Evaluation of Irrigation Management and Plastic Mulch Effects on Water Productivity and Quantitative and Qualitative Yield of Lemon Balm (Melissa officinalis L.)

Introduction Today, the crisis of water resources is one of the biggest challenges of human societies. Population growth and industrialization have increased the demand for water consumption. The agricultural sector is facing many problems to supply the required water resources. Irrigation management and the use of plastic mulches play a crucial role in raised-bed cultivation of horticultural and medicinal plants, serving various purposes such as enhancing quality and increasing yield. Moreover, given the current water scarcity conditions, improving the quantitative and qualitative yield of medicinal plants relies heavily on effective irrigation management and efficient cultivation practices.   Material and Methods In this research, a split-plot experiment was conducted using a randomized complete block design (RCBD). The experimental treatments included: the main factor of irrigation management (IM) with four levels (IM100, IM80, IM60 and IM40) and the sub-factor of plastic mulch (PM) with two levels (black plastic mulch (PM1) and no mulch (PM0)) with 3 replications during the growth season 2022-2023 and was conducted at Sari Agricultural Sciences and Natural Resources University (SANRU), Iran. The dimensions of the experimental plot was arranged as 1.80 × 1.20 meters (length and width). Lemon balm was planted in rows with a spacing of 30 cm between plants and 40 cm between rows. Soil moisture was measured by weight method and irrigation was done manually with a sprinkler. In the end, the data obtained from measuring water productivity (WP), relative water content (RWC), essential oil content, morphological and biochemical characteristics of lemon balm were analyzed using ANOVA of SAS software. The Duncan multiple range post hoc test was employed to compare treatment means.   Result Based on the findings, the interaction effect of irrigation management and plastic mulch on water productivity was significant. So, the IM100 conditions and PM1, the highest water productivity was observed with 38.5 kg.m-3 and the lowest amount was related to IM40 and PM0 25 kg m-3. The results showed that the simple effect of IM on the RWC in IM100 was very noticeable and obvious compared to other IMs. The highest (74.3%) RWC was observed in IM100, while the lowest RWC was related to IM40 (40.1%). In PM1, the highest amount of RWC (62.1%) was observed and the lowest amount was related to the PM0 (51.8%). The results showed that vegetative wet and dry weight, leaf area index (LAI) and the number of lateral branches of lemon balm increased under the IMs and PM1 affect compared to the control (PM0). The comparison of averages indicated that the highest wet and dry root weights were 217 kg ha-1 and 140.8 kg ha-1, respectively, observed in the IM100 treatment. Additionally, the highest wet and dry root weights under the plastic mulch treatment (PM1) were 151.1 kg ha-1 and 108.5 kg ha-1, respectively. The results also showed that the highest vegetative wet and dry weights were recorded at 647.2 kg ha-1 and 231 kg ha-1, respectively, in the IM100 treatment. Furthermore, the interaction effect of irrigation management (IM) and plastic mulch (PM) on the leaf area index revealed that the highest LAI value (2.8) was observed in the IM100 and PM1 combination. This was while the lowest value of the LAI was related to IM40 and PM0. The interaction effect of IM and PM on the number of lateral branches indicated that IM100 and PM1, the highest number of lateral branches (12) was generated. Based on the obtained results, the highest amount of phenol and flavonoids content were related to IM40 and PM0 with 13.4 mg.GA g-1 and 14.6 mg.QU g-1, respectively. While their lowest amount was observed under IM100 and PM1 with 10.2 mg.GA g-1 and 11.8 mg.QU g-1, respectively. Also, the results showed that the highest essential oil content was related to IM100 and IM80 conditions with 1.14% and 1.13 %, respectively, and the lowest was observed in IM40 condition with 0.53%.   Conclusion The evaluation of the irrigation management and plastic mulch effects on water productivity and quantitative and qualitative yield of lemon balm showed that the use of plastic mulch by maintaining soil moisture and improving cultivation conditions, increased the vegetative growth and some qualitative characteristics. In general, due to the crisis of lack of water resources and the challenge of water scarcity, irrigation under IM80 conditions and the use of black plastic mulch in the rise-bed of lemon balm are suggested.