Effect of Wood Vinegar and Humic Acid on Morphological and Biochemical Traits, Antioxidant Enzymes, and Essential Oil of Peppermint (Mentha piperita L.) under Water Deficit Stress Conditions

In order to investigate the effect of wood vinegar and humic acid on morphological and biochemical traits, antioxidant enzymes, and peppermint essential oil under water deficit stress conditions, an experiment was conducted as split plot based on randomized complete block design with three replications in the research farm of the Faculty of Agriculture of the Tarbiat Modares University. The experimental treatments were irrigation regimes at three levels (irrigation up to the field capacity and after depletion 25 (optimal irrigation), 40 (moderate water deficit stress), and 55 (severe water deficit stress) % of plant available water in the root zone, respectively) as main plots and foliar application at seven levels (humic acid (1, 2, and 3%), wood vinegar (5, 10, and 15%) and no foliar application (control)) as sub plots. The highest plant height (38.63 cm) was obtained from the treatment of using wood vinegar 15% under optimal irrigation conditions, while, the maximum leaf area index (3.01) was observed in the treatment using wood vinegar 15% under moderate water deficit stress conditions. The maximum essential oil yield was observed in the foliar application of humic acid 3% under optimal irrigation conditions with 37.48 kg h-1, and with the treatments of humic acid 2% under optimal irrigation conditions with 34.34 kg ha-1 and foliar application of 15% of wood vinegar under optimal irrigation conditions with 36.11 kg ha-1 was placed in a statistical group. An increase in water deficit stress caused a decrease in essential oil yield, so that in moderate and severe water deficit stress, 9.37% and 40.17% of essential oil yield decreased, respectively. The use of wood vinegar and humic acid increased the yield of peppermint essential oil, so that by increasing the foliar concentration of wood vinegar from 0 to 15%, the yield of essential oil increased by 81.29%, and increasing the concentration of humic acid foliar application from 0 to 3% increased the yield of essential oil by 69.28%. Considering the water limitation, the use of acid humic 3% is recommended in optimal irrigation conditions and moderate water deficit stress, and wood vinegar 15% in severe water deficit stress conditions is recommended.