Effect of water-deficit stress on secondary metabolites of Melissa officinalis L.: role of exogenous salicylic acid

Melissa officinalis is a perennial herbaceous plant from Lamiaceae family, widely used in food and pharmaceutical industries to add aroma. Drought stress in plants may happen due to the increase in water loss, insufficient water absorption or both. Salicylic acid (SA) is an important secondary metabolite in plants with hormone-like action in some biochemical pathways. Adding it during water-deficit stress, may alter a number of physiological processes, increasing the resistance of plant against possible damage by water loss. The aim of this study was to examine the effect of drought stress (DRU) on secondary metabolites of Melissa officinalis and also the efficacy of salicylic acid treatment to minimize the damages. According to the results, treatment of plant with 0.7 mM salicylic acid had considerable alteration in the quantity of important secondary metabolites. Protein concentration increased in all stages; Chlorophyll a reached to the highest amount; the amount of the MDA increased at the first and second stages in 1/3 fc drought treatment as well as at the third stage in 2/3 fc.  Besides, total phenol content increased at stage I in 1/3 fc drought and the flavonoid content increased in 1/3 fc drought at all stages. Finally, content of anthocyanin was increased at first and third stages 1/3 fc drought, as well as at second stage in 1/3 fc drought. In conclusion, application of salicylic acid (0.7 mM) was useful for improving quality of lemon balm subjected to stress due to water deficit.