Zinc oxide nanoparticles as a way to improve physiology, photosynthesis, and anti-oxidative systems in Salvia leriifolia

Abstract

Salvia leriifolia Benth is one of the vital herbs utilized in traditional medicine and pharmaceutical industries. Salinity stress has adverse effects on S. leriifolia. Zinc plays a significant role in plant tolerance to several environmental stresses. To investigate the interaction of salinity stress on various physiological and biochemical traits, different doses of NaCl (50, 100, 150, and 200 mM) and zinc oxide nanoparticles (ZnO NPs) in foliar form (2 and 4 mg/L) were used. A completely randomised design was conducted in four replications under greenhouse environs. The results displayed that compared to the control (without salinity stress), salinity stress at 200 mM NaCl caused a significant decrease in total chlorophyll (α ≤0.05). In addition, 150 and 200 mM NaCl led to a significant decrease in carotenoids and soluble sugars content. The amount of malondialdehyde (MDA), H2O2, catalase (CAT), superoxide dismutase (SOD), guaiacol peroxidase (GPx), polyphenol oxidase (PPO), and phenylalanine ammonialyase (PAL) was increased at all salinity levels studied (except for GPx and PAL at 200 Mm NaCl and MAL and PPO at 50 mM NaCl). Under salinity conditions (especially levels higher than 200 mM NaCl), foliar application of ZnO NPs (especially at 4 mg/L) caused a significant increase in the soluble sugar, total phenols, carotenoids, PAL, CAT, and GPx enzymes activity, and caused a substantial decrease in hydrogen peroxide and MDA (compared to the salinity stress treatment without ZnO NPs, α ≤0.05). In total, 32 types of compounds were identified in the essential oil of the plant. Under salinity conditions, some essential oil compounds (including α-pinene and α-muurolene) showed a significant decrease and some (including β-pinene and β-myrcene) showed a significant increase compared to the control. Under these conditions, the use of ZnO NPs caused significant changes in the essential oil compounds, which did not have a clear trend (α ≤0.05). It seems that foliar spraying of ZnO NPs, particularly at 4 mg/L, improved the tolerance to salinity of S. leriifolia by affecting antioxidant compounds, osmotic osmolytes, photosynthetic pigments, and membrane stability. These results suggest that 4 mg/L of ZnO NPs significantly improves the physiological and antioxidant activity of S. leriifolia under salinity stress conditions.