Effects of salicylic acid and nitrogen application on biochemistry, qualitative and quantitative yield of peppermint (Mentha piperita L.)

A range of methods are applied to enhance secondary metabolism in medicinal plants. Treatment of the plants with elicitors is one of these approaches. Salicylic acid (SA) exerts a positive impact on plant growth and yield. The present study, as a biannual field experiment, investigates the response of peppermint (Mentha piperita L.) to different rates of SA under different regimes of nitrogen nutrition during years of 2014-2015. The experiment was made as a random complete block design based on split-plot with three replicates. The treatments included nitrogen fertilization rate as the main factor (0, 70, 140, 210, and 280 kg N ha-1 applied as urea) and salicylic acid, as the sub factor, was spayed at different concentrations (0, 100, 200, and 300 μM). In both years, the highest nitrate reductase activity was found in the roots of the plants treated with 200 μM SA and fertilized with 280 kg N ha-1. Interestingly, the foliar activity of the enzyme was influenced only by nitrogen, whereas the highest rate of activity was determined using 280 kg N ha-1 (2.87 and 2.82 μM nitrite g-1 FW h-1, respectively in the first and second year). Higher concentration of SA caused to increased content of nitrogen and free amino acid in the plants fertilized with higher rates of nitrogen. Nitrate, total phenol, and soluble carbohydrates content of leaf was reduced in both years as the result of high rate of nitrogen and high concentration of SA applied. In the first and the second year, the highest fresh yields (3316.2 and 3480.7 g m-2, respectively), the highest dry yields (811.7 and 855.6 g m-2, respectively) and essence (13.01 and 15.2 mL m-2, respectively) were obtained through annual application of 210 kg N ha-1. Collectively, the application of 210 kg N ha-1 together with SA (200μM) is recommended to achieve desired quantitative and qualitative yield.