Effects of exogenous melatonin and glutathione on zinc toxicity in safflower (Carthamus tinctorius L.) seedlings.

The phytotoxicity caused by 500 μM ZnSO.7HO and its detoxifying by co-application of 100 μM of MT melatonin (MT) and glutathione (GSH) in 6-week-old safflower plants have been investigated. Reduced biomass production and total chlorophyll content on the one hand and increased content of hydrogen peroxide (HO), malondialdehyde (MDA) with increase in lipoxygenase activity, on the other hand, showed Zn- induced oxidative damage in safflower seedlings. When MT, GSH and especially MT + GSH exogenously were applied to Zn-stressed seedlings, the content of HO, MDA and the activity of lipoxygenase considerably decreased. In Zn- treated seedlings, the application of these signaling molecules led to a considerable increment in ascorbate (ASC), GSH and phytochelatin (PC) contents along with the induction of activity of antioxidant enzymes including ascorbate-glutathione cycle enzymes when compared with the plants stressed with Zn only. In Zn-stressed safflower seedlings treated with MT, GSH and MT + GSH, decreased activity of enzymes involved in glyoxalase system may be associated with the role of MT and GSH in reducing Zn uptake and reducing Zn-induced toxicity and subsequently, lower plant's defense responses. The data showed that the effects of MT and GSH, in particular, the combination of these two molecules in reducing Zn uptake and diminishing its accumulation in the shoots of safflower seedlings, and also the participation of MT and GSH on increasing plant ability to tolerate high amount of Zn through stimulation of various antioxidant defense systems suggest them as suitable candidates to better the survival of safflower in soils contaminated with Zn excess.