Dormancy-Related Bioactive Compounds and Antioxidant Activity during Optimization of Germination Conditions for Onopordum nervosum subsp. platylepis Murb. Using Response Surface Methodology

Abstract

Onopordum nervosum subsp. platylepis Murb. is an Asteraceae species found in Tunisia, Algeria, and Libya. It has been studied for its potential use as a vegetable rennet alternative to animal-derived rennet, making it important to understand its germination characteristics. This species often shows low germinability due to dormancy, which limits its large-scale cultivation. In preliminary experiments, many factors were analyzed to evaluate the seed germination of this thistle including physical stratification with abrasive paper and high-temperature water, chemical treatments with hydrogen peroxide (H2O2) or sulfuric acid (H2SO4), and the exogenous use of the growth regulator gibberellic acid (GA3). Based on the obtained results and subsequent analysis, GA3 and cold stratification durations were selected for the second experiment, which used a full factorial RSM design with three levels for each factor. Additionally, the total polyphenol content (TPC), total flavonoid content (TFC), condensed tannin content (CTC), and DPPH assay were analyzed during the optimization process and for ungerminated seeds. The findings revealed that 14 days of cold stratification followed by 750 ppm GA3 was the most efficient method for breaking the dormancy of these thistle seeds. The highest TPC was recorded in ungerminated seeds. However, germinated Onopordum nervosum ssp. platylepis seeds showed higher levels of TFC and CTC (14.83 mg QE/g DW and 6.49 mg EC/g DW, respectively) compared to the non-germinated ones and demonstrated the greatest potency in inhibiting free radicals (DPPH EC50 = 0.018 mg/mL) at the identified optimal germination conditions. Ungerminated seeds indicated significant phenolic content (TPC) and a limited ability to reduce oxidants, which could explain their low germination percentage. Our findings on the seed germination and dormancy characteristics of this endemic thistle will aid in the protection and development of its germplasm.