Protective Effects of Nitric Oxide on Photosynthetic Stability and Performance of Silybum marianum under Water Deficit Conditions

Core Ideas Milk thistle is a medicinal plant cultivating for production of silymarin and oil.The results our work showed that with the application of SNP photosynthesis, pigments content, and water status were improved in the drought stress condition and as a result significant increases in secondary metabolites production and grain yield was observed.Overall, this study suggest that application of 100 µM SNP and using Sari ecotype can be recommended for maximizing crop yield under drought stress in semiarid environment. Milk thistle is a medicinal plant cultivated for production of silymarin and oil. Nitric oxide as an active molecule, plays an important role in plant’s reaction to drought stress. In this study, we investigated the role of sodium nitroprusside (SNP) as a nitric oxide donor for tolerance enhancement of drought in two genotypes of milk thistle in a 2‐yr field experiment. With increasing drought severity, photosynthetic rate and stomatal conductance of milk thistle decreased significantly. The exogenous application of SNP significantly compensated for the negative effects of drought on photosynthetic rate by 40%, but also reduced stomatal conductance during 6 d after withholding irrigation. Drought stress significantly decreased pigments content and this affect was compensated by applying SNP in both genotypes. Among yield components, the number of heads per plant showed the highest sensitivity to drought stress followed by seed weight, while all yield components responded significantly to the application of SNP. Withholding irrigation significantly decreased seed yield in both genotypes and years. Averaged over 2 yr, application of 100 µM SNP in Sari ecotype at the stage of stem elongation and anthesis significantly improved seed yield by 38 and 33%, respectively, compared with the non‐application of SNP. However, in the Hungarian cultivar, SNP only led to a significant increase of 29% in seed yield during anthesis stress. Overall, this study suggest that application of SNP at a rate of 100 μM had potential to ameliorate the adverse effect of drought stress on photosynthetic rate and seed yield of milk thistle.