Foliar spraying of salicylic acid and melatonin enhances the nutritional value of Arugula (Eruca vesicaria subsp. sativa mill.)

Abstract Background Arugula (Eruca vesicaria subsp. Sativa Mill.) is a leafy vegetable rich in nutritional value, vitamins, minerals, and antioxidants. While previous studies showed that foliar applications of melatonin and salicylic acid influence plant nutrient levels, research on the individual and combined effects of these two compounds in Arugula is Limited. Therefore, this study aims to assess both the individual and combined effects of salicylic acid and melatonin on the concentrations of essential elements, such as nitrogen, phosphorus, potassium, and various micronutrients in Arugula. Treatments were initiated 45 days after planting and were conducted in greenhouse conditions, utilizing a completely randomized design with two factors—melatonin and salicylic acid—each at 0, 50, and 100 µM. After sampling, analysis of the nutritional elements was conducted. Results The findings revealed that the application of 50 µM salicylic acid significantly enhanced nitrogen concentration to 5.22 ± 0.10 mg g−1, representing an about 4.9% increase compared to the control. Application of 50 µM melatonin boosted potassium concentration to 18.9 ± 0.26 mg g−1. The concentration of phosphorus peaked at 100 µM of melatonin, showing around a 22.2% increase, while sulfur concentration rose to 1.94 ± 0.03 mg g−1, representing about a 60.3% increase. The highest concentration of iron, measuring 0.075 ± 0.001 mg g−1, was observed with the application of 50 µM salicylic acid. However, in the interaction involving 100 µM of salicylic acid and 100 µM of melatonin, the concentration of zinc declined from 0.098 ± 0.002 mg g−1 under control conditions to 0.03 ± 0.001 mg g−1. Additionally, copper concentration was highest in the condition without foliar spraying, with a concentration of 0.0074 ± 0.0001 mg g−1. Conclusions The study found that 50 µM salicylic acid significantly increased potassium, nitrogen, and iron levels in Arugula, while 50 µM melatonin enhanced potassium concentrations. The highest phosphorus levels were observed with 100 µM melatonin. However, combining melatonin and salicylic acid at higher concentrations led to a decrease in nitrogen and sulfur levels, indicating negative interactions. Higher levels of melatonin also improved iron and manganese concentrations. The results showed that the higher concentrations of salicylic acid and melatonin have a key impact on nutritional value and efficient nutrient concentrations in Arugula.