The Interactive Treatment of Alternating Stratification and GA₃ Provides a Viable Approach for Efficient Propagation of Magnolia biondii

Magnolia biondii, a species with traditional ornamental and medicinal value, is extensively cultivated in China. Owing to the advances in horticulture and pharmaceutical industries, the market demand for M. biondii seedlings has steadily risen in recent years. Therefore, how to efficiently obtain the high-quality seedlings is a challenging issue to be solved urgently. In this study, four interactive treatments were applied, combining different stratification temperatures (15 °C for 56 d; 5 °C for 28 d → 15 °C for 28 d) and exogenous GA₃ concentrations (300 mg·L⁻¹; 1200 mg·L⁻¹), to investigate their interactive effects on seed germination, especially the after-effects on subsequent seedling growth. Our results showed that a decrease in phenylalanine ammonia-lyase (PAL) activity and a synchronous increase in peroxidase (POD) and polyphenol oxidase (PPO) activities resulted in the decrease in total phenolic content of seed during stratification. The seeds treated with GA₃ (1200 mg·L⁻¹ sown for 36 h) and alternating stratification (5 °C for 28 d → 15 °C for 28 d) exhibited the most favorable germination traits, with the most vigorous subsequent seedling growth. Interaction analysis revealed that the stratification temperature and GA₃ had significant interactive effects on seed germination and the total mass (TM) of seedling. Actually, exogenous GA₃ exerted a more pronounced effect on seedling morphophysiological traits than stratification temperature, thus improving the seedling growth. In summary, pretreating the seeds with GA₃ and alternating stratification was an optimal approach for achieving higher germination rate and better seedling growth traits concurrently. These findings will provide viable technical guidance for efficient and high-quality cultivation of M. biondii seedlings.