Exogenous Sodium nitroprusside application on the antioxidant enzymes activity and active substance yield in milk thistle under drought stress

Milk thistle is a medicinal plant that is cultivated for the production of silymarin and oil. Silymarin composed of a different flavonolignans that have been used in treating liver disorders and other diseases. Although grain yield in the milk thistle decreases under drought stress, it may influenced on secondary metabolites yield. This study was conducted to investigate the effects of sodium nitroprusside on the enzymes antioxidant activity and active substance yield in milk thistle in the water deficit conditions.
The experiment was conducted in split-split plot restriction in a randomized complete block design with three replications. Foliar spray with sodium nitroprusside (SNP) 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, withholding of irrigation at stem elongation and withholding of irrigation at anthesis stage were regarded as secondary factors. Finally two genotypes of milk thistle (Hungarian and Sari) comprised the sub-sub factors. Measured traits were catalase, ascorbate peroxidase and peroxidase enzymes activity, RWC, cell membrane stability, silymarin yield and content and grain yield.
Drought stress decreased leaf relative water content since stem elongation and cell membrane stability during both times of withholding irrigation. Moreover, exogenous of sodium nitroprusside prevented from a further reduction RWC in stress conditions and improved cell membrane stability. Withholding irrigation enhanced catalase enzyme activity during stem elongation stage in both genotypes, While, drought stress in the Hungarian cultivar peroxidase and ascorbate peroxidase activity decreased and in Sari ecotype ascorbate peroxidase activity increased and had no effect on the peroxidase activity. Application of SNP in in the Hungarian cultivar particularly at the time of stem elongation stress significantly increased enzymes antioxidant activity. However in Sari ecotype, only led to a significant increase of ascorbate peroxidase activity during anthesis stress while, it caused a significant decrease in catalase in the stage of stem elongation stress and did not have any effect on peroxidase activity.
By increasing the intensity of stress, silymarin content increased, but had no influence on the silymarin yield. Application of up to 100 µM SNP had a increasing effect in silymarin content and improved silymarin yield in both withholding irrigation stages than plants which were solely under drought stress and did not receive SNP. Also drought stress significantly decreased grain yield while this decrease was compensated by exogenous application of 100 µM SNP in the stages of stem elongation and anthesis stress.
The results showed that SNP foliar application, particularly at the level of 100 µM increased activity of antioxidant enzymes and improved water use efficiency and stability of cell membrane. Hence prevented grain yield drop in water deficit conditions and enhanced milk thistle's active substance yield and content.