The Effect of Low Osmotic Potential on Phosphate Uptake and Metabolism by Beetroot Discs

The uptake of phosphate by aged beetroot discs was examined under a range of water deficits which were induced osmotically with mannitol or polyethylene glycol At low water potentials the absorption of labelled phosphate was enhanced when the ambient concentration of phosphate was high (10−3 M) and the time for absorption was short (30 mm): this occurred even when the metabolism was inhibited (by the water deficit), as indicated by lowered oxygen consumption and incorporation of 33P into hexose-phosphate, phosphoglycenc acid, a liquid fraction, and a precipitate consisting mainly of cell walls. The Q10 of the uptake process approached unity at these low water potentials. Accompanying the enhanced uptake there was an increase in leakage of previously absorbed phosphate. It is postulated that the stimulation in uptake of phosphate in low osmotic potentials was due to an increase in permeability Over long periods (6 h) uptake was inhibited by treatment at low osmotic potentials. This is interpreted in terms of inhibition of a metabolic component of phosphate uptake.