Effects of nickel nutrition in the mineral form and complexed with histidine in the nitrogen metabolism of onion bulb

Excess accumulation of nitrate in edible plants particularly onion is a great concern for human health. In this experiment, we investigated the possibility of reducing nitrate accumulation in onion bulb by application of Ni. Two onion cultivars (Allium cepa L. cvs. Dorrcheh and Cebolla Valenciana) were supplied with three levels of Ni (0, 25, and 50 A mu M) in the form of synthetic Ni-histidine complex [Ni(His)(2)] and NiCl2. Addition of Ni significantly increased fresh bulb mass of onion cultivars. In 'Valenciana', increasing Ni concentration from 25 to 50 A mu M reduced fresh bulb mass while in 'Dorrcheh', no significant difference was found between 25 and 50 A mu M Ni treatments. Supplement with Ni resulted in lower accumulation of nitrate in onion bulb. This reduction was associated with higher activity of nitrate reductase (NR) and glutamine synthetase (GS). Onion plants supplied with Ni had higher concentration of ammonium and total N in their bulb compared with those unsupplied with Ni. The [Ni(His)(2)] complex was more effective than NiCl2 in improving yield, stimulating activity of NR and GS, and reducing nitrate concentration in onion. According to the results obtained, application of 25 A mu M Ni particularly in the form of [Ni(His)(2)] can be effective in reducing nitrate accumulation and enhancing health quality of onion.