Effects of rice husk biochar and different irrigation regimes on growth, essential oil percentage, and concentration of some nutrients in peppermint (Mentha piperita L.)

To investigate the effects of rice husk biochar application on the various physiological and biochemical characteristics (fresh and aerial parts dry weight, plant height, percentage and essential oil yield,and  content of chlorophylls, carotenoids, nitrogen, phosphorus, and potassium) of peppermint (Mentha piperita L.) under different irrigation regimes, a factorial pot experiment with different levels of biochar (0, 3, and 6 t.ha-1) and irrigation (100% of full irrigation (control), 60, 80, and 120% of full irrigation) was conducted in a completely randomized design with four replications in the research greenhouse of Lorestan University in 2019. The results showed that the highest and lowest of aerial parts fresh weight were obtained in the 3 t.ha-1 biochar + 120% of full irrigation (24.2 g.pot-1) and 3 t.ha-1 biochar + 60% of full irrigation (13.3 g.pot-1) treatments, respectively. At the 60% of full irrigation level, the dry weight, height, essential oil yield, and N and P concentration of the plant aerial parts significantly decreased by 39.8, 33.3, 26.2, 17.2, and 8.7%, respectively compared to the control; while the essential oil percentage and K concentration of aerial parts had an increasing trend. The amount of a, b, and total chlorophyll, and carotenoids decreased under the water stress conditions. The highest amount of total chlorophyll was observed in the 6 t.ha-1 biochar + 100% of full irrigation treatment. The application of 6 t.ha-1 biochar increased the fresh and aerial parts dry weight, plant height, percentage and essential oil yield, photosynthetic pigments, and N, P, and K concentration of aerial parts compared to the no biochar application, significantly. The results showed that the 6 t.ha-1 biochar + 120% of full irrigation treatment increased most of the traits studied. However, due to the importance of peppermint in terms of essential oil production, the highest essential oil percentage was obtained at the highest level of water stress.