The effect of foliar application of nickel in the mineral form and urea-Ni complex on fresh weight and nitrogen metabolism of lettuce

• Foliar application of Ni enhanced growth and yield of lettuce.
• The effectiveness of urea-Ni complex was in general, higher than NiCl2.
• Foliar spray of Ni improved activity of urease and thereby reduced shoot accumulation of endogenous urea in NH4NO3-fed lettuce plants.
• Ni reduced tissue concentration of ammonium by enhancing activity of glutaminee synthetase.

The positive effect of nickel (Ni) on nitrogen metabolism of several plants supplied with urea as nitrogen source has been reported while limited information is available about the role of Ni in NH4NO3-fed plants. In this study, we evaluated the effectiveness of foliar application of Ni on growth, fresh weight, and nitrogen metabolism of two commercial lettuce cultivars (Lactuca sativa L. cvs. Conco Istador and Grizzly) in nutrient solution culture. Nickel was sprayed onto lettuce plants in the form of NiCl2 or urea-Ni complex. Foliar application of Ni resulted in significant increase of shoot concentration of Ni. The effect of urea-Ni and NiCl2 on shoot fresh weight of Conco-istador cultivar was similar while in Grizzly cultivar, the urea-Ni complex was more effective than NiCl2. Exposure to Ni of lettuce significantly increased activity of urease and reduced shoot concentration of endogenous urea. Application of Ni reduced tissue concentration of ammonium (NH4+) in Grizzly cultivar by enhancing activity of glutamine synthetase (GS). In addition, Ni-treated lettuce plants accumulated lower concentrations of nitrate (NO3−) in their leaves compared with untreated plants. According to the results, the effectiveness of urea-Ni complex in increasing fresh weight and improving nitrogen metabolism of lettuce was dependent on plant cultivar.