Exogenous β-Aminobutyric Acid (BABA) Improves the Growth, Essential Oil Content, and Composition of Grapefruit Mint (Mentha suaveolens x piperita) under Water Deficit Stress Conditions

Grapefruit mint, Mentha suaveolens x M. piperita, belonging to the Lamiaceae family, is an important medicinal plant with applications in the cosmetic, pharmaceutical, food, and nutraceutical industries. Environmental factors such as cold, salinity, and water deficit significantly impact the quantity and quality of the active compounds of medicinal plants. To examine the effects of drought stress and beta-aminobutyric acid (BABA) as an elicitor on the biochemical characteristics and essential oil (EO) profile of grapefruit mint, a factorial experiment was conducted in a completely randomized design (CRD) with two factor and three replications under greenhouse conditions. The first factor included field moisture capacity (FC) as the control (100% FC), mild (75% FC), moderate (55% FC), and severe water deficit stress (35% FC), while the second factor consisted of 0 (control plants without BABA), 0.8, 1.6, and 2.4 mM of BABA foliar application. Water stress and BABA application significantly affected the EO content and composition of grapefruit mint. The highest content of EO was observed in mild drought stress and BABA spraying at 1.6 to 2.4 mM, which increased by about 140% compared with the control condition. The EO components were identified using GC-FID and GC-MS analysis. Linalool (33.7-47.3%) and linalool acetate (31.2-52%) were the most abundant compounds. The highest content of linalool acetate was observed in severe drought stress (35% FC) with foliar application of BABA (1.6 mM), which increased by 33.86% compared with the control condition. However, the highest content of linalool was observed under normal irrigation with foliar application of 0.8 to 1.6 mM BABA. Based on the results, severe drought stress reduced the total chlorophyll and carotenoids by 81.76 and 64.6% compared with the control condition, respectively. Water stress and the foliar application of BABA significantly affected the activity of antioxidant enzymes (ascorbate peroxidase, APX; guaiacol peroxides, GPX; and superoxide dismutase, SOD). The application of 1.6 mM BABA significantly increased the activity of antioxidant enzymes under water stress conditions. Finally, our results showed that the application of BABA (mainly at 1.6 mM) can improve the grapefruit mint yield and EO profile under water stress conditions.