SUPPRESSION OF ANNUALS BY TRIBULUS-TERRESTRIS IN AN ABANDONED FIELD IN THE SANDY DESERT OF KUWAIT

In an abandoned field in the sandy desert of Kuwait annual plants were less numerous in stands dominated by Tribulus terrestris than in adjacent stands dominated by other species. In the Tribulus stands, annuals were smaller in phytomass and stature. Possible physical and biotic factors were investigated in both types of stands in order to determine the causative agent(s) for the lesser density and development of annuals in the Tribulus stands. Physical conditions of moisture, light and soil characteristics did not appear to be limiting to annuals in the Tribulus stands. Experimental evidence is presented for the leaching from T. terrestris shoots of water-soluble substances which strongly inhibit the germination and radicle elongation of most of the associated annual species. The effect of these substances is fairly specific. Preliminary analysis indicates the presence of phenolic compounds in the leachate of T. terrestris, which are believed to play a significant role in the growth inhibition. The possible role of chemical inhibition on certain annuals in stands of T. terrestris is discussed.