The objective of this study was to determine the effect of foliar spermidine (Spd) application on biochemical and nutritional response of valerian grown under different water stress (100, 70, 50 and 30% Available water content). The study was conducted in a pot experiment under greenhouse conditions. Water - stressed valerian seedlings were treated with foliar Spd application at different concentrations (0, 0.5 and 1 mM).The results showed that water stress significantly affected most biochemical and physiological characteristics as well as nutritional status of valerian plants. Leaf relative water content, chlorophyll a and b contents decreased while carotenoids and electrolyte leakage increased with the increase of water stress. Nutritional imbalance was also observed in water-stressed plants. For the ameliorating the adverse effect of water stress, among the investigated defense traits, plants increased proline levels and catalase and ascorbate peroxidase activities. Foliar application of Spd provoked the antioxidant enzymes activity and potassium accumulation that resulted in membrane damages alleviation. Therefore, exogenously applied Spd increased photosynthetic pigments content which supply energy for plant growth and production. Results also revealed that in most cases, effectiveness of Spd concentration had reverse trend with water shortage increasing, so, under moderate water stress, 1mM Spd was better than 0.5 mM.
