Exogenous melatonin modulates physiological responses, phytochemical profiles and essential oil production in grapefruit mint under drought stress

Currently, the use of anti-stress elicitors such as melatonin, N-acetyl-5-methoxytryptamine, is known as an effective method to reduce the negative impacts of stressful environments. Therefore, a two factor- five level (irrigation levels: 25-75% field capacity (FC) and melatonin: 0-300 mM) dry yield optimization and modeling were done using central composite design, essential oil quantity and quality, total phenol, flavonoid and chlorophyll content and also some physiological properties of grapefruit mint (Mentha suaveolens × Mentha piperita). The results showed that the grapefruit mint biomass in severe drought stress was reduced by 73% in comparison with normal irrigation. However, the application of 155 mM melatonin enhanced biomass yield, phenol, flavonoid, essential oil content and essential oil yield by 92.2, 33.4, 55.1, 45.3 and 143%, in comparison with control. Optimization analysis predicted that under FC 65% irrigation level and application of 240 mM melatonin would positively assist in achieving the maximum chlorophyll, phenol, flavonoid content and essential oil yield of grapefruit mint. In addition, the maximum content of linalool and linalool acetate was observed in FC 50% treated with 155 mM melatonin. Generally, it can be concluded that the application of melatonin, especially in 155 mM level, improved plant growth, essential oil quantity and quality and also plant physiological characteristics under drought stress.