Introduction: Milk thistle (Silybum marianum L.) is one of the most important medicinal plants that has been considered due to its importance in the pharmaceutical industry. This medicinal plant contains a group of compounds called silymarin, which have antioxidant and anti-inflammatory properties. Milk thistle has liver-protective flavonolignans. Also, the seeds of this plant contain 20-25% oil, of which 50-60% is oleic acid. Drought is one of the most important problems in crop production, especially in arid and semi-arid regions of the world such as Iran. Under drought stress, crop growth and economic yield are intensely declined. Ascorbic acid antioxidant is a key growth mediator that protects plant tissues against free oxygen radicals in plants. Also, this antioxidant is known as a growth regulator due to its effects on cell division and differentiation. This study was conducted to investigate the deleterious effects of water deficit stress on physiological characteristics, growth and seed yield of milk thistle by foliar spraying plants with the antioxidant ascorbic acid.

Materials and Methods: This study was conducted on milk thistle in the research farm of the Agricultural Research Institute of Zabol University. The experiment was performed in split plots and in a randomized complete block design with three replicates in the 2014-2015 crop year. The main factor included three levels of irrigation treatment (irrigation after 60 mm of evaporation from the surface of the evaporation pan of class A (control), irrigation after 120 mm of evaporation from the surface of the evaporation pan of class A (moderate water stress), and irrigation after 180 mm of evaporation from the surface of the evaporation pan of class A (severe water stress)) and the subfactor included three levels of ascorbic acid (0, 1, and 2 mM). Traits such as chlorophyll a and b and proline contents, plant height, biological yield, number of capitulum per plant, number of seeds per capitulum, 1000-seeds weight, seed yield, and harvest index were measured.

Results and Discussion: The effects of irrigation treatment and ascorbic acid on all measured traits was significant except the effect of ascorbic acid on harvest index. Significant interaction effect of irrigation treatment and ascorbic acid was observed on the chlorophyll a and proline contents, number of capitulum per plant, number of seeds per capitulum, and seed yield. Moderate and severe water stress reduced all studied traits except proline content. Proline content increased under water moderate and severe stress conditions. Foliar spraying of plants with ascorbic acid at both concentrations resulted in an increase in the values of the studied traits except for the harvest index. The highest seed yield (976.33 Kg/ha) was obtained under non-water stress condition and foliar spraying of plants with ascorbic acid at a higher concentration (2 mM).

Conclusion: Water stress had negative effects on chlorophyll content, growth and seed yield of milk thistle. Foliar spraying of milk thistle plants with ascorbic acid resulted in a reduction of the deleterious effects of water stress on these traits. Therefore, the use of ascorbic acid is recommended as a suitable strategy to improve the growth and seed yield of milk thistle under non-stress and water stress conditions.