Hamid Hatami Maleki

experiment was conducted using randomized complete block design with 3 replications. Drought stress levels were measured using control plants with 100% water availability, 70%, 50% and 30% of available water. The results showed that water stress significantly affected most biochemical characteristics 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. For the ameliorating the adverse effect of water stress, among the investigated defense traits, plants increased proline levels and catalase and ascorbate peroxidase activities. Potassium, zinc and iron content in leaves was gradually increased by decreasing water depletion from 100 to 70% but further increase in drought stress decreased their values compared with control. Plant aerial biomass as well as root biomass was severely inhibited under water stress, but essential oil percentage increased with decreasing the water content from 100 to 50%. From this study, it was observed that lowering the soil water from 100 to 50% AWC is the best