Comparison of physiological and molecular responses of Melissa officinalis L. to osmotic stress imposed by drought and salinity with iso-osmotic potential in hydroponic culture

This study was performed to compare some physiological and molecular responses of lemon balm to osmotic stress imposed by drought and salinity stresses with equal osmotic potential in hydroponic culture. Growing lemon balm plants in hydroponic culture were treated with drought and salinity stress using polyethylene glycol 6000 and sodium chloride for 14 days. Then, seedlings were sampled to evaluate the amount of photosynthetic pigments, proline, phenolic compounds, malondialdehyde, ion leakage percentage, phenylalanine ammonialyase activity and the amount of rosmarinic acid. Also, the expression of phenylalanine ammonialyase and rosmarinic acid synthase genes was examined by real-time PCR. This experiment was performed as a completely randomized design with three replications. The results showed that photosynthetic pigments, phenolic compounds, free proline, malondialdehyde, ion leakage, enzyme activity and relative expression of phenylalanine ammonialyase gene, relative expression of rosmarinic acid synthase and rosmarinic acid content affected by drought and salinity stresses at one percent level of probability. Also, a significant reduction in photosynthetic pigments was observed in drought and salinity stresses. The phenolic compounds content in plants under salinity stress increased compared to the plants under drought stress and untreated. The increase in free proline in plants under drought and salinity stresses was 11% and 25%, respectively. The amount of malondialdehyde and the percentage of ion leakage in plants under drought and salinity stress increased compared to the control plants. Relative gene expression and phenylalanine ammonialyase activity increased in salinity stress. Increasing the relative gene expression of rosmarinic acid synthase in drought and salinity stresses increased rosmarinic acid content by 42 and 60% compared to untreated plants. According to the results, lemon balm responded differently to drought and salinity with iso-osmotic potential. In another words, the plant had a better response to osmotic stress induced by salinity than drought with more increase in proline, phenolic compounds, rosmarinic acid contents, relative gene expression and phenylalanine ammonialyase enzyme activity.