The Effect of Some Plant Growth Stimulants on Some Physiological and Biochemical Properties of Lemon Balm (Melissa officinalis. L.) under Different Irrigation Regimes

Medicinal plants are one of the most valuable resources in Iran's wide range of natural resources, which can play an important role in society's health, job creation, and non-oil exports if they are scientifically recognized, cultivated, developed, and exploited properly. Iran is considered one of the world's best regions in terms of climate, geographical location, and medicinal plant growth, and it has been a source of production and consumption of medicinal plants in the past. Lemon balm is generally used in traditional medicine to treat digestive problems, pain, and mental disorders. Drought stress is one of the most serious and widespread problems that limit plant productivity because it negatively affects plant physiology. The effects of drought stress depend on the duration, intensity and stage of growth and the genetic tolerance capacity of plants, which can reduce the growth of plants. It causes a change in morphological and physiological structures and the pattern of biomass distribution or even death.

To study the effect of foliar different growth stimulants on the growth characteristics and essential oil of balm at different levels of drought stress, an experiment was carried out in the crop year 2021. The experiment was a split plot based on randomized complete blocks with four replications, where different levels of irrigation include irrigation after 80, 60, and 40 percent of the field crop capacity (FC) in the main plots and foliar application of different growth stimulants including control, melatonin, amino acid, and folic acid were placed in sub-plots. In this research, total chlorophyll, dry matter yield, relative water content, phenoline content, soluble sugars, essential oil content, essential oil yield, total phenol, and flavonoid were measured. The data obtained from the experiment was analyzed using SAS.9.4 software. The obtaineds averages were statistically compared using Duncan's method (LSR) and at the five percent probability level.

The results of variance analysis of the data showed that the effect of irrigation and foliar spraying on growth stimulants on all investigated traits were significant at the probability level of 1%. There was a significant difference between the interaction treatments in terms of chlorophyll content, proline content, essential oil percentage, essential oil yield, and flavonoid content at the probability level of one percent and in terms of relative leaf water content, dry matter yield, soluble sugars content and total phenol content at the probability level of five percent. The results showed the highest chlorophyll content (2.25 mg g-1 FW), relative leaf water content (88.12 percent), and dry matter yield (793.79 kg ha-1) was recorded in melatonin foliar application and irrigation of 80% of FC treatments. Also, the highest essential oil content (0.81%) and essential oil yield (6.46 kg ha-1) were observed in treating foliar application with melatonin under the irrigation treatment of 60% FC. This study allocated the highest total phenol content to melatonin foliar application at 40% of the FC conditions. In this study, 40% FC irrigation treatment increased the flavonoid content by 29.90% and 93.23%, respectively, compared to 60% and 80% FC. Furthermore, foliar application of melatonin, proline amino acid, and folic acid increased the flavonoid content compared to the control treatment by 17.64, 23.35 and 32.35%, respectively.

In this study, melatonin foliar application in all three irrigation conditions increased essential oil yield compared to the corresponding control. Under irrigation and foliar spraying of melatonin, the synthesis of secondary metabolites and essential oil has been accelerated and produced the maximum yield of essential oil. Therefore, foliar spraying of melatonin and mild water deficit stress can be recommended to achieve maximum economic yield in Lemon balm.