Endogenous hormonal status of embryogenic callus in cocos (Cocos nucifera L.) explants

Abstract

Endogenous hormonal concentrations of Z/ZR, IP/IPA, ABA and free IAA were determined in Cocos in vitro cultures during the period of embryogenic callus (calloid) formation and in selected tissues after 16 weeks grown on calloid induction medium. Using immature inflorescence of the Cocos nucifera L. cv. Laguna Tall as explant source rachilla explants were cultured on semi-solid medium supplemented with 2,4-D in combination with either 2iP or BAP. For hormonal analyses samples were taken at every subculture or at the end of the calloid induction period only. Then explant tissues were separated into embryogenic callus (calloid), calloid forming and non-calloid forming explants. Compared to the initial status the main hormonal changes of Z/ZR, IP/IPA and ABA occurred in the first four weeks after culture initiation. Measurements at every subculture revealed a decrease of the endogenous Z/ZR and ABA concentrations during the four months of culturing and increased endogenous IP/IPA levels in week 16. Often a slight increase of the Z/ZR concentration could be observed when calloid formation became visible. The levels of free endogenous IAA fluctuated during the whole culture period following no clear pattern. After 16 weeks on calloid induction medium there were no distinct differences detectable in the endogenous Z/ZR level of calloid forming and non-calloid forming explants but higher IP/IPA concentrations could be found in calloid forming explants. Besides this rachilla explants having a certain IP/IPA/IAA ratio tended to form calloids at a higher percentage. In general, percentage of calloid formation was quite variable among the inflorescences derived from different palms. Calloid formation was significantly not affected by the type of applied cytokinin but influenced by the exogenous 2,4-D concentration added to the culture medium.