Potential allelopathic effects of sunflower (Helianthus annuus L.) on microorganisms

Potential allelopathic effects of sunflower (Helianthus annuus L.) cultivar Hysun 38 on Rhizobium, Azospirillum, and phosphate-solubilizing bacteria (PSB) were studied by applying oxidase, QTS tests and Gram staining. Microorganisms for the control group were isolated from soil sampled before sowing sunflower. A commonly observed effect of allelochemicals was a decrease, compared to control, in the number of colonies of the three groups of microorganisms, isolated from the roots of sunflower in the case of Azospirillum and PSB and from the rhizosphere of sunflower in the case of Rhizobium. Isolates of Azospirillum and PSB from the rhizosphere of sunflower differed in their response: the number of colonies of Azospirillum remained the same as that in control but that of PSB was greater than that in control. QTS tests showed a range of responses, which varied with the group of microorganisms as well as with the habitat from which they were collected, namely roots of sunflower, rhizosphere of sunflower, and control. Allelopathy also influenced the results of Gram staining test but not those of the oxidase test.