Effect of drought stress on seed yield, essential oil yield and ability of reactive oxygen species scavenging in Nigella sativa L. ecotypes

In order to investigate the effect of drought stress on grain and essential oil yield, antioxidant properties of essential oil, and accumulation of conformal osmolites in the leaves of two Nigella sativa L., this research was conducted at the research farm of Islamic Azad University, Shoushtar Branch during 2016-2017. The experiment was a split plot based on randomized complete block design with four replications. The main plot included four levels of irrigation including optimal irrigation, mild, moderate and severe stress (based on irrigation under 90, 80, 70 and 60% soil field capacity, respectively), and ecotypes of Kashmar and Shahreza were considered as subplots. Results indicated that the grain yield of Kashmar ecotype was reduced 2.4%, 20.2% and 40.1% under mild, moderate and sever stress compared to optimum conditions, respectively. The grain yield of Shahreza ecotype was reduced 6.2%, 38.1% and 63%, respectively. Reduction of grain yield under severe stress conditions was due to a significant decrease in the number of capsules and seeds per capsule. Essential oil yield of Kashmar and Shahreza ecotypes decreased by 18% and 54% in severe drought stress conditions, respectively. Kashmar ecotype had higher levels of carbohydrate and proline than those of Shahraza under stress conditions. This property caused the Kashmar ecotype to have more relative water content and lower slope change of grain yield and essential oil yield as compared with Shahreza. In normal conditions and severe drought stress, 85% and 93% of essential oil components of Kashmar and 82% and 93% essential oil components of Shahreza ecotype were identified, respectively. Drought stress increased the percentage of essential oil in both ecotypes and the essential oil of Kashmar contained more antioxidant. In general, under mild stress conditions, the highest essential oil yield was obtained due to an increase in essential oil content and no significant reduction in grain yield. Thus, it seems that the application of mild controlled stress improves the quality of Nigella sativa without affecting its quantitative performance.