Root Initiation in Cuttings ofPinus radiataSeedlings

Effects of exogenous growth regulators are related to the histological events in adventitious root initiation in hypocotyls of Pinus radiata D. Don. Indolebutyric acid (IBA) is required both for the pre-initiative and post-initiative phases in root development. Kinetin has a marked inhibitory effect over the pre-initiative phase, with escape from inhibition following establishment of meristemoids. Gibberellic acid (GA3) has three distinct effects depending on time of application. While inhibitory during the pre-initiative phase, it strongly enhances rooting if applied at a time coincident with the first observable stage of root initiation. If applied following establishment of meristemoids, GA3 inhibits root formation.