ACTION OF CYTOKININS ON TRANSPORT AND PARTITIONING OF SUBSTANCES IN DETACHED LEAVES .2. CONDITIONS FOR DEVELOPING THE CYTOKININ EFFECTS

The detached leaves of broad bean and pumpkin plants were employed to study the time-course and developmental changes in phloem transport an d assimilate redistribution as affected by cytokinin applied to the ha lf-leaf. The retention activity of cytokinin (inhibition of metabolite export from the phytohormone-treated leaf blade area into the midrib and petiole) reached its maximum 24 h after phytohormone application, whereas the development of import capacity (induction of mobile metabo lite inflow from the untreated half-leaf to the site of cytokinin appl ication) took another 24 h. Using leaves of four different ages, a rel ationship was outlined between the functional state of the leaf (rated by its source or sink activity) and the magnitude of cytokinin effect on the metabolite import or retention. In juvenile sink leaves, this phytohormone effect was missing; it appeared and retention activities grew up along with the development of source activity in the leaf and diminished upon its aging. The cytokinin effect depended on several fa ctors: (1) the rate and direction of mesophyll cell metabolism affecti ng metabolite partitioning between the intracellular consumption and t he formation of transport pool; (2) competition for assimilates betwee n two sinks - the artificial (the cytokinin-treated half-leaf) and the natural (the petiole and midrib); and (3) leaf competence toward cyto kinin.