The roles of nickel in nitrogen assimilation and metabolism via biological fixation in soybean

Abstract

Nickel (Ni), a cofactor for urease and hydrogenase, is the most recent micronutrient to be recognized as an essential element for plants. However, there are no reports of Ni deficiency in annual species grown under field conditions, possibly because such a deficiency is hidden (or latent) and lacks obvious and distinct symptoms. In Brazil, soybean cultivation is highly dependent on Biological Nitrogen Fixation (BNF), in which Ni plays a fundamental role. The treatment of seeds with other micronutrients that are essential to BNF, such as cobalt and molybdenum, and annual inoculation with strains of Bradyrhizobium spp. are widespread practices in Brazil, but information on the potential role of Ni is scarce. Given the possibility of hidden Ni deficiency, we hypothesize that fertilization with Ni will optimize the efficiency of BNF, plant physiological parameters, and nitrogen metabolism and provide greater grain productivity and quality. To verify this hypothesis, a greenhouse experiment at Lageado Experimental Farm, College of Agricultural Sciences/UNESP, Botucatu-SP, and three field experiments at (I) Embrapa Soja, Londrina-PR, (II) Lageado Experimental Farm, College of Agricultural Sciences/UNESP, Botucatu-SP, and (III) Experimental Farm, Faculty of Engineering/UNESP, Selvíria-MS (Cerrado region), will be carried out. Ni efficiency in BNF will be assessed by analyzing the reduction of acetylene gas under continuous flow in a non-destructive system not previously used in Brazil in order to determine the activity of nitrogenase. In addition, the correlations of physiological parameters (photosynthetic pigments, gas exchange and efficiency of photosystem II) related to BNF, enzymatic parameters (urease and hydrogenase activity), and N metabolism (N-ureides) with grain yield, reserve protein and Ni content will be evaluated. The results are expected to define the need for the inclusion of Ni in fertilization programs, which may bring significant benefits to the symbiotic process and thus improve the productive capacity of this legume in tropical soils. (AU)