EFFECT OF PHYTOHORMONES ON TRANSPORT AND DISTRIBUTION OF C-14 SUCROSE IN EXCISED LEAVES OF SUGAR-BEET

We investigated the possibility and conditions of using excised leaves of sugar beet as a model on which to study regulatory mechanisms of assimilate transport. Preservation of the ability to export substances to the midrib in isolated source leaves is demonstrated with the aid of C-14-sucrose. The cytokinin BAP and phytotoxin fusicoccin (but not GA, IAA, or ABA) reverse this process, causing inhibition of export in the treated half and induction of import of substances from the untreated half of the leaf. In lower-story leaves the extent of sucrose outflow and effectiveness of BAP in its regulation decline rapidly with increase of calendar age. In middle-story leaves both indices are maintained on a constant high level for a fairly long time. The results produced by BAP in these leaves declined sharply when the petiole was preserved in them, which indicates that the hormonal effect on transport depends on the source-sink balance of the experimental system. It is concluded that middle-story leaves without petioles constitute a suitable model system possessing high sensitivity to cytokinin and ensuring good reproducibility of results.