Abstract: Salicylic acid (SA) play important roles in different physiological
processes of plants such as plant growth, stress response, plant adaptation and
secondary metabolite accumulation. This study was conducted to evaluate the
effect of exogenous SA applications on the growth measurements such as fresh
and dry weight of aerial part and lead dry weight, biochemical properties (membrane permeability, lipid peroxidation, Proline content and ROS scavenger
enzymes) and secondary metabolite accumulation (total phenolic and the
flavonoid and essential oil content) in peppermint (Mentha piperita L.) plants
grown at different levels of water deficit stress (Field capacity: FC). For this aim,
three different water deficit stress [no stress (100% FC), mild stress (75% FC) and
moderate stress (50% FC)] and four different SA concentrations (0, 1, 2 and 2.5
mM) were applied to peppermint plants. Results showed that all of the measured
parameters were affected by the water deficit stress and SA application. By elevating the level of water deficit stress, fresh and dry weights of aerial parts and
dry leaf weight decreased. Increasing in the water deficit stress level from mild to
moderate stress resulted to reduce the essential oil content while proline, lipid
peroxidation, total phenolic contents, flavonoid content, and antioxidant enzyme
activities increased depending on water deficit stress. Exogenous application of
SA obstructed the negative effects of water deficit stress by decreasing the lipid
peroxidation and membrane permeability and improving the antioxidant
enzymes activities. Essential oil content increased significantly in plant treated
with SA grown under water deficit stress conditions. Application of 2 or 2.5 mM
of SA enhanced the plant growth and development without any toxic effects and
increased significantly the total phenolic content, dry leaf weight, and the essential oil content in stressed and even in control (100% FC) peppermint plants.
