Allelopathic effects ofCitrus aurantium L.

Field observations on undisturbed stands of sour orange revealed thatCynodon dactylon, Chenopodium album, Avena sativa, andAmaranthus retroflexus were not able to grow normally and complete their life cycles under its canopies, although the same species grow well under adjacent trees of date palm. Investigations revealed that the failure of the test species to grow normally under sour orange was not due to competition for light, moisture and minerals or to differences in soil texture or pH. Soil under sour orange trees drastically reduced seed germination and/or seedling growth of test species. Aqueous extracts, decaying materials, and volatile compounds of senescent and nonsenescent sour orange leaves were found to inhibit seed germination and/or seedling growth of test species. Therefore, allelopathy appeared to be the basic factor responsible for the reduction in plant growth with competition propably accentuating its effects.