One of the least known of biocenosis factors is the allelopathic effect of higher plants. This investigation's results indicate that the phytosociological systems of most plant associations are developed by these properties. Allelopathy could be used so that the agroecosystem biodiversity could be extended. Some investigations need to be performed to get to know the allelopathic mechanisms as well as to separate them from other plant interactions, mainly competition. Field experiments are extremely difficult to carry out, that is why attempts are made to determine these relationships under the laboratory conditions. To define the operation of an allelopathic factor of various grass species was decided to use a density dependent phytotoxicity model. The model is based on the fact that along with acceptor plant density growth, the response to allelopathic compounds decreases, while the negative competition effects increase. A higher growth of acceptor plants accompanying their density increase in a population is discordant with competition rules and is likely to imply the allelopathic background of the changes recorded. These investigations were designed to evaluate the allelopathic properties of grass donors by using extracts of leaves collected at various developmental stages. A tested acceptor species
