RESPONSE OF CYTOKININ CONCENTRATION IN THE XYLEM EXUDATE OF BEAN (PHASEOLUS-VULGARIS L) PLANTS TO DECAPITATION AND AUXIN TREATMENT, AND RELATIONSHIP TO APICAL DOMINANCE

When xylem exudate of previously untreated Phaseolus vulgaris plants w as analysed for cytokinins by radioimmunoassay, a low concentration (a bout 5 ng.ml(-1)) was found. However, when the plants were decapitated about 16 h before the xylem exudate was collected, an almost 25-fold increase in cytokinin concentration was observed. Twenty-four hours af ter decapitation this increase even reached 4000% compared to control plants. Applying naphthaleneacetic acid (NAA) to the shoot of decapita ted plants almost eliminated the effect of shoot tip removal on cytoki nin concentration, suggesting that cytokinins in the xylem exudate of intact plants are under the control of the polar auxin transport syste m. Other xylem constituents, such as potassium or free amino acids did not show this strong increase after decapitation and did not respond to NAA application. It is concluded that the observed auxin/cytokinin interaction has an important regulatory role to play, not only in apic al dominance but in many other correlative events as well.