Various Forms of Foliar Applied Zinc Improve Drought Acclimation in Pomegranate: Response of Photosynthesis, Osmoregulation and Antioxidant Defense

Pomegranate (Punica granatum L.) is considered as a source of antioxidants with anticancer and antiviral activities. Optimal nutrition, especially the use of various forms of minerals can alleviate drought stress as a major challenge for pomegranate production. In this study, the effect of foliar application of various forms of zinc [zinc sulfate (5 and 10 mg L-1 of ZnSO4) and zinc oxide nanoparticles (5 and 10 mg L-1 of ZnO-NPs) and distilled water as a control] on some physiological and biochemical attributes of pomegranate plants under various levels of drought stress (100, 60 and 30% field capacity, FC) was assessed. SPAD, relative water content (RWC), content of chlorophyll a and b (Chl a and b) decreased by 16, 7, 46, and 14% under severe stress, respectively. In contrast, ZnSO4 (10 mg L-1) significantly increased the amount of SPAD, RWC and total Chl compared to the control. In addition, ZnSO4 (10 mg L-1) reduced the content of electrolyte leakage (23%) and malondialdehyde (13%) while ZnO-NPs (5 mg L-1) reduced the amount of hydrogen peroxide (9%) under severe drought. Foliar application of ZnSO4 (10 mg L-1) under severe drought stress also led to an increase in antioxidant enzymes including superoxide dismutase (1%), proline (27%), and carbohydrates (12%). Polyphenol oxidase (7%) also increased with the use of ZnO-NPs (5 mg L-1). It seems that the foliar application of Zn, especially ZnSO4 (10 mg L-1) can be a useful solution to improve the tolerance of pomegranate plants to drought stress, especially in areas with zinc deficiency.