Impact of foliar nickel application on urease activity, antioxidant metabolism and control of powdery mildew (Microsphaera diffusa) in soybean plants

Nickel (Ni) is a cofactor for urease, an enzyme that breaks down urea into ammonia and carbon dioxide. This study aimed to evaluate the physiological impact of Ni on urea, antioxidant metabolism and powdery mildew severity in soybean plants. Seven levels of Ni (0, 10, 20, 40, 60, 80 and 100gha(-1)) alone or combined with the fungicides fluxapyroxad and pyraclostrobin were applied to soybean plants. The total Ni concentration ranged from 3.8 to 38.0mgkg(-1) in leaves and 3.0 to 18.0mgkg(-1) in seeds. A strong correlation was observed between Ni concentration in the leaves and seeds, indicating translocation of Ni from leaves to seeds. Application of Ni above 60gha(-1) increased lipid peroxidation in the leaf tissues, indicative of oxidative stress. Application of 40gha(-1) Ni combined with 300 mLha(-1) of fungicide reduced powdery mildew severity by up to 99%. Superoxide dismutase, catalase, peroxidase and urease enzyme activity were greatest under these conditions. Urea concentration decreased in response to Ni application. Urease activity in soybean leaves showed a negative correlation with powdery mildew severity. The leaf Ni concentration showed a positive correlation with the urease and a negative correlation with powdery mildew severity. The results of this study suggest that urease is a key enzyme regulated by Ni and has a role in host defence against powdery mildew by stimulating antioxidant metabolism in soybean plants.