Cold stress induces biochemical and physiological changes in plants. Therefore, it is important to identify and apply compounds that can increase plant tolerance to cold. Nowadays, the use of nanoparticles as a suitable solution to increase plant resistance to environmental stresses is considered. Besides, polyamines play an important role in regulating plant defense responses to environmental stresses. This study aimed to investigate the effects of spermine and titanium dioxide nanoparticles (TiO2 NPs) on physiological responses of licorice (Glycyrrhiza glabra L.) plants to cold stress. Results showed that cold stress increased the content of total protein, phenolic compounds, glycine betaine, soluble sugars, glycyrrhizin, proline, hydrogen peroxide and malondialdehyde and also the activity of antioxidant enzymes, but decreased photosynthetic pigments content and growth indices. Spermine increased most of the features measured at low temperature but decreased the content of hydrogen peroxide and malondialdehyde. The production of glycyrrhizin was stimulated by cold stress and also by the application of spermine and TiO2 NPs.
