Effects of cytokinins on the growth of primary roots of Zea mays

Intact primary roots of Zea mays seedlings, apical 6-mm segments isolated from the intact primary roots and 5-mm detipped segments, prepared from the 6-mm apical segments by removal of the apical 1-mm meristematic region, were incubated in potassium-phosphate buffer that contained various concentrations of kinetin, 6-benzylaminopurine (BAP) or zeatin. These cytokinins inhibited the growth of intact primary roots but they promoted the growth of both tipped and detipped apical segments. In other words, they promoted the growth of root segments irrespective of the presence or absence of apical meristematic regions. Detipped segments were stood vertically, with their apical or basal cutends in contact with an agar plate that contained the above-mentioned buffer and various concentrations of kinetin, BAP or zeatin so that cytokinins were supplied either from the apical or basal cut-ends. Cytokinins supplied from the top promoted the growth of the segments, while those supplied from the base did not. These results indicate that the response of roots to the exogenously applied cytokinins is not influenced by the presence of the root meristem but is significantly affected by the way in which cytokinins are supplied.