Cytokinin-Induced Mitosis in Cultured Explants of Helianthus tuberosus L. Tuber Tissue

Freshly excised explants of H. tuberosus tuber parenchyma tissue were cultured for up to 42 h in aqueous solutions of 0.6 �M indole-3-acetic acid (IAA; 0.6 �M) or one of four different cytokinins (0.4 or 2.0 �M) in the absence of IAA. All phytohormone treatments were effective in inducing partially synchronous mitosis, but mitosis was not observed in control explants cultured in water alone. IAA induced mitosis earlier and in a greater proportion of cells than did any cytokinin treatment. In most cases, cytokinins were more effective at the higher concentration. The relative effectiveness of the different cytokinins appeared to be benzyladenine > zeatin ≥ kinetin > zeatin riboside. Microdensitometric measurements of relative nuclear DNA levels indicated that DNA synthesis had not occurred in control explants, but was induced by both IAA and cytokinins before the mitotic activity that was observed in the later stages of culture. These responses of freshly excised explants were compared to the effect of zeatin on IAA-induced mitosis in prewashed explants, in which zeatin increased the synchrony of IAA-induced mitosis but not the proportion of cells undergoing mitosis.