Regulatory effects of combined application of exogenous IAA and 6-BA on sprout development and endogenous hormone balance in Pinus yunnanensis seedlings after stumping

Abstract Background To reveal the physiological mechanism by which exogenous indole-3-acetic acid (IAA) and 6-benzyladenine (6-BA) regulate sprout number in stumped Pinus yunnanensis seedlings, optimize hormone application schemes, and provide a theoretical basis for its asexual cutting propagation, this study adopted a two-factor three-level completely randomized block design. Different concentrations of IAA (0, 150, 300 mg/L) and 6-BA (0, 100, 200 mg/L) were sprayed on 1-year-old stumped P. yunnanensis seedlings. The contents of endogenous IAA, abscisic acid (ABA), gibberellin (GA₃), and zeatin (ZA) were determined, and the correlations between endogenous hormones and their ratios with sprout number and sprout length were analyzed. Results The results showed that single application of IAA exerted an overall dose-dependent inhibitory effect on sprout number. Endogenous IAA content decreased continuously with rising exogenous IAA concentration, while GA₃ and ZA showed a non-monotonic response characterized by “promotion at low concentration and inhibition at high concentration”. Under single 6-BA application, the responses of sprout number and sprout length were inconsistent, and endogenous ZA content decreased continuously with increasing 6-BA concentration. The combined application of IAA and 6-BA exhibited a significant non-additive interaction effect, and endogenous IAA content was generally low under combined treatments, decreasing in all combined application groups. Path analysis showed that sprout number had the strongest correlation with the GA₃/ZA ratio, while sprout length was significantly positively correlated with ZA + GA₃+IAA. The relative ratios of hormones had stronger explanatory power for sprout phenotypes than the absolute content of a single hormone. Conclusions The germination and growth of sprouts in stumped P. yunnanensis are closely related to the relative balance of endogenous hormones. The combined application of exogenous IAA and 6-BA may affect sprout occurrence by regulating key ratios such as GA₃/ZA. These results provide quantitative parameters for hormone regulation in the efficient propagation of superior P. yunnanensis clonal scions.