Relationships between endogenous hormonal levels and axillary bud development of Ananas comosus nodal segments

The effects of some endogenous hormones on the control of axillary bud development of pineapple Ananas comosus (L.) Merr. nodal segments cultivated in vitro were verified. Nodal segments with the apex (control) and decapitated nodal segments were used as explants and were cultured on hormone-free medium. Histological modifications occurring during the developmental process were also observed. Endogenous levels of indole-3-acetic acid (IAA) and four cytokinins (Cks) (isopentenyladenine (iP), isopentenyladenine 9-riboside (iPR), zeatin (Z) and zeatin riboside (ZR)) were quantified by means of high performance liquid chromatography (HPLC) and enzyme-linked immunosorbent assay (ELISA). During the first 4 h of culture, a marked reduction in the level of IAA (309%) of the decapitated nodal segment was detected and this coincided with the beginning of cell division in the leaf axil. Cks levels decreased during the first 12 h of culture in both explants. However, this reduction was 150% higher in the control, and occurred mainly due to a decrease in the level of ZR. The reduction in the level of IAA probably favored Cks biosynthesis and/or inhibited its degradation in the nodal tissue of the decapitated segment. After 16 h of culture, there was an increase in the level of IAA probably as a consequence of the establishment of the new auxin-producing shoot apex. After 1 d, a progressive increase in ZR occurred, suggesting that Z-type Cks have an important role in the leaf development of the new plant. Comparatively, auxin/total Cks ratio was always lower in the decapitated nodal segment throughout the process of the axillary bud development.