Effects of foliar nickel (Ni) application on mineral nutrition status, urease activity and physiological quality of soybean seeds
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Nickel (Ni) at low concentrations is an essential micronutrient for higher plants. Nickel is a cofactor of the enzyme urease and plays a critical role in the germination of seeds. This study aims to evaluate the effect of foliar Ni application on mineral nutrition status, urease activity and the physiological quality of soybean seeds. The study was conducted using different levels of Ni (0, 10, 20, 40, 60, 80 and 100 g ha-1) were applied in the presence or absence (0 or 75 g i.a. ha-1) of the fungicide pyraclostrobin. Leaf nitrogen concentrations showed a linear increase in response to Ni application irrespective of pyraclostrobin application. Foliar sulfur concentrations inversely proportional to Ni application in the absence of pyroclostrobin, but were proportional to Ni application when pyroclostrobin was also applied. Nickel application showed no effects on foliar concentrations of other macronutrients (P, K+, Ca2+, Mg2+). Nickel concentrations in the leaves and seeds were proportional to nickel applications levels. There was a highly significant correlation between foliar Ni concentrations in the leaves and seed Ni levels (r = 0.99), indicating extensive translocation of Ni from leaf to seed. Urease activity increased proportionally up to 20 g ha-1, with peak activity between 20 and 40 g ha-1 of Ni. Urease activity correlated highly with seed yield (r = 0.88) and pod number (r = 0.84). Seed germination and emergence rate increased proportionally with Ni application. However, when Ni was combined with pyraclostrobin, the germination percentage displayed a parabolic curve with an increase of germination up to 40 g ha-1 of Ni. The electrical conductivity for a maximum was between 40 and 60 g ha-1 of Ni. Nickel treatments had no significant effect on seedling length or seedling dry weight. Foliar application of Ni up to 20 g ha-1 in the presence or absence of pyraclostrobin was beneficial to soybean plants based on seed yield, mineral nutrition status and physiological quality of soybean seeds.