Melatonin and TiO2 NPs Application-Induced Changes in Growth, Photosynthesis, Antioxidant Enzymes Activities and Secondary Metabolites in Stevia (Stevia rebaudiana Bertoni) Under Drought Stress Conditions

Drought stress is one of the major constraints on the growth and productivity of crop plants. Recently, new approaches have been employed in order to cope with the drought. Out of the approaches available, nanomaterials have the great interests in the relevant fields. In this regard, considering the effects of melatonin and nano-titanium dioxide nano particles (TiO2 NPs) against abiotic stresses, the current study was designed to compare the effects of a well-known and recently studied molecules, melatonin (0, 50, 100 mu M) and TiO2 NPs (0, 10, 100 mg L-1) on an array of agronomic, physiological and biochemical traits of stevia (Stevia rebaudiana Bertoni) under moderate and severe drought stress. The drought stress was submitted according to the field capacity of the soil, ranging from 90 to 60%. As expected, growth and photosynthetic pigments of the plants decreased with the severity of the stress but the relevant traits (growth and photosynthetic pigments) were improved with the applications of melatonin and TiO2 NPs under mid and severe stress. The improved traits of the plants and their tolerance might be consequences of induced antioxidant activities and high content of proline and diminished levels of MDA, as observed in the current work. Also, secondary metabolites of stevia were also increased by the treatments of melatonin and TiO2 NPs. Of the concentration used for treatments, generally, foliar application of melatonin (100 mu M) and TiO2 NPs (10 mg L-1) was the best treatment for decreasing negative effect of drought stress and increasing secondary metabolites in stevia plant.

Automated summary

Drought stress is a major constraint on crop growth and productivity. This study investigated the effects of melatonin and nano-titanium dioxide nanoparticles (TiO2 NPs) on the agronomic, physiological, and biochemical traits of stevia under moderate and severe drought stress. The results showed that the application of melatonin and TiO2 NPs improved plant traits, increased antioxidant activities, proline content, and secondary metabolites, and decreased malondialdehyde levels. The best treatment for alleviating the negative effects of drought stress and increasing secondary metabolites in stevia was foliar application of 100 μM melatonin and 10 mg/L TiO2 NPs.