Sink Effect of Cytokinin in Detached Leaves Is Not Caused by Structural Rearrangements of Phloem

The sink effect of cytokinin is manifested as a decrease in source capacity and the induction of sink activity in the phytohormone-treated region of a mature excised leaf. In order to find out whether this effect was due to the direct action of cytokinin on the phloem structure, two types of phloem terminals were examined. In pumpkin (Cucurbita pepo L.) leaves, the phloem terminals are open; i.e., they are linked to mesophyll by numerous symplastic connections, which are located in narrow areas called plasmodesmal pit fields. In broad bean (Vicia faba L.) leaves, the phloem terminals belong to the closed type and have no symplastic links with mesophyll. The electron microscopic study of terminal phloem did not reveal any structural changes in the companion cells, which could account for the suppression of assimilate export. The treatment of leaves with cytokinin neither disturbed the structure of plasmodesmal pit fields in pumpkin leaves nor eliminated the wall protuberances (the ingrowths promoting phloem loading) in bean leaves. No evidence was obtained that the cytokinin-induced import of assimilates in mature leaves is caused by the recovery of meristematic activity, i.e., by either formation of new phloem terminals having immature sieve elements capable of unloading or by the development of new sieve elements within the existing veins. Cytokinin did not induce de novo formation of phloem elements. Structural characteristics of the leaf phloem, such as the number of branching orders in the venation pattern, the number of vein endings per areole, the number of areoles per leaf, the area of one areole, and the number of sieve elements per bundle remained unaltered. It is concluded that the sink effect of cytokinin in excised leaves cannot be determined by alteration of the phloem structure.