Nonosmotic effects of polyethylene glycols upon sodium transport and sodium-potassium selectivity by rice roots

Addition of polyethylene glycol (PEG) as an osmotic agent (at -230 kilopascals) dramatically lessened the toxicity of NaCl (at 50 moles per cubic meter) to rice (Oryza sativa L.) seedlings. This was explained by a reduction in the uptake of NaCl. This reduction was much greater than could be accounted for by the lowered transpiration rate resulting from the solute potential changes due to the PEG. Low concentrations of PEG (-33 kilopascals and less) had no effect upon transpiration rate but reduced sodium uptake (from 10-50 moles per cubic meter NaCl) by up to 80%. PEG (at -33 kilopascals) also reduced chloride uptake but had no effect upon the uptake of potassium from low (0.5-2.0 moles per cubic meter) external concentrations. However, the increased uptake of potassium occurring between 2 and 10 moles per cubic meter external concentration was abolished by PEG. Similar concentrations of mannitol had no effect upon sodium uptake in rice. PEG, in similar conditions, had much less effect upon sodium uptake by the more salt-resistant species, barley. 22Na studies showed that PEG reduced the transport of sodium from root to shoot, but had a long half time for maximal effect (several days). 14C-labeled PEG was shown to bind to microsomal membranes isolated from rice roots; it is suggested that this is due to multipoint attachment of the complex ions of PEG which exist in aqueous solutions. It is argued that this reduces passive membrane permeability, which accounts for the large effect of PEG on sodium influx in rice and the different effects on sodium influx and (carrier-dependent) potassium influx.