Evidence for Energy-Dependent 14C-Photoassimilate Retention in Isolated Tobacco Mesophyll Cells

14C-photoassimilate release from isolated tobacco (Nicotiana tabacum L. cv Xanthi nc.)mesophyll cells was gradual and much of the photoassimilate was retained for several hours. 14C-product retention was enhanced by Ca2' and impaired by EDTA and ATP. Mg2' reduced the ATP-enhanced "C-efflux suggesting that ATP impairs product retention by modifying cell membrane permeability, possibly as a result of its chelating properties. Various metabolic inhibitors increased "4C-efflux indicating an energy requirement for "C-product retention. Profiles of "4C-product distribution in the cells and suspending medium indicated that "C-efflux from cells treated with carbonylcyanide p-trifluoromethoxyphenylhydrazone occurred by passive diffusion. Coupled with the observation that exogenous sucrose increased IC-efflux and the demonstration of C-sucrose uptake, the results suggest that product retention is partially due to the activity of an energy-dependent uptake process which recovers much of the photoassimilate lost from the cells. Under certain conditions, carbonylcyanide p-trifluoromethoxyphenylhydrazone reduced "4C-efflux. The apparently contradictory effects of the uncoupler appeared to be related to the intracellular photoassimilate concentration and may be explained by the partitioning of photoassimilate within the cells