The Effects of Fusicoccin, Abscisic Acid, and Benzyladenine on the Transport and Partitioning of Substances as Related to Cytokinin Sink Activity

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We tested the possible cytokinin effect on the functioning of the active transport system involved in the assimilate loading into the phloem as a cause for the cytokinin sink and retention effect. This effect is manifested in the deceleration of substance export from and the stimulation of substance import to the sites of local phytohormone application to the mature detached leaf from untreated leaf areas. To affect the membrane mechanisms of the substance transport, we used leaf treatment with the phytotoxin fusicoccin, an enhancer of plasmalemmal H+-ATPase and a potential stimulator of assimilates export, and with the phytohormone ABA affecting transport, metabolism, and plant growth. However, fusicoccin did not enhance 14C-sucrose export from the leaf blade and did not interfere with the cytokinin-induced export deceleration. ABA reduced substantially 14C export from the leaf but eliminated the cytokinin effect on this process. Similar results were obtained for broad bean (Vicia faba L.) leaves with apoplastic phloem loading, involving H+-ATPase activity, and pumpkin (Cucurbita pepo L.) leaves with symplastic phloem loading, that is, occurring without sucrose transmembrane translocation and without H+-ATPase involvement. The conclusion is that the cytokinin-induced development of sink zones in source leaves is not related to the membrane mechanisms of the substance transport in the mesophyll–phloem system. The data obtained support the idea that the cause for the cytokinin sink and retention effect is the enhancement of elongation growth and total activation of metabolism in the mesophyll cells of the detached leaf.

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