Interactive effect of deficit irrigation and soil organic amendments on seed yield and flavonolignan production of milk thistle (Silybum marianum L. Gaertn.)

Milk thistle is a recognized medicinal plant cultivating for production of silymarin and oil. Responses of milk thistle to deficit irrigation and organic soil amendments were studied in a 2-year field experiment. Three levels of irrigation regimes including normal irrigation (I100), moderate deficit (I75) and severe deficit irrigation (I50) and two sources of soil amendments consisted of vermicompost (VM) and poultry manure (PM) plus no amendments comprised nine treatments laid out in a factorial pattern in a randomized complete block design. Moderate and severe deficit irrigation resulted in 7 and 27% seed yield reduction, respectively. Among yield components, number of seeds head−1 showed the highest sensitivity to deficit irrigation followed by number of heads plant−1 while seed weight remained stable. As irrigation deficiency intensified, Irrigation water use efficiency followed an increasing trend. Among the two soil organic amendments, only PM significantly improved the seed yield where plots amended with PM produced 8, 12 and 17% higher seed yield under I100, I75 and I50, respectively. Application of PM also improved irrigation water use efficiency. Implementation of deficit irrigation improved silymarin content of the seeds; however, silymarin yield declined due to the lower productivity in stressed conditions. Oil yield was also adversely affected by both moderate and severe deficit irrigation and reduced by 9 and 32%, respectively. Application of soil amendments had no significant effect on silymarin and oil content of the seeds, but PM improved silymarin and oil yield due to its beneficial effect on seed yield. The results of this study indicated that milk thistle can be grown under moderate deficit irrigation (saving 25% of irrigation water) and amending soil with PM can improve the yield and minimize the negative impact of drought stress.