Evaluation of the rainfed and irrigated conditions on biomass and essential oil yield of German chamomile (Matricaria chamomilla) in response to melatonin foliar application.

German chamomile (Matricaria chamomilla L.) is a valuable medicinal plant known for its rich content of bioactive compounds, including chamazulene, α-bisabolol, and α-bisabololoxide A. Drought stress poses a significant challenge to agricultural productivity and negatively affects the growth and yield of chamomile. This study aimed to investigate the effects of foliar-applied melatonin on the antioxidant capacity and secondary metabolite production of M. chamomilla under both rainfed and irrigated conditions. The research was conducted using a randomized complete block design with three replications during the 2022-2023 period. To impose drought stress, we compared rainfed and irrigated treatments, while melatonin was applied at three concentrations (0, 100, and 200 mM). The composition of essential oils was analyzed using gas chromatography-mass spectrometry (GC/MS). Results indicated that the application of melatonin significantly improved essential oil content and plant performance under drought conditions. In rainfed situations, applying 100 mM of melatonin increased the essential oil rate by 45%, reaching 1.45% compared to the control group. Under irrigated conditions, the highest essential oil yield of 0.33 g per plant was also achieved with 100 mM melatonin. The maximum proline content of 4.05 mg/g of fresh weight was found in rainfed cultivation with 200 mM melatonin. Plants receiving irrigation with 200 mM melatonin demonstrated the highest values for relative water content (83.2 mg/g fresh weight), total chlorophyll (3.99 mg/g fresh weight), and dry matter (16.61 g). GC and GC-MS analyses revealed that secondary metabolites ranged from 39.85 to 56.97%, with α-bisabolol, chamazulene, and α-bisabololoxide B being the major components. Path analysis showed strong direct effects of essential oil rate (0.921), proline (1.397), and relative water content (1.115) on dry mass, with R² values reaching 95.4% in the final model. The analysis indicated that all measured traits influenced dry mass, with relative water content directly affecting dry mass and essential oil rate positively influencing both proline and dry mass. Based on these findings, combining rainfed cultivation with melatonin application is a promising and environmentally friendly strategy to enhance drought tolerance, growth, and biochemical content in German chamomile.