Cytokinin‐induced switch in development in excised cotyledons of radiata pine cultured in vitro

Cotyledons of Pinus radiata D. Don were cultured under shoot‐forming (plus cytokinin) and elongating (minus cytokinin) conditions. Using. autoradiographic and precursor incorporation techniques, the sites and rate of macromolecular synthesis were examined during the first five days in culture. Active incorporation of 3H‐thymidine, 3 H‐uridine and 3H‐leucine occurred. In shoot‐forming cotyledons the incorporation became preferentially located in the epidermal and sub‐epidermal cell layers in contact with the medium. In elongating cotyledons, in contrast, incorporation was randomly distributed, and the amount of incorporation declined with time. Biochemically, differences in DNA, RNA and total protein synthetic patterns were observed. In elongating cotyledons the rates of RNA and protein synthesis were higher during the first 48 h than in shoot‐forming tissues, after which the synthetic rates were similar. Two peaks of newly formed DNA were observed in both tissues. These findings indicate that the cytokinin‐induced changes in developmental pathways began within 24 h in culture.