Excretion of cytokinins into the cultivation medium by suspension-cultured tobacco cells

The dynamics of individual cytokinins were determined in both the cells and the cultivation medium during the subculture interval of cell suspension cultures of Nicotiana tabacum L., line VBI-O. The amounts of cytokinins detected in the cultivation medium were less than 1 pmol ml(-1) of suspension. In the late stationary phase, the levels of isopentenyladenosine, as well as that of dihydrozeatin and its riboside, increased significantly. However, when expressed per cell number, the levels of zeatin- and isopentenytadenine-type cytokinins in both the cells and medium were at a maximum at the beginning of the subculture interval and then gradually decreased. Cytokinins were excreted from the cells during the whole subcultivation period, and their concentrations in the cultivation medium were found to be approximately in proportion to their momentary levels inside the cells. The excretion might thus represent one of the mechanisms controlling endogenous cytokinin concentrations.