Transport of Solutes and Water by Resetting Bladders of Utricularia

Abstract

The bladder of the insectivorous aquatic plant Utricularia, after stimulation and consequent sudden increase in volume, slowly resets, transporting solutes, mainly Na+, K+ and C1-, and water, from the lumen to the outside solution. The resetting process, which involves the movement of both water and chloride against the direction of passive driving forces, requires energy. As the resetting rate is diminished by the application of respiratory inhibitors and by lowered temperature, but is not affected by darkness, it is clear that respiration and not photosynthesis provides the energy for transport. Experiments with bladders resetting under paraffin oil show that the transported luminal fluid emerges from the mouth region of the bladder. The transport of solution from lumen to outside is essentially a one-way process. Changes of osmotic potential in the luminal solution affect transport rate, but similar changes in the outside solution do not. Bladders which are fully set appear to be almost completely impermeable to water. The transport process can be accounted for as an active transport of chloride ions, with sodium as the predominant accompanying ion, from the lumen to some intermediate region in the cellular bladder wall. Between this region and the lumen an osmotic gradient could be set up that would cause a passive flow of water from the lumen. An increase in hydrostatic pressure in the space could then expel solution, possibly through a subcuticular space to the outside.