Foliar Application of Chitosan and Salicylic Acid Improved Milk Thistle's Tolerance to Drought Stress

Drought is the main abiotic agent for growth limiters and crop production. Therefore, in this study, we investigated the effect of chitosan and salicylic acid on morphological and physiological parameters and fatty acids content under drought stress. The first factor was drought stress levels (Field capacity (F.C), 70% of F.C and 40% of F.C), the second factor was elicitors (control (C), 0.5 mM salicylic acid, 1 mM salicylic acid, 100 mg/L chitosan and 200 mg/L chitosan) and the third factor was sampling times (3, 7, 14, 21 days after drought stress) with three replications. According to the results, comparing the three drought stress levels, severe stress treatment (40% F.C) significantly reduced the growth traits of milk thistle. Foliar application of chitosan and salicylic acid significantly compensated for the reduction of growth traits in plants under drought stress, in which chitosan frequently had higher growth and tolerance than salicylic acid. The role of elicitors in reducing the stress negative effects on plant growth is often due to stimulating chlorophylls, reducing hydrogen peroxide levels, and production of antioxidant enzymes. In our findings, chitosan demonstrated an increase of 28.4, 10, and 8% in relative water content (RWC), chlorophylls, and catalase activity, respectively, compared to the control. Similarly, salicylic acid revealed an increase of 29.3, 4, and 2% in the same parameters compared to the control. Besides, hydrogen peroxide in plants treated with chitosan and salicylic acid decreased by 20 and 15%, respectively. Besides, drought stress significantly reduced the predominant fatty acids in the seed, which elicitor application reduced the effect of stress slightly. Chitosan had a higher effect than salicylic acid in increasing fatty acids content. Foliar application of CHT and SA increased plant tolerance under drought stress. A comparison of two elicitors showed that CHT had a clear effect on plant development and amplification of defense systems against SA.