Differential Cytokinin Structure-Activity Relationships in Phaseolus

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 2,3-position of the isoprenoid N(6) side chain. In Kingston, trans-zeatin was 3-fold more active than dihydrozeatin and 30-fold more active than cis-zeatin. The activities of N(6)-(Delta(2)-isopentenyl)adenine and N(6)-isopentyladenine were nearly the same. In Great Northern, however, dihydrozeatin was at least 30-fold more active than both trans-zeatin and cis-zeatin, and N(6)-isopentyladenine was 100-fold more active than N(6)-(Delta(2)-isopentenyl)adenine. The results suggest the possibility of employing cytokinin structure-activity relationships in distinguishing genotypic differences in cytokinin function and metabolism.