THE ACTION OF CYTOKININS ON THE TRANSPORT AND PARTITIONING OF SUBSTANCES IN DETACHED LEAVES .1. SINK COMPONENTS OF CYTOKININ EFFECTS ON PLANTS WITH DIFFERENT TYPES OF TERMINAL PHLOEM

Source-sink transitions in a mature detached leaf in response to local ly applied cytokinin (BA) were studied. In the phytohormone-treated ha lf of a leaf, the export of C-14-sucrose from the leaf blade into the midrib and petiole was inhibited. In addition, BA induced sucrose impo rt form the untreated areas of the leaf. This means that the phytohorm one exerted sink effects, including both induced import and retention. As a result, the direction of phloem transport reversed. Basically, w e have found the direction of the transport to change in pumpkin leave s with the open terminal phloem, which is linked to the mesophyll by n umerous symplastic connections, and in the leaves of broad beans and k idney beans with their closed terminal phloem lacking symplastic conne ctions with the mesophyll. The magnitude of the cytokinin effect may d epend on the structural organization of mesophyll-phloem connections b ecause, as a result of the BA-inducted import, 52% of the C-14 accumul ated in the treated half of the pumpkin leaf, whereas only 32-35% of t he label accumulated in the leaves of broad and kidney beans. It is su ggested that the reversal of transport flow in the leaves with the ope n terminal phloem occurs much more easily than in the leaves with the closed terminal phloem.