The activities of eight cytokinins in promoting callus growth were tested In two Phaseolus genotypes, P. vulgaris L. var. Great Northern, and P. lunatus L. var. Kingston. The structural feature which contributes to the major genotypic difference in cytokinin structure-activity relationships is the presence or absence of a double bond at the 2,3position of the isoprenoid Nf side chain. In Kingst...