of gene transcript levels between the control group and the group inoculated with AMF and treated with melatonin at 100 μmol·L^-1 revealed increased transcript levels of Cu/Zn-SOD （by 33.9%）， POD （by 16.8%）， and CAT （by 6.4%） in the latter group. The activities of superoxide dismutase， peroxidase， and catalase were increased to different degrees in the group inoculated with AMF and treated with melatonin at 100 μmol·L^-1. We concluded that AMF and melatonin can synergistically increase the chlorophyll a and b contents and chlorophyll fluorescence parameters of alfalfa， improve photosynthesis， increase the absorption of nutrients such as nitrogen， phosphorus， and potassium， reduce the absorption of sodium， and upregulate the expression of Cu/Zn-SOD， POD， and CAT， leading to increased activities of superoxide dismutase， peroxidase， and catalase. Among the treatments tested here， AMF inoculation combined with root application of 100 μmol·L^-1 melatonin had the strongest effect to promote plant growth under salt stress and improve salt resistance.

Key words: arbuscular mycorrhizal fungi, melatonin, Medicago sativa, photosynthesis, chlorophyll, nutrient