Osmotic Factors Affecting the Mobilisation of Sucrose from Vacuoles of Red Beet Storage Root Tissue

Abstract

The accumulation of solutes in the vacuoles of plant cells is a reversible process and vacuolar solutes can be made available both for metabolism and to buffer cytoplasmic solute concentrations. However, some of the storage compounds in the vacuole may also make important contributions to sap osmotic pressure and hence turgor pressure. This raises the question of what happens to turgor pressure when these solutes are exported from the vacuole. To test this, the maintenance of sap osmotic pressure and turgor pressure during the mobilisation of sucrose from red beet storage root vacuoles was studied. Sucrose is localised in the vacuoles of this tissue (Leigh et al., 1979) and contributes about 40% of the sap osmotic pressure (Tomos et al., 1984). Thus if it was completely mobilised without a compensatory accumulation of other solutes, there would be a large decrease in turgor pressure. Mobilisation of sucrose can be induced by washing disks of beet tissue in aerated solutions for several days. This induces a vacuolar acid invertase and the concentration of sucrose in the vacuole declines (Leigh et al., 1979). In addition, washing causes an increase in the rate of salt uptake by the tissue (Van Steveninck, 1975), therefore the effect of salts on turgor opressure and solute content were also studied. The capacity of any turgor pressure regulation mechanism was tested by washing disks at three different external osmotic pressures.