A split plot experiment with three replications on the bases of randomized complete block design was conducted in the research field of the university of Zanjan in 2013, to evaluate the effects of sodium nitroprusside (SNP) application rates as a NO donor (0, 100 and 200 µM) in improvement of growth and yield parameters of milk thistle under different times of withholding irrigation (Control, stem elongation stress and anthesis stress). Results of this study showed that with increasing drought severity, leaf RWC, Capitula per plant, 1000 grain weight, plant height, branch per plant, capitula diameter and biological and grain yield of milk thistle decreased significantly while the number of grains per capitula increased compared with the control. Application of SNP treatment had no influence on improvement of milk thistle yield parameters except for capitula diameter under drought stress. The exogenous application of 100 µM SNP significantly compensated the negative effects of leaf RWC and yield decline in the period of drought, so that spraying with 100 µM SNP enhanced RWC and grain yield in both stages of withholding irrigation and biological yield under anthesis stress compared with its non-application. Furthermore, exogenous of 100 µM SNP is more efficient than level 200 µM in improvement of milk thistle growth and yield under water stress in farming conditions. Keywords : Active constituents, Grain yield, Milk thistle, Nitric oxide, Water stress

Keywords: Active constituents, grain yield, milk thistle, nitric oxide, water stress