Abstract:

This study aimed to investigate the effects of exogenous melatonin （MT） on endogenous hormones and organic acids in alfalfa （Medicago sativa） seedlings under low-phosphorus stress. Two alfalfa varieties， one tolerant to low-phosphorus conditions （7890） and one sensitive to phosphorus deficiency （15578）， were subjected to seven treatments： normal phosphorus （1.0 mmol·L?1， NP）， low phosphorus （0.01 mmol·L?1， LP， with KH?PO? as the phosphorus source）， and LP combined with root irrigation with MT at concentrations of 30， 60， 90， 120， and 150 μmol·L?1. Fourteen characteristics of alfalfa seedlings were evaluated： plant height， leaf area， aboveground biomass， underground biomass， leaf relative water content， and root zeatin， auxin， gibberellin acid， oxalic acid， lactic acid， citric acid， succinic acid， fumaric acid， and malic acid contents. The values of these 14 characteristics initially increased and then decreased with increasing MT concentration， reaching a maximum in the 90 μmol·L^-1 MT treatment. For alfalfa variety 7890， compared with the plants under LP stress， those in the 90 μmol·L^-1 MT treatment showed increases of 16.55%， 34.62%， 32.41%， 86.78%， 15.77%， 10.45%， 49.56%， 15.74%， 37.34%， 20.05%， 61.21%， 50.14%， 11.50%， and 39.06%， respectively， in the above 14 measured characteristics. For alfalfa 15578， these values were increased by 26.75%， 32.00%， 44.88%， 108.91%， 27.45%， 28.64%， 70.21%， 107.20%， 29.50%， 9.17%， 55.24%， 56.09%， 12.00%， and 30.71% respectively， in the 90 μmol·L^-1 MT treatment compared with the LP treatment. Abscisic acid content exhibited a trend of initially decreasing then increasing with rising MT concentrations， reaching its maximum in the 150 μmol·L^-1 MT treatment. The membership function analysis indicated that the 90 μmol·L^-1 MT treatment exerted the strongest effect to alleviate the symptoms of low-phosphorus stress in alfalfa plants. The trends in the responses of variety 7890 （tolerant to low phosphorus） and variety 15578 （sensitive to low phosphorus） to MT were the same， but variety 15578 showed a greater level of improvement with MT treatment under LP stress， indicating that the alleviating effect of MT is greater for materials that are sensitive to low-phosphorus conditions. The results of this study provide a theoretical basis and technical approach for using MT to improve the phosphorus-use efficiency of crops.

Key words: alfalfa, melatonin, low phosphorus stress, endogenous hormone, organic acid