Foliar Nickel Nutrient Application for Improved Urea Conversion and Abiotic Stress Mitigation in Tomato

Tomato plants (Solanum lycopersicum L.) in controlled climate greenhouse setting were treated with 25 mg plant⁻¹ of foliar nickel (Ni), calcium (Ca) nutrient management levels of high calcium (HC) or low calcium (LC), while under full water (W) or simulated drought (D) irrigation conditions. This study compared overall plant growth and yield, nutrient intake, and gene expression of Ni-associated urease (Ure), stress-related glyoxalase I, II (Gly I, II), GSH1 encoding gamma-EC synthase and GSH2 encoding glutathione synthetase (the two enzymes synthesizing glutathione) for abiotic stress mitigation. Results demonstrated that foliar Ni increased tomato biomass for both irrigation Ca level treatments by ≤ 70%. Tomato plants treated with foliar Ni expressed higher levels of GlyI, GlyII, GSH1, GSH2, and Ure by up to eight-fold. Foliar Ni application resulted in a maximum increase in tomato leaf nitrogen (N) content from 15 to 41 g kg⁻¹ dry weight when W + HC conditions were applied. Tomato plants treated with Ni also had improved leaf potassium (K) content (≤ 30%), Ca content (≤ 20%), and iron (Fe) content (≤ 100%), demonstrating the potential benefit of Ni application to overall plant health. Evaluation of Ni concentration in fruit and leaves did not show elevated Ni accumulation in fruit (5.7–2.7 mg kg⁻¹ dry weight) or plant leaf (2.2–13.2 mg kg⁻¹ dry weight). The observed lower Ni loading in most plants treated with foliar Ni and W conditions indicated that foliar Ni application does not affect fruit quality or food safety considerations.