Histological, Morpho-Physiological, and Biochemical Changes during Adventitious Rooting Induced by Exogenous Auxin in Magnolia wufengensis Cuttings

Magnolia wufengensis, a rare ornamental tree species, is now in a huge gap between market demand and actual supply of seedlings. As cutting propagation is one of the most important means to solve the shortage of seedling supply, this study developed an efficient cutting propagation procedure of M. wufengensis, revealed the morphological and histological changes of adventitious root formation, and explored the rhythm correlation between rooting process and physiological and biochemical changes. Cuttings pre-treated with NAA:IBA (2:1) exhibited the best rooting performance. Anatomical analysis demonstrated that adventitious root primordia of M. wufengensis were initiated from cambial and parenchyma cells of xylem, with no relationship to the callus formed on the epidermis. The rooting process of M. wufengenis can be divided into four periods: induction phase (0–8 dap) (dap means days after planting), initiation phase (8–13 dap), expression phase (13–18 dap), and extension phase (18–28 dap). NAA:IBA (2:1) induced the accumulations of 3-indoleacetic-acid and increased the contents of peroxidase and polyphenol-oxidase near the wounding at induction phase. The initiation phase, with the first histological modifications to the formation of meristemoids, correspond to the increase of peroxidase, polyphenol-oxidase, and soluble protein contents. The synergistic reaction of low 3-indoleacetic-acid and high levels of gibberellins and zeatin also stimulates the initiation phase. In the expression and extension phase, high activities of polyphenol-oxidase, IAA-oxidase, and increased contents of soluble protein co-stimulate the emergence and outgrowth of adventitious roots. The present study not only provides optimized protocol by application of auxin combination but also presents insights in the histological, morpho-physiological, and biochemical changes in stem cuttings of M. wufengensis.