The combined effect of melatonin and salicylic acid improved the tolerance of safflower seedlings to zinc toxicity

A pot experiment was carried out to assess the effect of exogenous melatonin (MT) and salicylic acid (SA), alone or mixed together on the alleviation of phytotoxicity caused by Zn excess (500 µM ZnSO 4) in safflower ( Carthamus tinctorius L.) seedlings. The use of MT, SA, and especially MT + SA caused a marked raise in the production of biomass and chlorophyll amount of Zn-treated plants. A considerable reduction in the level and the activity of oxidative stress markers was recorded by using MT, SA and their combination. A remarkable enhancement in ascorbate (ASC) level was noted by using MT and SA in Zn-treated seedlings related to plants grown under Zn alone, while the glutathione (GSH) levels exhibited a noticeable raise only in SA-treated samples. The level of phytochelatin (PC) increased significantly in different parts of seedlings treated by Zn excess, though, by applying MT, SA and MT + SA, a further increase was noted only in PC level of leaves. MT and SA usage in seedlings treated with Zn considerably promoted the activity of enzymes involved in glyoxalase system. Based on our findings, it was found that the synergistic use of MT and SA can lead to enhanced survivability of safflower plants in soils contaminated with a large level of Zn by diminishing the absorption of Zn and its accumulation in aboveground parts of plants, also with the stimulation of phytochelatin biosynthesis and the modification of glyoxalase and antioxidant defense systems.