The effect of nickel supply on bulb yield, urease and glutamine synthetase activity and concentrations of urea, amino acids and nitrogen of urea-fed onion plants

Abstract

The role of nickel (Ni) on urea metabolism of certain plants has been documented, but little is known regarding the growth and physiological response of onion to Ni nutrition, particularly when urea is used as nitrogen source. In this research study, we investigated the effects of Ni on urea metabolism and growth of two onion cultivars (Allium cepa L., cvs. Dorrcheh and Cebolla Valenciana) supplied with urea as nitrogen source. Three levels of Ni (0, 25, and 50 µM) were used in the form of NiCl2 or Ni-histidine [Ni(His)2] complex. Addition of Ni positively affected nitrogen metabolism in onion plants fed with urea and thus was correlated with increase of the bulb yield. Regardless of the plant cultivar and the applied Ni source [NiCl2 or Ni(His)2 complex], an increase in urease activity and reduction in bulb urea concentration was observed by Ni nutrition. An increase in hydrolysis of urea and production of NH4+ in the presence of Ni was correlated with higher concentration of the total amino acids (AAs) and nitrogen in onion bulbs. The efficiency of Ni(His)2 complex in improving Ni uptake and increasing activity of urease and glutamine synthetase, two enzymes involved in urea metabolism, was in general greater than NiCl2. Accordingly, higher concentration of AAs was measured in the onion plants supplied with Ni(His)2 complex than those supplied with NiCl2.