EFFECT OF HERBICIDE DIURON ON GROWTH AND SYMBIOTIC BEHAVIOR OF RHIZOBIUM AND BRADYRHIZOBIUM SPECIES

Herbicide diuron was tested for toxicity to the nodule forming bacteri a Rhizobium loti, Rhizobium meliloti and Bradyrhizobium japonicum, and to the nitrogen-fixing symbiosis of these organisms. Neither growth n or symbiotic behaviour of the Rhizobium loti strain assayed was affect ed by addition of diuron to the culture medium. In contrast, cultures of R. meliloti showed optical densities 26, 42 and 58% lower than cont rol at 30, 50 and 100 ppm of diuron respectively. Although nodulation percentage and nodule nitrogenase activity remained unaffected by diur on pretreatment, the number of nodules developed by alfalfa roots was halved when inoculum was exposed to 50 and 100 ppm of diuron. In agree ment, leaf chlorophyll contents were proportionately reduced. In vitro growth rate of B. japonicum strain was not affected by addition of di uron; however, its subsequent nodulation ability was significantly dim inished with increasing herbicide concentrations. Thus, infectivity pa rameters observed when B. japonicum was pretreated with 50 ppm of herb icide were 75% lower than those of controls. Effectiveness of this str ain, measured as nodule nitrogenase activity, decreased when exposed t o 50 ppm of diuron. However, leaf chlorophyll content was significantl y reduced by 30 and 50 ppm pretreatment, while 100 ppm of herbicide co mpletely inhibited plant growth.