ON THE BIOGENESIS OF CYTOKININS IN ROOTS OF PHASEOLUS-VULGARIS

Roots of intact bean plants were supplied with [14C]adenine by pulse-chase experiments. The rate of incorporation of radioactivity into tRNA and oligonucleotides of roots and the content of radioactive labeled cytokinin nucleotides in these RNA fractions were determined. On the average, 1/70 of the radioactivity incorporated into tRNA was localized in N6(.DELTA.2-isopentenyl)adenosine. The half-life of tRNA was estimated to be 65-70 h. Shortly after the pulse period, oligonucleotides contained zeatin riboside at a ratio of 1:800, on the basis of radioactivity. The half-life of these oligonucleotides was determined to be .apprx. 8 h. The main free radioactive cytokinin of roots and leaves was zeatin. In comparing the rate of degradation of 14C-labeled tRNA and the oligonucleotides of roots and the rate of appearance of radioactive cytokinins in roots and leaves, strong indications for their dependency were found. The results contradict the hypothesis of de novo synthesis of cytokinins in roots of intact bean plants.