Study the Effect of Melatonin on Early Growth and Some Physiological and Germination Characteristics of Seed and Moldavian Balm (Dracocephalum moldavica) Seedling under Osmotic Stress

To evaluate the effect of melatonin pretreatment on germination, early seedling growth and some physiological characteristics of Dracocephalum moldavica under osmotic stress, an experiment was conducted in a factorial arrangement based on completely randomized design with 25 treatments and three replications. The experimental treatments were including different concentrations of melatonin (0, 5, 10, 50 and 100 µmol) and osmotic stress (0, -0.2, -0.4, -0.6 and -0.8 MPa). The seeds of moldavian balm which were primed with different concentrations of melatonin for 24 hours, then in order to germination test under osmotic stress, they transferred into sterile petri dishes which contain 10 ml PEG. Results showed that increasing in stress caused reducing of germination percentage and rate, seedling dry weight, seed vigor index, radicle length, radicle dry weight, shoot length, shoot dry weight, photosynthetic pigments, protein content ,increasing and soluble sugar content. Seed priming with melatonin caused an increase of plant tolerance to osmotic stress. Melatonin caused the increment of germination percentage and rate of germination, seed vigor index and ultimately enhanced dry weight of moldavian balm seedlings under stress condition. The seeds which were pretreated with higher concentrations of melatonin (50 and 100 µmol) showed an increasing of physiological characteristics (photosynthetic pigments, soluble sugar content and protein) under stress condition compared to control. No germination was observed in seeds which were pretreated with distilled water at the levels of -0.6 and -0.8 MPa, while the concentrations of 50 and 100 µmol melatonin caused a seed germination at all levels of osmotic stress.