The co-application of exogenous 100 µM melatonin (MT) and 100 µM salicylic acid (SA) on 21-day-old safflower seedlings grown in the presence of cadmium (Cd, 100 µM) toxicity was investigated. The application of MT, SA, or MT + SA efficiently improved toxicity symptoms and declined Cd toxicity as shown by a considerable rise in plant biomass production and chlorophyll content accompanied by decreased level of oxidative stress markers. In Cd stressed plants, the simultaneous application of MT and SA led to sharp decreases in MDA and H2O2 amounts (61.04 and 49.11%, respectively), related to plants treated with Cd alone. With respect to the control, a 41 and 48% increment in reduced glutathione (GSH) and ascorbate (ASC) content was recorded in Cd-treated seedlings. Though, with the addition of MT, SA, or MT + SA, the content of GSH and ASC increased more. The application of MT, SA, or MT + SA caused a sharp induction in phytochelatin content of the leaves of Cd-treated seedlings, while in roots, the highest PC content was recorded only in the presence of MT, which was about 1.8-fold greater than in plant treated with Cd alone. The activity of enzymes responsible for the ascorbate-glutathione cycle and glyoxalase system considerably improved by using MT, SA, or the combination of MT and SA. Our findings suggest a possible synergic interaction between MT and SA in tolerating Cd toxicity by reducing Cd uptake, improving chlorophyll biosynthesis and accelerating ascorbate-glutathione cycle as well as the modulation of glyoxalase system.
Keywords: Antioxidant system; Cadmium; Carthamus tinctorius L.; Melatonin; Salicylic acid.