The Effect of Melatonin on Some Physiological and Morphological Characteristics of Coriandrum sativum L. and Anethum graveolens L. under Salt Stress

Introduction      Salinity is the most important environmental parameter limiting plant growth and productivity. The detrimental effects of high salinity on plants can be observed at the whole-plant level as the death of plants and/or decreases in productivity. Increasing salinity is accompanied by significant reductions in number of leaves per plant, shoot weight, root weight, shoot length, and root length. With an increase in salinity, water potential and osmotic potential of plants become more negative. Two medicinal species of Coriandrum sativum L. and Anethum graveolens L. are herbaceous and annual plants of the Apiaceae family, which have many uses in the pharmaceutical and food industries. Considering the importance of these two medicinal species and the increase of environmental stresses including salinity stress in recent years, this research aims to investigate the effect of external application of melatonin on resistance to salinity stress in Coriandrum sativum L. and Anethum graveolens L. species and its effect on some morphological and physiological characteristics of these two species under salt stress.   Materials and Methods      This research was conducted in a factorial experimental format based on a randomized complete block design with three replications. Experimental treatments include five levels of salinity (0, 40, 80, 120 and 160 mM) and two levels of melatonin foliar spraying (0 and 100 μM). After the end of the treatment period, the morphological and physiological characteristics of the plant were measured by the different methods. Data analyses were performed using SPSS software version 20. Results were analyzed using one-way analysis of variance (ANOVA) followed by Tukey’s multiple comparison test. The results were expressed as mean values and standard error (SE) of the means.   Results and Discussion      The results of variance analysis indicated that species, melatonin and salinity stress have a significant effect on all morphological factors at the p < 0.05. The results of compare means showed that the number of leaves in both plants has a significant decrease at the probability level of 5% with the increase in salinity. However, the amount of this decrease in the samples that have been affected by melatonin is lower than the samples without melatonin. The use of melatonin has reduced the negative effects of salinity stress in two plants, so that at the salinity level of 160 mM sodium chloride, the use of melatonin has increased the fresh and dry weight of coriandrum sativum L. shoots by 7 and 3.61 times, respectively. The results of variance analysis showed that melatonin and salinity stress have a significant effect on all pigments. The results shown that with the increase in the level of salinity stress, a significant decrease (p < 0.05) was observed in the amount of chlorophyll and anthocyanin pigments of two species. The results of variance analysis showed that species and melatonin have a significant effect at the p < 0.01 on all physiological parameters, and salt stress has a significant effect at the p < 0.01 on all the physiological parameters except of relative water content. Also, the interaction effects of species with salinity, species with melatonin, melatonin with salinity and the interaction of all three factors have a significant effect at the 1% probability level on the parameters of proline and total phenol.With the increase in salinity, the amount of total protein in both species decreased, but the amount of this decrease was lower in the plants that were treated with melatonin. In coriandrum sativum L. plant, the amount of total protein reduction at 160 mM salinity level is 42.31% compared to the control, but this reduction was 28.9% in the plants that were treated with melatonin. Also, in the Anethum graveolens L., the amount of total protein reduction at the salinity level of 160 mM was 29.78% and 21.06% respectively, in the samples without melatonin treatment and under melatonin treatment.   Conclusions      The results of variance analysis of the data showed that melatonin has a significant effect at the probability level of 1 and 5% on all morphological and physiological parameters measured in both plants. Also, the compare means showed that with the increase in the level of salinity stress, a significant decrease in the probability level of 5% was observed in the parameters measured in two plants. In general, the external application of melatonin moderates the negative effects of salinity stress, and therefore melatonin can be used to improve the growth of plants under stress