Ontogenetic Variation of Four Cytokinins in Soybean Root Pressure Exudate

Abstract

Cytokinins exported from the root may be involved in the correlative control of plant development. To test this hypothesis in soybean (G. max [L.] Merr. [cultivars] McCall, Chippewa 64 and Hodgson 78), cytokinins were intercepted en route from the root to the shoot by collecting root pressure exudate from detopped roots. The quantities of 4 cytokinins in the exudate were studied throughout the development of plants grown in the field and in controlled environment chambers. Zeatin, zeatin riboside, and their dihydro derivatives, dihydrozeatin and dihydrozeatin riboside, were isolated and quantitated using high-performance liquid chromatography. Cytokinin fluxes (pmol/plant per h) were independent of exudate flux (g/plant per h). All fluxes are averages for a 6- or 8-h collection period. The ribosides accounted for the majority of the observed cytokinin transport. The fluxes of zeatin riboside and dihydrozeatin riboside increased from low levels during vegetative growth to maxima during late flowering or early pod formation. Before the seeds began rapid dry matter accumulation, zeatin riboside and dihydrozeatin riboside fluxes decreased and remained at low levels through maturation. The fluxes of zeatin and dihydrozeatin were low throughout development. No correlation was found between cytokinin fluxes and nodule dry weight or specific nodule activity (acetylene reduction). The timing of distinct peaks in zeatin riboside and dihydrozeatin riboside fluxes during flowering or pod formation suggests that cytokinins exported from the root may function in the regulation of reproductive growth in soybean.