Licorice (Glycyrrhiza glabra L.) is known as an important medicinal plant throughout the world. Glycyrrhizin is one of the most important specialized metabolites produced by licorice. In order to study the effect of TiO2 nanoparticles (TiO2 NPs) and spermine on physiological and biochemical traits of licorice under cold stress conditions, a factorial experiment was conducted in a completely randomized design with three replications. Plants were exposed to optimum temperature (26 ºC) as control and low temperature (4 ºC) as cold stress conditions and also treated with TiO2 NPs (2 and 5 ppm) and spermine (1 mM), separately. Results from physiological and biochemical analyses of the aerial parts of licorice seedlings showed that the growth parameters and the content of photosynthetic pigments decreased in response to low temperature. TiO2 NPs and spermine treatments increased plant resistance to cold stress and decreased the level of oxidative damage by reduction of malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents. In other hand, TiO2 NPs and spermine caused increase of phenolics, total protein and osmolytes contents under cold stress conditions. An increase in glycyrrhizin content was significantly induced by low temperature, TiO2 NPs and spermine.
