Soybean seed treatment and foliar application with nickel sources and Ni-particle sizes: an appraisal of biological nitrogen fixation and plant growth.

Nickel (Ni) is an essential element to N metabolism due to the fact it is a structural component of the enzymes urease and hidrogenase. Thus, this study was carried out in three different experiments with the aims of: (i) to investigate the pathway of Ni sources (NiSO4; Ni(OH)2 micrometric ~ 24 µm; Ni(OH)2 nanometric ~ 5 nm), by µ - XRF and SEM, after dressing soybean seed with Ni in soybean plants grown on rhizotrons; (ii) to investigate the effects of Ni sources in soybean plants, under greenhouse conditions, applied via seed or leaf on BFN (by 15N natural abundance method), urease, reductase nitrate, nitrogenase activity, nitrate, ammonia, ureides concentration, photosynthetic parameters and biomass in soybean plants; (iii) to examine the distribution and translocation of foliar-applied Ni in soybean leaves grown in hydroponic solutions by Synchrotron micro-X-ray fluorescence analysis and also to determine the effect of seed Ni concentration on urease activity and to establish whether an internal (presence of cotyledons) or to external (absence of cotyledons) supply of Ni can compensate for Ni nickel within the seed. The results of µ - XRF and SEM revealed that the hilum of soybean seeds coated with Ni showed areas of high spot of Ni concentrated and the seeds that received Ni based on nanoparticles had lower germination rate compared to treatments of Ni sulfate and Ni micrometric. The soybean seedlings grown in the rhizotrons showed higher quantity of Ni in roots and in the rhizosphere soil. Therefore, the Ni applied in the seed is either transported throughout the imbibition or provides a fertile microenvironment that favors the primary seedling development. Foliar application associated with nanoparticles had more positive impact on soybean growth, physiology and biological nitrogen fixation than sulfate Ni fertilizers. Regarding soybean cultivated in hydroponic solution, the results showed that leaf Ni concentrations, shoot biomass and urease activity were augmented by increasing either internal (from cotyledon seed store) or external (in solution) nickel supply. Synchrotron micro-X-ray fluorescence showed that trichomes are an important pathway for foliar Ni absorption in soybean (AU)