Co does not replace Ni with respect to urease activity in zucchini (Cucurbita pepo convar. giromontiina) and soybean (Glycine max)

In order to evaluate whether Co can functionally replace Ni in urease, as suggested by Watanabe et al. (Soil Sci. Plant Nutr. (1994) 40, 545-548), zucchini and soybean plants were grown in purified nutrient solutions containing nitrate and ammonium as sole N source supplemented either with 0.05 mmol m(-3) Ni or Co or with no supplementation (control). In addition, isolated soybean cotyledons were incubated axenically with either 1 mmol m(-3) Ni or Co supplements. Plant growth was not affected by the Ni and Co additions, but urease activity was only detected in Ni-supplemented tissue, and in no case did Co raise the urease activity level. Only Ni-deprived plants (-Ni - Co and -Ni + Co) accumulated appreciable amounts of urea in the leaves. These results strongly suggest, that Co does not replace Ni with respect to functional urease.