ZINC AND CADMIUM EFFECTS ON THE EARLY STAGES OF NODULATION IN WHITE CLOVER

Addition of heavy metals to soils from sources such as biosolid applic ation, smelter emissions, fertilizers, and the like may produce soil s olution concentrations of Zn and Cd that can potentially restrict nodu lation of legume crops such as white clover, To assess the effects of Zn and Cd on the early stages of nodulation of white clover, a solutio n culture study was conducted, Rhizobia [Rhizobium leguminosarum by. t rifolii] of a metal-sensitive strain (USDA 2063) and a metal-tolerant strain (USDA 2046) were inoculated into nutrient solution with white c lover seedlings containing EGTA to buffer Zn2+ and Cd2+ activities, Th e calculated activities of Zn2+ were p = 8.00, 5.70, 5.25, and 5.00; t he calculated activities of Cd2+ were p = 10.50, 9.50, 8.75, and 8.50, Phase-contrast microscopy was used after staining roots with methylen e blue to observe root hair infection at intervals of 1, 12, 24, 72, a nd 168 h, Few differences were observed between the sensitive and tole rant rhizobial strains. At the lowest Zn concentration, initial attach ment of bacteria to root hairs was observed within 1 h, At the highest Zn concentration, attachment was not observed until 12 h, Also, at th e highest Zn concentration, a delay tvas observed for up to 60 h and 4 d, respectively, for the first signs of root hair curling and infecti on thread formation, Shoot weight decreased with increasing Zn concent ration, Few Cd-induced effects on early stages of nodule development w ere observed. Cadmium was more toxic to plant growth than Zn, These re sults show that in addition to direct effects on the macro and microsy mbiont, the process of nodulation is susceptible to the toxic effects of heavy metals.