REVERSAL OF THE DIRECTION OF PHOTOSYNTHATE ALLOCATION IN URTICA-DIOICA L PLANTS BY INCREASING CYTOKININ IMPORT INTO THE SHOOT

The natural cytokinin import from the root into the shoot of Urtica dioica plants was enhanced by supplying zeatin riboside (ZR) solutions of various concentrations to a portion less than 10 % of the root system after removal of their tips. After 6 h ZR pretreatment of the plants, (CO2)-C-14 was supplied for 3 h to a mature (source) leaf or to an expanding leaf and the C-14-distribution in the whole plant was determined after a subsequent dark period of 14 h. ZR substantially increased C-14 fixation by the expanding leaves and also enhanced export of carbon and transport to the shoot apex. The effect of the hormone treatment was, however, more pronounced when the (CO2)-C-14 was supplied to a mature leaf. In the control plants these leaves exported carbon only to the roots: When the amount of the natural daily ZR input from the roots to the shoot was enhanced by 20 %, the bulk of the C-14 exported from a mature leaf moved to the shoot apex and only a minor portion of C-14 was still detected in the root fraction. A several-fold increase of the natural daily ZR input into the shoot resulted in a flow of C-14 only to the growing parts of the shoot. The results suggest control of the sink strength of the shoot apex by ZR in Urtica diocia.