Osmotically induced volume changes in protoplasts isolated from Cucurbita ficifolia

Free membrane-bound protoplasts are formed when turgid stem segments are cross-sectioned into 2-mm-thick slices and placed in a nonplasmolysing solution of electrolyte or mannitol. The protoplasts range in diameter from less than 5 μm to a few more than 80 μm. The larger protoplasts contain a fuller complement of organelles than the small ones, and some large protoplasts form transvacuolar strands and restart cyclosis after 2 to 3 days. The protoplasts change diameter when they are exposed to fusicoccin or 2,4-dinitrophenol; from this we conclude that some of the solute transporters in the bounding membranes are still functioning. The pattern of swelling and shrinking of the protoplasts in response to the change in concentration and composition of the bathing medium leads us to believe that the membranes are very leaky to solutes causing a large coupling between solute and water flow. In addition high capacity net ion transport occurs at rates of up to 2 μosmol∙s−1∙m−2 causing shrinkage of the protoplasts in hypotonic KCl solution even though osmotic considerations force the prediction that the protoplasts will swell. Ion transporters working at the net rate of 0.5 μosmol∙s−1∙m−2 also appear to be responsible for swelling of protoplasts that previously shrank in hypertonic KCl or NaCl.