A glimpse into the physiological, biochemical and nutritional status of soybean plants under Ni-stress conditions

Nickel (Ni) toxicity has been reported to decrease productivity in soybean (Glycine max L.). However, soybean responses to Ni toxicity are not well understood. The aim of the present study was to describe Ni toxicity in soybean plants through physiological, nutritional, and ultrastructural analyses. Plants were grown in nutrient solution containing increasing Ni concentrations (0, 0.05, 0.1, 0.5, 10, and 20 μmol L−1), and nutritional, anatomical, physiological and biochemical features were determined. The results revealed previously unreported detrimental effects of Ni toxicity on soybean plants. CO2 assimilation rates, stomatal conductance and transpiration decreased, resulting in lower biomass in soybean plants exposed to the highest Ni levels. Nitrate reductase activity increased with up to 0.05 μmol L−1 Ni and then decreased, indicating halted N-metabolism. Urease activity increased with increasing Ni availability in the nutrient solution, and peroxidase and superoxide dismutase activities were higher in plants grown at higher Ni levels. Leaf epidermal thickness (abaxial and adaxial), as well as root xylem and phloem diameter, decreased starting at 0.1 μmol L−1 Ni. Mean Ni concentrations varied from 77.5 to 17,797.4 mg kg−1 in roots and 2.3 to 16,774.5 mg kg−1 in shoots. Soybean plants exhibited symptoms of Ni toxicity starting at 0.1 μmol L−1 Ni, presenting mean shoot Ni concentration of 28.9 mg kg−1, along with leaf water loss until complete drying. The results contribute to our understanding of several physiological, biochemical and histological mechanisms of Ni toxicity in soybean, which is still poorly understood.