Is Modulation of the Rate of Proton Pumping a Key Event in Osmoregulation?

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Abstract
The net uptake of 3-O-methylglucose into leaf segments obtained from Senecio mikanioides Otto, and net proton efflux from the segments, were both promoted when the osmotic potential of the medium was decreased by addition of mannitol, sorbitol, or polyethylene glycol (optimal osmolarity, 0.3 Osmolar for mannitol and sorbitol). The effect was not due to promotion of `aging', since the antibiotic cerulenin suppressed aging without reducing the size of the mannitol stimulation; further, mannitol did not accelerate aging. Neither was the effect ascribable to diminished efflux (i.e. reduced `leak' because: first, visualization of the unidirectional sugar fluxes by double labeling indicated that the effect of added osmoticum was to promote influx rather than to reduce efflux; second, compartment analysis did not suggest any effect of mannitol on the rate constants for efflux from either the slowly equilibrating or more rapidly equilibrating compartment. The effect was not specific to poly-ols since it was also obtained with betaine and choline chloride. Since methyl glucose is not taken up into the phloem it could not be ascribed to a turgor effect on phloem loading. We conclude that the effect may reflect osmoregulation. As the sugar flux is probably driven by protonmotive force, it is likely that the effects on proton flux and on sugar flux are related. We suggest that the plasmalemma-sited proton pump is sensitive to the hydrostatic pressure gradient across the plasmalemma-cell wall complex, and functions both as detector and as effector in osmoregulation.
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