Enhancement of drought stress tolerance in two genotypes of milk thistle (Silybium marianum (L.) Gaertn.) by exogenous application of sodium nitroprusside

To evaluate the effects of sodium nitroprusside (SNP) on tolerance enhancement towards drought in Milk thistle (Silybium marianum (L.) Gaertn.), an experiment was conducted in a split split plot restriction in a randomized complete block design with three replications. The study was conducted in the research field of the University of Zanjan in 2014. Sodium nitroprusside at three levels of 0, 100 and 200 µmol l-1 was considered as the main factor, while drought stress at three levels of control and withholding irrigation at stem elongation and anthesis stages were regarded as sub factors. Finally, two genotypes of milk thistle (Hungarian and sari) comprised sub-sub factors. With increasing drought severity, leaf photosynthesis rate decreased significantly up to 45 percent compared with the control in both genotypes. Application of 100 µM SNP increased the leaf photosynthesis during stem elongation by roughly 80 and 100 percent in the Hungarian cultivar and Sari ecotype, respectively compared to non-application of SNP. Drought stress significantly decreased the pigment content in Sari ecotype, while this decrease was compensated by applying SNP during stem elongation. Drought stress in both stages of irrigation withholding reduced the grain yield in both genotypes. However, the exogenous application of 100 µM SNP compensated the reduction of grain yield, particularly in Sari ecotype. In addition, withholding of irrigation reduced the active ingredient yield; however, this reduction was compensated by increasing the content of silymarin and grain yield especially in the stage of anthesis stress with application of 100 µM SNP. Research findings showed that concentration of 100 µM SNP was more efficient to protect the milk thistle with increasing drought severity and cultivation of this plant in the water deficit systems.