| Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial (positive allelopathy) or detrimental (negative allelopathy) effects on the target organisms. Alfalfa (Medicago fativa L.) is known to have an allelopathic chemical which is believed to affects alfalfa germination and seedling growth and therefore is considered to be autotoxic or toxic to itself. Field and greenhouse experiments were conducted to study the allelopathic effects of alfalfa on alfalfa plants (autotoxicity) and plants of grass family (heterotoxicity).1. The effect of the interval between tillage and sowing on autotoxicity of alfalfa.In order to make clear the autotoxicity of continuous cropping alfalfa for five years in the field,the effects of different durations of tillage and sowing on seedlings growth and physiological characteristics of follow-cropping alfalfa were studied. The results showed that the alfalfa root and shoot length and chlorophyll contents in the leaves under the treatment of 90-d duration were significantly (p<0.05) more than other treatments. The activities of SOD and POD protective enzymes system were getting declined with the elongation of tillage durations.2. Experiment on the spatial extent of autotoxicity in old alfalfa field.A five year old alfalfa field was fully ploughed and alfalfa plants were removed but only one plant was left. Alfalfa was reseeded in concentric circles with the old exiting alfalfa plant in the center and concentric circles were spaced 20 cm, 40 cm, 60 cm and 80 cm from the centered old plant. The results showed that the weight and length of alfalfa root and shoots increased with the distance to the centered old alfalfa, with those of treatment of 80cm were significantly (p<0.05) higher than the other treatments. 3. Experiment on the special extent of autotoxicity in field without alfalfa growing before.An alfalfa plant five years old was transplanted to a field without alfalfa growing before and alfalfa was reseeded in concentric circles with the old exiting alfalfa plant in the center and concentric circles were spaced 20 cm, 40 cm,60 cm and 80 cm from the centered old plant. The results showed that the weight and length of alfalfa root and shoots increased with the distance to the centered old alfalfa, with those of treatment of 80cm were significantly (p<0.05) higher than the other treatments.4. The effect of leaching liquor on seedling growth of grass family.Different concentrations of alfalfa leaching solution have certain effect to the growth of orchardgrass. With the increase of concentration,Seedling weight and root weight and root length of orchardgrass is gradually reduced,and Low concentrations promote,high concentrations restrain for Seedling weight,seedlinglength and root length of Dahurian Wildrye.With the increase of concentration,germination percentage is gradually reduced,germination percentage of contrast is highest,and germination percentage of the concentration of 5% is lowest.Thus,alfalfa extracts contains allelochemicals,Different concentrations have different content allelochemicals,and these allelochemicals have the influence of different level for the grasses. 5. The effect of separating on seedling growth of grass family.Planting orchardgrass and Dahurian Wildrye in the same basin,Half of planting alfalfa,Half of planting orchardgrass,and separate use impervious clapboard .the contrast is not separate. Results: The ground and underground alfalfa both can produce allelochemicals. Seedling length,root length,Seedling weight of the not separate's are significantly higher than separate's.separate use impervious clapboard in the same basin,and the contrast is not separate. Half put in ever grow alfalfa soil,and half put in ever not grow alfalfa soil. the contrast is not separate. Planting orchardgrass.Seedling length,root length,Seedling weight and root weight and the content of chlorophyll of ordinary soil of separate's are significantly higher than not separate's,and POD significantly lower.Thus,there is some allelochemicals in ever grow alfalfa soils. The allelochemicals by osmosis into ordinary soil in the treatment of not separated,and influence of orchardgrass.The content of chlorophyll of ordinary soil of separate's are significantly higher than ever grow alfalfa soil separate's.In the separated treatment,the allelochemicals in ever grow alfalfa soils is more than ever not grow alfalfa ordinary soils,and the inhibition is stronger to orchardgrass.6. The effect of replacement on seedling growth of grass family.Planting different proportion of alfalfa and grasses in the same basin.contrast is grasses.different proportion have certain effect to Seed germination and growth of orchardgrass and Dahurian Wildrye. With the increase of proportion of Alfalfa,Seedling length,root length,Seedling weight and root weight and of orchardgrass and Dahurian Wildrye is gradually reduced.With the increase of proportion of Alfalfa,The chlorophyll content of orchardgrass and Dahurian Wildrye,First increases,and Then decreased. When alfalfa and orchardgrass proportion with 1-3,The content of chlorophyll b,the content of chlorophyll a and b is highest,and Significant higher than other treatment. When alfalfa and Dahurian Wildrye proportion with 1-3,the content of chlorophyll a,The content of chlorophyll b,The content of chlorophyll a and b is highest. When alfalfa and orchardgrass proportion with 1-3,Biomass is the highest.In the germination experiment,in each treatment,seedling length,root length,seedling weight and root weight of orchardgrass and Dahurian Wildrye is smaller than contrast Significantly.
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