Evidence for energy-dependent C-photoassimilate retention in isolated tobacco mesophyll cells.

(14)C-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. (14)C-product retention was enhanced by Ca(2+) and impaired by EDTA and ATP. Mg(2+) reduced the ATP-enhanced (14)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 (14)C-efflux indicating an energy requirement for (14)C-product retention. Profiles of (14)C-product distribution in the cells and suspending medium indicated that (14)C-efflux from cells treated with carbonylcyanide p-trifluoromethoxyphenylhydrazone occurred by passive diffusion. Coupled with the observation that exogenous sucrose increased (14)C-efflux and the demonstration of (14)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 (14)C-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.