Primary leaves of young and old bean plants (Phaseolus vulgarisL.) were treated with benzyladenine (BA) after cell division had been completed. Changes in the synthesis and amount of DNA in individual nuclei in mesophyll cells shortly after BA application were studied. Cytofluorometric determination of nuclear DNA with 4′,6-diamidino-2-phenylindole (DAPI) showed that cells containing 4C nuclei had appeared in both young and old leaves by 24 h after BA application, while the nuclear DNA content in control leaves remained at 2C. The number of 4C nuclei increased until 48 h in young leaves, but not in old leaves. Autoradiographic analysis showed that nuclei labelled with 6-3Hthymidine increased over the control level 12 h after BA application. This effect peaked at 24 h followed by a decline. There was no difference in the initial effect between young and old leaves, but the effect diminished more rapidly in old leaves than in young ones. The results are discussed in relation to those obtained from DNA measurements long after BA application in previous studie
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