How does Ni fertilization affect a responsive soybean genotype? A dose study

Background and aims Nickel affects a wide range of physiological processes in plants, which may result in higher growth and yield. Recent studies demonstrate that soybean genotypes show positive, but differential, responsiveness to Ni-fertilization. Thus, this study investigated the impact of supplying this micronutrient to a soybean genotype previously known to be responsive to Ni-fertilization, with the aim of identifying a suitable range of Ni for the adequate development of this crop plant. Methods We tested, under greenhouse conditions, the effect of applying six Ni doses (0.00, 0.25, 0.50, 1.00, 3.00, and 9.00 mg kg(-1)) via soil to a commonly-grown soybean genotype. Soybean plants were evaluated for nutrition, physiology, growth, N-urea metabolism, and efficiency of the N-2 fixation process. Results Nickel supply positively affected the physiology and growth of the genotype, indicating a deficiency of this micronutrient. Ni-application also led to an increased N metabolism; in particular, nodules had a higher nitrogenase activity associated with greater Ni-concentrations in areas of highest metabolic activity. By exploiting the data acquired in a growth response curve (grain yield x Ni concentration), the greatest beneficial effects were observed following an application of 3.35 mg of Ni kg(-1). A supply of more than 4.25 mg of Ni kg(-1) was toxic to the plants. Conclusions Ni-fertilization improved the N-mineral and N-biological assimilation pathways of soybean, optimizing the overall development of the plants. Through this study, we defined a reliable and adequate Ni-range for both plants and soil, which can be used as a basis for agricultural practice. Moreover, supplying plants with Ni at the recommended doses was revealed to be safe for in natura human consumption of soybean grains; however, more studies are required to establish the appropriate safety limit standards for soybean-based food products.