The perennial alfalfa (Medicago sativa) is presently used as a forage crop or cover plant to reduce soil erosion caused by wind and water and improve soil quality by nitrogen fixation in many regions of China. But when alfalfa is planted continuously in the same field, its yield will decrease. This is considered to be caused by alfalfa autotoxicity. Allelopathy is one of the important factors which reduce the production of alfalfa. It was reported that some allelochemicals in alfalfa could inhibit seed germination and seedling growth and change root morphology, but the antioxidant enzyme of alfalfa autotoxicity remains largely unknown.
According to previous studies, extracts of alfalfa roots, fresh leaves, and litter include phenolics such as vanillic and ferulic acid, p-hydroxybenzoic, o-coumaric, and p-coumaric acid. Ferulic acid, coumarin, vanillic acid and p-coumaric acid were used to test their allelopathic effect on germination, growth and physiological characteristics of alfalfa in this study. The four allelochemicals were dissolved in distilled water to concentrations of 10^-3mol•L^-1, from which, solutions of 10^-4, 10^-5, 10^-6 mol•L^-1 were prepared by adding distilled water. The distilled water was used as control. The objective of our research was to test the alteration of antioxidant defense system by using sand culture method. So as to better understand the allelopathic action mechanism of different allelochemicals.
The results showed that the four allelochemicals had significant allelopathic effects on seed germination, seedling growth and the activity of antioxidant enzyme, and that the physiological activities were significantly related with the concentration of the allelochemical. The seed germination of alfalfa was significantly inhibited under the treatment of 10^-3 mol•L^-1 of the four allelochemicals. Ferulic acid, coumarin and vanillic acid at 10-3 mol•L^-1 significantly reduced the activities of superoxide dismutase, catalase, peroxidase and ascorbate peroxidase, while the content of MDA in alfalfa seedling was significantly increased. However, ferulic acid, coumarin and vanillic acid could increase the activities of antioxidant enzyme at 10^-6mol•L^-1. In generally, the synthetic allelopathic effects of the four allelochemicals on the growth and antioxidant enzymes activities of seedling ranked, from the strongest to the weakest, in the order of ferulic acid, coumarin, vanillic acid and ρ-coumaric acid.