Effect of harvest time, nitrogen and plant density on yield and some physiological traits of pennyroyal (Mentha pulegium L.)

Pennyroyal (Mentha pulegium L.), Labiatae family, is widely used as a vegetable and medicinal herb. This plant commonly grows in the wet soils such as ditches, riversides and ponds. It is indigenous to the Central, Southern and Western Asia, North Africa, Ethiopia and the Canary Islands. It can also be found in Alborz, North and Northeast regions of Iran. The aerial parts of the plant are used as pharmaceutical and flavoring substances. The aim of this research was to find the best level of nitrogen fertilizer regime, harvesting time and plant density for pennyroyal plant in Rasht area.

This study was conducted as a split-plot experiment based on randomized complete block design with three replications in Faculty of Agriculture, University of Guilan. The main plots included two harvesting times, the subplots were four levels of Nitrogen fertilizer including control (without fertilizer), 50, 100 and 150 kg ha-1 Nitrogen and the sub-subplots were plant density at 3 levels; 10, 14 and 18 plants m-2.

The results showed that triple interaction of nitrogen, plant density and harvest time had significant effect on dry weight. The highest dry weight (1909 kg ha-1) were obtained by 150 kg ha-1 net nitrogen with the density of 18 plants per m2 for the first harvest. Moreover, 50 kg ha-1 nitrogen fertilizer treatment with 10 plants per m2 showed lowest amount of dry weight (396.66 kg ha-1) in the second harvest time. However, the interaction of nitrogen and planting density revealed significant effect on fresh yield and leaf area. The highest yield (8071 kg ha-1) was related to 150 kg ha-1 nitrogen treatment with the density of 18 plants m-2. About physiological traits, the results claimed that treatments significantly affected the essential oil percentage, total chlorophyll, total phenol, nitrogen and calcium elements. The highest essential oil content (2.66%) was exhibited by the treatment of 150 kg ha-1 nitrogen with plant density of 10 plants m-2. In addition, the essential oil yield was also influenced by the interaction effect of harvest time and planting density. The highest yield (25665/075 milliliters ha-1) was found by the plant density of 18 plants m-2 from the first harvesting time.

Finally, according to our results, the best treatment was first harvesting time, 150 kg ha-1 nitrogen with the density of 18 plants m-2, which can be recommended for the planting program of aromatic pennyroyal.