Zinc and cadmium effects on the early stages of nodulation in white clover

Addition of heavy metals to soils from sources such as biosolid application, smelter emissions, fertilizers, and the like may produce soil solution concentrations of Zn and Cd that can potentially restrict nodulation of legume crops such as white dover. To assess the effects of Zn and Cd on the early stages of nodulation of white clover, a solution culture study was conducted. Rhizobia [Rhizobium leguminosarum bv. trifolii] of a metal-sensitive strain (USDA 2063) and a metal-tolerant strain (USDA 2046) were inoculated into nutrient solution with white dover seedlings containing EGTA to buffer Zn2+ and Cd2+ activities. The calculated activities of Zn2+ were p = 8.00, 5.70, 5.25, and 5.00; the calculated activities of Cd2+ were p = 10.50, 9.50, 8.75, and 8.50. Phase-contrast microscopy was used after staining roots with methylene blue to observe root hair infection at intervals of 1, 12, 24, 72, and 168 h. Few differences were observed between the sensitive and tolerant rhizobial strains. At the lowest Zn concentration, initial attachment of bacteria to root hairs was observed within 1 h. At the highest Zn concentration, attachment was not observed until 12 h. Also, at the highest Zn concentration, a delay was observed for up to 60 h and 4 d, respectively, for the first signs of root hair curling and infection thread formation. Shoot weight decreased with increasing Zn concentration. Few Cd-induced effects on early stages of nodule development were observed. Cadmium was more toxic to plant growth than Zn. These results show that in addition to direct effects on the macro and microsymbiont, the process of nodulation is susceptible to the toxic effects of heavy metals.