The combined effects of salinity and heat on the yield of rosmarinic acid and expression of some genes involved in its biosynthesis in Mentha piperita L. at different time intervals

The growth and yield of plants in the arid and semiarid areas are limited by salinity stress. In these areas, salinity intensifies by high temperatures. Peppermint (Mentha piperita L.) is of great economic importance in the most of world areas due to their valuable essential oils. This study investigated the simultaneous effects of salinity and heat on rosmarinic acid production and expression of three effective genes (C4H, HPPR, and RAS) in rosmarinic biosynthesis in peppermint.

The effect of two levels of salinity 60 and 120 mM of sodium chloride (irrigation water with salinity of 10 Mm as control) and heat stress at 35°C (25°C as control) at three interval times (24, 48 and 72 hours) after treatments were measured in a completely randomized design with three replications.

The results showed that salinity stress decreased the relative level of rosmarinic acid. The process intensified over time so that it reached its lowest level at salinity of 60 mM at 72 hours. RAS genes expression was significantly reduced under salinity of 120 mM and 60 mM under 25˚C and 35 ˚C. Changes in the expression level of C4H in peppermint seedlings at the salinity levels of 60 mM and 120 mM at 25˚C showed a significant increase (4.1 and 3.6 times respectively) at the first 24 hours compared to control. The expression of HPPR gene in peppermint seedlings at 35 ° C increased 2.68, 1.79 and 2.55 times at 24, 48 and 72 hours, respectively, compared to the control.

Simultaneous salinity and heat stress are a limitation factor for growing Mentha piperita L. These factors had a negative effect on the biosynthesis of rosmarinic acid in this plant. Salinity and heat stress decreased the expression of some genes and increased the expression of others in peppermint.