Abstract

The present research attempted to investigate the role of sodium nitroprusside (SNP) as a NO donor in yield and accumulation of active substances in milk thistle under drought stress in a 2-year farm experiment. In this experiment, two genotypes of milk thistle (Hungarin and Sari) were sprayed with SNP solutions (0, 100, and 200 µM) under water stress (control or withholding of irrigation from the stages of stem elongation and anthesis). Withholding of irrigation decreased the grain and silymarin yield and increased malondialdehyde (MDA) and hydrogen peroxide (H2O2) content. Application of 100 µM SNP decreased the content of MDA and H2O2 in plants submitted to water deficit and as a result, prevented silymarin yield reduction under stress by increasing grain yield. In both years, with increasing the intensity of stress, taxifolin content, silychristin, silybin A and B, as well as isosilybin B were increased in both genotypes. While, exogenous NO further enhanced taxifolin, silychristin, silybin B, and isosilybin B accumulation, compared with plants solely grown under water deficit without SNP. The results of this study indicated that SNP application, especially at the dose of 100 µM, prevented the leaf water loss and reduces irrigation times from eight times to six or four times under drought stress from the stages of anthesis and stem elongation, respectively, and thus improved grain yield in milk thistle. Moreover, exogenous application of SNP to increasing taxifolin synthesis further enhanced secondary metabolites’ performance under conditions with low water availability.