UPTAKE, TRANSPORT AND STORAGE OF CARDENOLIDES IN FOXGLOVE - CARDENOLIDE SINKS AND OCCURRENCE OF CARDENOLIDES IN THE SIEVE TUBES OF DIGITALIS-LANATA

Cardiac glycoside transport was investigated on the organ and whole pl ant level. Uptake experiments were carried out with shoot and root cul tures of Digitalis lanata. In both systems primary cardenolides, i.e., those with a terminal glucose in their oligosaccharide side chain, we re taken up against their concentration gradient, whereas the glucose- free secondary cardenolides were not. Active uptake of primary cardeno lides was further evidenced by KCN inhibition of uptake. Using plantle ts grown in vitro the long-distance transport of primary cardenolides from the leaves to the roots was demonstrated. Cardenolides were also detected in etiolated leaves, induced on plants with green leaves, whi ch are supposed to be unable to synthezise cardenolides de novo, provi ding further evidence for long-distance transport. Several primary car denolides were detected in the honeydew excreted by aphids fed on Digi talis lanata leaves, indicating that the phloem is a transporting tiss ue for cardenolides. On the other hand, the xylem sap obtained by appl ying the pressure-chamber technique was cardenolide-free. It was concl uded that in Digitalis primary cardenolides serve as both the transpor t and the storage form of cardenolides. After their synthesis they are either stored in the vacuoles of the source tissue or loaded into the sieve tubes, from which they are unloaded at other sites where they a re trapped in the vacuoles of the respective sink tissue.