Effects of Slowly and Rapidly Permeating Osmotica on Permeability of Excised Roots of Zea mays

Abstract

Excised roots of Z. mays were treated at a water potential of – 20.8 atm, using slowly and rapidly permeating solutes both as osmotica (800 mM ) and as permeants (10 mM ) . The temperature during all experiments was 2°C. The solutes were mannitol, glycerol, and ethylene glycol. In highly vacuolated root tissues there were at least three phases for the influx and efflux of the permeants. These phases presumably represented the free space, the 'cytoplasm', and the 'vacuoIe'.

Treatment with slowly permeating osmotica increased the free space of both highly and slightly vacuolated tissues. In highly vacuolated tissues all osmotica doubled the rate of exchange for the 'cytoplasm', but had very little effect on the rate of exchange for the 'vacuole'. The latter observation suggests that the permeability of the tonoplast changed little during plasmolysis. In contrast to highly vacuolated tissues, exposure of slightly vacuolated tissue to osmotica decreased the rates of exchange of the permeant mannitol.

Removal of slowly permeating osmotica dramatically increased the permeability of highly vacuolated tissues. Moreover, permeability to rapidly permeating ethylene glycol increased much less than permeability to slowly permeating mannitol and glycerol. These observations suggest that deplasmolysis changed the structure of cell membranes.

Permeability of slightly vacuolated tissues was not greatly increased by removal of osmotica. These different effects on the permeability of highly and slightly vacuolated tissues are consistent with earlier results, which showed that removal of slowly permeating osmotica restored metabolism of slightly vacuolated tissues, while it strongly depressed cellular activity of highly vacuolated tissues.