Isatis indigotica Fort, a member of the Brassicaceae family, is widely distributed in China. I. indigotica is a biennial herbaceous plant. In the first year, the root and leaf are harvested to be processed into indigowoad root and indigowoad leaf that are believed to have antibacterial, antiviral, and immune-modulating effects and used as a traditional Chinese medicine (TCM) in China. In the second year, seeds are collected to be used for the cultivation of the plant. Currently, propagation by seeds is considered the most efficient method of I. indigotica production to fulfill an increasing demand.
Salinity and abiotic stress are limiting factors for plant growth. At least 20% of irrigated fields across the world are affected by salt (Pitman and Läuchli, 2002) due to improper watering and drainage systems, little rain, and evaporation (Munns and Tester, 2008). In China, according to the second national soil survey, the total area of saline soil is 9.913 × 107 ha. Although salinity stress appears to be an inhibitory factor at every stage of plant growth, the most sensitive phase comprises the germination and seedling periods (Patade et al., 2011), which are essential for plant growth (Hubbard et al., 2012). Therefore, effective and suitable measures should be developed to enhance seed germination in saline soils and facilitate plant establishment.
Seed priming is considered a critical and vital technique for enhancing stress tolerance through pretreatment with stimulating factors to adapt seeds more resistant to subsequent adverse factors (Tanou et al., 2012). Priming causes some metabolic changes to prepare seeds for plumule emergence (Farooq et al., 2007). It has been well-documented that priming treatment has enhanced salinity resistance in Cassia obtusifolia L. (Zhang et al., 2012, 2013, 2016), Glycyrrhizauralensis (Zhang et al., 2015), Cynanchum bungei Decne (Zhang, 2015), maize (Panuccio et al., 2018), wheat (Ali et al., 2017), broccoli and cauliflower (Wu et al., 2019), and lettuce (Ouhibi et al., 2014). The catalytic role of seed priming is related to a series of physiological, biochemical, and molecular changes. Among them, inducing the antioxidant system is a typical stress-avoidance response. Priming treatments increases the expression of antioxidant enzymes (Panuccio et al., 2018), which can optimize host defense mechanisms and reduce the oxidative damage caused by the accumulation of reactive oxygen species (ROS).
Although there have been many studies regarding the impact of seed priming on alleviating salt stress in a variety of plants, no investigation of the roles of priming in seed germination and seedling growth of I. indigotica under salt stress has been reported. Therefore, this study was designed to investigate the roles of three priming agents at different concentrations in seed germination, early seedling growth, and antioxidant enzyme activities of I. indigotica under salinity conditions.
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