Effects of salinity stress on calendula (Calendula officinalis L.) by exogenous application of melatonin

Calendula officinalis L. is an annual plant of the Asteraceae family that has medicinal, food and ornamental uses. In order to evaluate the effect of melatonin on some morphological and physiological characteristics of Calendula officinalis L. under salt stress, a factorial experiment was conducted using a completely randomized design with three replications. In this study, the effects of 5 levels of salinity (0, 40, 80, 120 and 160 mM sodium chloride) and two levels of foliar spraying of melatonin (0 and 100 μM) were evaluated. The results showed that salinity stress caused a decrease in all the investigated parameters in C. officinalis L. under salinity stress. The results of variance analysis of the data showed that salinity stress has a significant effect (p ≥ 0.01) on all measured parameters except chlorophyll b and anthocyanin. However, melatonin caused a significant increase (p ≥ 0.01) in a number of morphological (number of leaves, number of flowers, length of shoots, fresh and dry weight of shoots) and physiological parameters (relative water content, chlorophyll a, total chlorophyll, carotenoids, total protein and total phenol). The results of this study showed that salinity stress reduced the morphological and physiological characteristics compared to control, while the use of melatonin improved the morphological and physiological characteristics of the Calendula officinalis L., and melatonin can be used to moderate the negative effects of stress.