Hidden deficiency of nickel in soybean: assessment and control

Abstract

Studies on nickel (Ni) fertilization in cultivated plants are remarkably new, although their positive effects have been reported since the early 1980s. This element, emerging as a micronutrient, takes place in the enzymes urease, in plants, and hydrogenase, in microorganisms. So, legume plants, due to their symbiotic association with diazotrophic bacteria, may be one of the species most affected by Ni deficiency in agricultural soils. The most important legume species currently cultivated is soybean, an important source of food on the world`s scenario. Thus, practices that lead to a more efficient and sustainable production of this species are aimed to ensuring the current and the future food security. In the first paper, we followed the evidence of Ni deficiency presented in the literature and associate them with the low natural contents of this element commonly found in agricultural soils, and, thus, we confirmed the occurrence of Ni deficiency also in soybean plants. However, in this species, Ni deficiency occurred as a hidden deficiency. To detect this event, we cultivated 17 soybean genotypes, which are commonly cultivated by farmers, with and without the supply of Ni in greenhouse and field, evaluating their physiology, metabolism and production. Therefore, we noticed that fertilization with this micronutrient maximized all evaluated points, even without expressing leaf symptoms of the Ni deficiency. In the second paper, we started from the selection of genotypes previously done, which discriminated them as to their responsiveness to Ni fertilization. To the most responsive genotype, we offered the fertilization with increasing doses of this micronutrient in order to establish the concentrations that would provide the ranges of undernutrition, adequate development and toxicity. As expected, the genotype tested required a higher concentration of Ni to reach its maximum development, and curiously, the nitrogen-fixing bacteria associated with these, too. To reach this conclusion, we evaluated the genotype development following the supply of the Ni doses and how the concentrations of Ni and activity of the root nodules accompanied them. With the developed works, we provide not only ways to detect the hidden deficiency that occurs in the new soybean genotypes, but also how to circumvent this limitation by provision of adequate doses for the maximum development of soybean plants.