The external application of putrescine (Put) overcomes the adverse impact of different abiotic stresses. The current study assesses the impact of Put on the growth and antioxidative responses of oilseed rape (Brassica napus L.) seedlings under excess levels of zinc (Zn). The experiment was done in three different treatments including Hoagland nutrient solution containing supplemental concentrations of 20 μM ZnCl2 (Zn treatment), 0.2 mM Put, 20 μM ZnCl2 along with 0.2 mM Put (Put + Zn), and a control treatment without extra Zn and Put. The excess Zn led to the reduction of plant growth and pigment content. The content of total phenols, flavonoids, and flavonols was not significantly changed except that of the flavonols in the roots which was enhanced under Zn toxicity. Likewise, phenylalanine ammonia-lyase (PAL) activity has been significantly increased upon exposure to excess Zn. The content of hydrogen peroxide (H2O2), malondialdehyde (MDA), and the activity of antioxidant enzymes has been increased upon exposure to Zn. The Put improved growth and the content of pigments of oilseed rape under Zn stress. The content of H2O2 and MDA in oilseed rape exposed to Put was reduced under Zn excess. In contrast, Put enhanced the content of flavonoids and flavonols under Zn toxicity. The activity of PAL and polyphenol oxidase was induced but that of the superoxide dismutase and peroxidase was reduced by treatment of oilseed rape with Put under Zn stress. Indeed, Put alleviated the negative impact of Zn toxicity partially due to the minimized endogenous levels of Zn in the root, the reduction of the content of H2O2 and MDA, and the modulation of the antioxidant system.
