Question of the Ribosyl Moiety in the Promotion of Callus Growth by Exogenously Added Cytokinins

We have examined the role of the 9 position and the influence of the 9-substituents on the cytokinin activity of N⁶-(isopent(en)yl)adenine derivatives. Five new cytokinin analogs have been employed in this study: 7-(3-methyl-2-butenylamino)pyrazolo[4,3-d]pyrimidine, 3-methyl-7-(3-methyl-2-butenylamino)pyrazolo[4,3-d]pyrimidine, 7-(3-methy 1-2- butenylamino)-3-β-D-ribofuranosylpyrazolo[4,3 - d]pyrimidine, 7-(3-methylbutylamino)pyrazolo[4,5 -d]pyrimidine, and 3-methyl-7-(3-methylbutylamino)pyrazolo[4,3-d]pyrimidine. Their activities have been compared with those of the corresponding purine derivatives. The relative biological activities in the tobacco bioassay suggested that the addition of a ribosyl moiety in the 9 position of exogenously supplied cytokinins is not a prerequisite for their promotion of cell division and growth of plant tissue.