The allelopathic potential of Coridothymus capitatus L. (Labiatae). Preliminary studies on the roles of the shrub in the inhibition of annuals germination and/or to promote allelopathically active actinomycetes

Suppression of annuals at various intensities was observed around some shrubs of Coridothymus capitatus growing on kurkar formation in the coastal hills of Israel. The phenomenon was clearly observed as annuals-free belts of 15-20 cm around 'aggressive' shrubs. Quantitatively, density of annuals decreased by 16 fold in the annual-free belts as compared to a distance of 60-80 cm from the canopies of the shrubs. Their dry matter was decreased by 5.4 fold around the shrubs. Suppression rate of emergence of planted seeds of annuals (Plantago psyllium and Erucaria hispanica) early in the season was 45% higher around 'aggressive'C. capitatus than that around 'non-aggressive' ones. In the laboratory, seed germination of the annuals was strongly suppressed by diffusates and volatiles from shoots, as well as from their water extracts and their essential oils. Incubation of fresh shoots of C. capitatus in soil collected from around 'non-aggressive' shrubs, for 7 days, increased population levels of actinomycetes by 9.6 fold and by 36.7 fold when soil was collected from around 'aggressive' shrubs. Isolates of some soil-borne actinomycetes inhibited germination of the test plants Lactuca sativa and Anastatica hierochuntica on agar plates (4-98%). The preliminary results indicate a possible synergistic inhibitory effect induced by essential oils of the aromatic shrub and the phytototic activity of actinomycetes.