The effect of humic acid on endogenous hormone levels and antioxidant enzyme activity during in vitro rooting of evergreen azalea

It is important to produce roots in vitro for woody plants, such as azaleas. The influence of humic acid (HA) on histological development, antioxidant enzyme changes and endogenous hormone levels was evaluated during adventitious root formation in evergreen azalea microshoots. Explants (microshoots) were transferred to Anderson rooting medium supplemented with HA at 0, 0.5, 1, 2 and 5 mg L−1 for 56 days. HA at 1 and 2 mg L−1 improved the morphological root character of microshoots, such as root length, root number and rooting percentage compared with other treatments. The data collected during anatomical evaluation indicated that the cell division occurred on the third day of culture in the phloem adjacent to the cambium, which led to differentiation of the root primordium after ten days. Both treatments of HA (1 and 2 mg L−1) increased the endogenous hormone levels of indole-3-acetic acid (IAA) and gibberellic acid (GA) in rooted shoots, especially at the first period of root development. However, the increase of zeatin riboside (ZR) and isopentenyl adenosine (iPA) levels was shown during the in vitro rooting process. Moreover, HA contributed to higher activities in peroxidase (POD), superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT), polyphenol oxidase (PPO) and total soluble protein compared with other treatments at the same concentration. The results demonstrated that HA is effective for rooting in evergreen azaleas, and this effect was related to physiological and metabolic changes during adventitious root formation. Therefore, this information could help in developing a new type of rooting stimulator to reduce the high cost of plant growth hormones that are used for micropropagation.