Objective Cutting propagation is an important way to propagate excellent seedlings, and the application of suitable exogenous substances can promote rooting of cuttings. The rooting of Quercus mongolica cuttings is difficult. Exploring the effects of different concentrations of exogenous substances and their combine applications on the rooting of Quercus mongolica softwood cuttings can provide technical support for large-scale propagation by cutting propagation.

Method Using rapid dipping or soaking methods, different concentrations of indole butyrate potassium (IBA-K), rooting powder (ABT₁), K599 Agrobacterium rhizogenes, propiconazole (PCZ), and their combination were used to treat cuttings. The rooting rate, callus rate, rooting number, root length, nutrient content, enzyme activity, and changes in endogenous hormone content of cuttings were measured.

Result When different exogenous substances were used separately, the rooting rate of K599 Agrobacterium rhizogenes (OD₆₀₀ = 0.8) was the highest after soaking for 30 minutes, reaching 24.71%. After soaking in 50 μM PCZ for 2 hours, the root callus rate of cuttings was the highest, reaching 43.62%. For other exogenous substances alone, the rooting effect of cuttings was poor. When different combinations of exogenous substances were used, 50 μmol·L⁻¹ PCZ soaking for 2 hours combined with K599 Agrobacterium rhizogenes (OD₆₀₀ = 0.8) soaking for 30 minutes significantly improved the rooting rate of Quercus mongolica cuttings, reaching 36.37% for rooting and 51.14% for callus. The average number of roots and root length were 2.25 and 5.72 cm, respectively. In addition, the results indicated that the use of PCZ can accelerate the consumption of soluble sugars and proteins during the rooting process of cuttings, promote the activities of peroxidase (POD) and polyphenol oxidase (PPO), increase the content of IAA during the induction period of adventitious roots, and increase the IAA/ABA and IAA/ZR ratios.

Conclusion The combined application of PCZ and Agrobacterium rhizogenes K599 can significantly improve the rooting rate of Quercus mongolica softwood cuttings. Slow consumption of nutrients (soluble sugars, proteins), weak activity of antioxidant enzymes (PPO, POD), and low content of IAA may be the reasons of the difficulty in rooting of Quercus mongolica cuttings. PCZ has an important inhibitory effect on this, and can play an important role in difficult rooting species such as Quercus mongolica cuttings.