STAGES IN THE INITIATION OF ROOT AND SHOOT ORGANOGENESIS IN CULTURED LEAF EXPLANTS OF NICOTIANA-TABACUM CV XANTHI NC

Reciprocal transfers of Nicotiana tabacum cv. Xanthi nc. leaf explants were made daily between root inducing medium (RIM) and shoot inducing medium (SIM), SIM and a basal medium containing no growth regulators (BM), and RIM and BM. It was found that the explants became determined for shoot production after 6 d, while roots were produced after only 1 d on RIM before transfer to BM. The competence of the explant to produce roots was greatly reduced by culture on BM prior to culture on RIM. There was far less reduction in shoot numbers with preculture on BM. Explants were found to be only weakly canalized for both caulogenesis and rhizogenesis for the first 2 d after determination. Thereafter they became strongly canalized. Transfers were also made from BM to SIM and back to BM, which revealed that the explants became competent for caulogenesis in the absence of cytokinins prior to determination. The period for which SIM is required can be reduced to only 1 d.